SIO Technical Reference Manual

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By Ray Gwinn

Date
January 1, 1997


INTRODUCTION

Documentation of software is often more difficult than writing the program itself (at least it is for this author). I will do my best though.

But who do I document for? The user that is setting up a BBS just wants to get SIO installed and to move on to more important things. The user that is having problems needs information about serial I/O communications in general and the PC specifically. The application developer (programmer) needs detailed information about each individual function and the information generated.

The answer is, more than one manual is needed. This manual is intended as a reference for use by application programmers that desires to use SIO. A separate Users Manual is included in the distribution file(s).

This manual and the software described is provided as is and with no guarantees. Use at your own risk.


    WHAT IS SIO . . . . . . . . . . . . . . . . . . . . . . . . . . .    1


    Development and Debugging Tools . . . . . . . . . . . . . . . . .    4
         The Poor Man's Line Monitor (PMLM.EXE) . . . . . . . . . . .    4
         VIEWPMLM.EXE . . . . . . . . . . . . . . . . . . . . . . . .    5
         SIO's Utility (SU.EXE) . . . . . . . . . . . . . . . . . . .    5
              IRQs that currently free for use  . . . . . . . . . . .    5
    File System Requests  . . . . . . . . . . . . . . . . . . . . . .    6
         Driver Initialization  . . . . . . . . . . . . . . . . . . .    6
         Read . . . . . . . . . . . . . . . . . . . . . . . . . . . .    6
         Input Status . . . . . . . . . . . . . . . . . . . . . . . .    6
         Input Flush  . . . . . . . . . . . . . . . . . . . . . . . .    7
         Write  . . . . . . . . . . . . . . . . . . . . . . . . . . .    7
         Output Status  . . . . . . . . . . . . . . . . . . . . . . .    7
         Output Flush . . . . . . . . . . . . . . . . . . . . . . . .    7
         Open . . . . . . . . . . . . . . . . . . . . . . . . . . . .    7
              Initial Open  . . . . . . . . . . . . . . . . . . . . .    7
              Additional Open . . . . . . . . . . . . . . . . . . . .    8
         Close  . . . . . . . . . . . . . . . . . . . . . . . . . . .    8
              Intermediate Close  . . . . . . . . . . . . . . . . . .    8
              Last Close  . . . . . . . . . . . . . . . . . . . . . .    8
    Device I/O Control (IOCtl) Commands . . . . . . . . . . . . . . .    9
         IOCtl Functions From a DOS Session . . . . . . . . . . . . .   10
         41h - Set Bit Rate . . . . . . . . . . . . . . . . . . . . .   11
         42h - Set Line Characteristics . . . . . . . . . . . . . . .   12
         43h - Extended Set Bit Rate  . . . . . . . . . . . . . . . .   13
         44h - Transmit Byte Immediate  . . . . . . . . . . . . . . .   14
         45h - Set Break Off  . . . . . . . . . . . . . . . . . . . .   14
         46h - Set Modem Control Signals  . . . . . . . . . . . . . .   15
         47h - Hold Transmit  . . . . . . . . . . . . . . . . . . . .   16
         48h - Start Transmit . . . . . . . . . . . . . . . . . . . .   16
         4Bh - Set Break on . . . . . . . . . . . . . . . . . . . . .   17
         53h - Write Device Control Block (DCB)   . . . . . . . . . .   17
         54h - Write Extended Mode Parameters . . . . . . . . . . . .   20
         61h - Query Current Bit Rate . . . . . . . . . . . . . . . .   21
         62h - Query Line Characteristics . . . . . . . . . . . . . .   21
         63h - Query Extended Bit Rate  . . . . . . . . . . . . . . .   22
         64h - Query SIO Status . . . . . . . . . . . . . . . . . . .   23
         65h - Query Transmit Data Status . . . . . . . . . . . . . .   24
         66h - Query Current Modem Output Control Signals . . . . . .   25
         67h - Query Current Modem Input Signals  . . . . . . . . . .   25
         68h - Query Number of Characters in Receive Buffer . . . . .   26
         6Dh - Query SIO Error  . . . . . . . . . . . . . . . . . . .   27
         72h - Query SIO Event Information  . . . . . . . . . . . . .   28


                                                                       iii
         73h - Read Device Control Block (DCB)  . . . . . . . . . . .   29
         74h - Read Enhanced Mode Parameters  . . . . . . . . . . . .   29
    Differences between SIO/VSIO and COM/VCOM . . . . . . . . . . . .   30
    Appendix A, Hardware Port and IRQ Assignments . . . . . . . . . .   31
         Addresses and IRQs for COM1 and COM2 . . . . . . . . . . . .   31
         Defacto standard for COM3 and COM4 . . . . . . . . . . . . .   31
         Addresses and IRQ for COM3 through COM8 on the PS/2  . . . .   31
    Appendix B, SIO Chips . . . . . . . . . . . . . . . . . . . . . .   32
         8250 . . . . . . . . . . . . . . . . . . . . . . . . . . . .   32
         8250A  . . . . . . . . . . . . . . . . . . . . . . . . . . .   32
         16450  . . . . . . . . . . . . . . . . . . . . . . . . . . .   33
         16C451 . . . . . . . . . . . . . . . . . . . . . . . . . . .   33
         16550 (Non A)  . . . . . . . . . . . . . . . . . . . . . . .   33
         16550A . . . . . . . . . . . . . . . . . . . . . . . . . . .   33
         16550AF  . . . . . . . . . . . . . . . . . . . . . . . . . .   33
         16550AFN . . . . . . . . . . . . . . . . . . . . . . . . . .   33
         16550s Made by Western Digital . . . . . . . . . . . . . . .   34
         16C551 . . . . . . . . . . . . . . . . . . . . . . . . . . .   34
         16C552 . . . . . . . . . . . . . . . . . . . . . . . . . . .   34
         16C554 . . . . . . . . . . . . . . . . . . . . . . . . . . .   34
         82510  . . . . . . . . . . . . . . . . . . . . . . . . . . .   34
         16650  . . . . . . . . . . . . . . . . . . . . . . . . . . .   35
         16750 from Texas Instruments . . . . . . . . . . . . . . . .   35
         ComBic . . . . . . . . . . . . . . . . . . . . . . . . . . .   36
    Appendix C, Multi-Port Serial I/0 Cards . . . . . . . . . . . . .   37
         Hayes ESP  . . . . . . . . . . . . . . . . . . . . . . . . .   37
              ESP in Compatibility Mode . . . . . . . . . . . . . . .   37
         Telcor Tport . . . . . . . . . . . . . . . . . . . . . . . .   37
    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   38
         Gtek BBS550  . . . . . . . . . . . . . . . . . . . . . . . .   38
         Neotech Quad Serial (MCA bus)  . . . . . . . . . . . . . . .   39
    Appendix D, Advanced SIO Options  . . . . . . . . . . . . . . . .   40
         Protection . . . . . . . . . . . . . . . . . . . . . . . . .   40
         Locked Baud Rate . . . . . . . . . . . . . . . . . . . . . .   40
         IRQ Reflection to DOS Process  . . . . . . . . . . . . . . .   40
         I/O Port Mapping to DOS Process  . . . . . . . . . . . . . .   40
    Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   41


WHAT IS SIO

SIO is a Serial Input/Output (SIO) communications character device driver. It provides an interface between application programs and the serial communications hardware.

SIO had been designed as a high performance replacement for the OS/2 device driver COM.SYS. This manual documents the known differences between COM.SYS and SIO.SYS.

SIO only works with 8250 (type) serial I/O devices. Such devices include, but is not limited to the 8250A, 16450, 16550, 16550A and the 82510. If you have a PC that is an IBM or near compatible which has a serial communication port, it is likely that it contains one of these devices. SIO will identify the type of serial devices that it finds at load time.

Like other Device Drivers, SIO will do very little standing alone. There must be an application(s) program that makes use of SIO's functions before one will benefit from the use of SIO.

Development and Debugging Tools

The Poor Man's Line Monitor (PMLM.EXE)

PMLM is basically of interest to developers of comm applications and those providing technical support for those applications. This SIO utility is released largely (in self defense) in the hope that some users can diagnose their own problems. PMLM's feature of saving trace information to disk will only work with registered and beta versions of SIO.

PMLM provides basic line monitoring for OS/2, DOS and Windows comm programs. All characters sent or received by application programs are displayed in the upper portion of the screen. Receive characters are displayed as "white on blue" and transmitted characters are displayed "yellow on violet". Line signals, such as CTS are displayed as "dim white on red" when turned off and "bright white on red" when turned on. In addition, a signals transition to OFF is displayed in lower case and a signals transition to ON is displayed in upper case.

Status information is displayed in the lower part of the screen. A bright white display means the signal is on, a dim white indicates that the signal is off. In addition, when bright, Rxoff and Txoff indicate the SIO has sent (Txoff) or received (Rxoff) an Xoff, and an Xon is pending.

When a DOS or Windows session uses a comm port, an additional line of signal status appears. This status line shows the state of the virtual UART's signals. Only the signals which MAY be different from the real UART's signals are shown.

"Rx Chars" and "Tx Chars" reflects the number of characters in SIO's receive and transmit buffers respectively.

The last line on the screen shows the keys (from the keyboard) that PMLM will recognize. The bright character shows the key to hit for the associated function. The font keys will not work in a windowed OS/2 session.

PMLM's command line requires one parameter, the comm port number, and optionally accepts an additional parameter giving a save trace file name. For example, PMLM 1 COM1INFO.TEC will monitor COM1 and save ALL of the communications traffic in the file COM1INFO.TRC. The trace files created by PMLM contain exactly the same information that is displayed by PMLM, video attributes and all.

VIEWPMLM.EXE

VIEWPMLM.EXE may be used to view trace files created by PMLM. VIEWPMLM.EXE requires one parameter, the trace file name. The user can use the keys UP, DOWN, PAGEUP, PAGEDOWN, HOME, and END to navigate the saved trace file. Registered SIO users may send VIEWPMLM.EXE (along with saved trace files) to anyone for technical support purposes.

VIEWPMLM can be used in native DOS, in DOS sessions under OS/2, and in OS/2 sessions. This means that even those developers and support people need not have OS/2 to view the captured communications data created by PMLM.

SIO's Utility (SU.EXE)

This utility program has several useful commands. You can obtain a list of the available commands by typing SU at any command line prompt without any parameters. One very useful command is SU IRQS. This command will show IRQs that currently free for use. Commands are provided allowing user control (missing from MODE) are available. Status commands provide a wealth of diagnostic information.

File System Requests

SIO processes all file system requests made to it in one way or another. The following file system requests and how SIO processes them may be of use to the application programmer.

  • INIT Driver initialization.
  • Read Data transfer from driver to program.
  • Input Status Get input status.
  • Input Flush Terminate all pending input requests.
  • Write Data transfer from program to driver.
  • Output Status Get output status.
  • Output Flush Terminate all pending output requests.
  • Open Open the device for use.
  • Close Close the device.
  • IOCTL Device I/O control commands.

Each of these file system requests are discussed in more detail in the following.

Driver Initialization

During device driver initialization the command line is parsed, memory for data and control buffers is allocated and the system environment is examined.

Buffers are allocated for the maximum number of ports that the installed version of SIO can support (up to 16).

The mouse port is determined and if it is a serial mouse, that port is excluded from use by SIO.

In the absence of any command line parameters, SIO will only support (look for) COM1 and COM2 at the standard addresses. COM3 and up will not be supported (or looked at) unless they are specifically identified in the SIO command line.

  Read

If another read is already in progress, the process issuing the additional read is blocked (suspended) until the previous read command completes. When a read is started, that read is not interrupted by additional reads. If multiple reads are issued, they are not necessarily processed in the order they were issued. Thus, applications may not see the read requests complete in the order in which they were issued.

The conditions that control when the read will complete are controlled by the active (current) DCB when the read is started.

   Input Status

This function returns with "device busy" set if there is no data in the receive buffer. That is, busy is returned if the process may be blocked if the application issues a read command.

Input Flush

This function causes the receive buffer to be cleared (made empty). Any threads that are blocked, waiting for a read, will be restarted when this function is issued.


  Write

If another write is already in progress, the process issuing the additional write is blocked (suspended) until the previous write command completes. When a write is started, that write is not interrupted by additional writes. If multiple writes are issued, they are not necessarily processed in the order they were issued. Thus, applications may not see the write requests complete in the order in which they were issued.

The conditions that control when the write will complete are controlled by the current DCB when the write is started.

Output Status

This function returns with "device busy" set if the output buffer is more than three quarters full. That is, busy is returned if the process may be blocked if the application issues a write command.

Output Flush

This function causes the transmit buffer to be cleared (made empty). Any threads that are blocked, waiting for a write, will be restarted when this function is issued.

Open

There are basically two kinds of open. They are an Initial Open and Additional (shared) Opens. The IBM documentation calls the initial open a First Level Open. An Initial Open is an open that occurs when no other process has the communications port open. An Additional Open is when an open occurs and another process already has the communications port open. For an Additional Open to occur, the Initial Opener must have issued an open allowing shared access to the port.

Initial Open

The IRQ associated with the comm port is claimed (taken) when the Initial Open occurs. If the associated IRQ is not available a "Port in Use" error is returned. At the time of this writing, the IRQ is claimed allowing sharing. However, the shared IRQ claiming is done to circumvent anomalies in the OS, and in actuality SIO will not concurrently share an IRQ with another device driver. The author found it necessary to claim (reset) all interrupts directed to the interrupt service routine in order to prevent lock ups of a communications port. The IRQ mess may be cleaned up at a future date when the OS problems are corrected.

During Initial Opens, DTR and RTS are set in accordance with the current Device Control Block (DCB) settings. If DTR Control Mode is the "Enabled" option ( bits 1 and 0 of the DCB Flags1 byte equal 01), then DTR is turned on. Otherwise, DTR is unchanged by the Initial Open. Similarly, if RTS Control Mode is the "Enabled" option ( bits 7 and 6 of the DCB Flags1 byte equal 01), then RTS is turned on. Otherwise, RTS is unchanged by the Initial Open.

The following actions also occur during an Initial Open:

  • The Xon and Xoff characters are set to 11h and 13h respectively.
  • The transmit and receive buffers and structures are cleared and re-initialized.
  • Status bits are reset.

If the Initial Opener is an OS/2 process (not a DOS process) then the Read time out processing is set to "Normal" in the current DCB.

Additional Open

A count of active opens is incremented. No changes are made to current comm settings.

Close

There are two basic kinds of closes. They are an Intermediate Close and a Last Close.

Intermediate Close

An intermediate close occurs when a process issues a close and other processes still have the communications port opened. This type of close simply decrements an open count and returns. No action or changes take place at the communications port.

Last Close

The last close occurs when a process issues a close and no other process has the communications port open. If there are any writes pending or incomplete, the process is blocked (suspended) until the writes complete or time out. If 60 seconds pass with no activity at the communications port, the port is forced to close and all writes are terminated. If any processes are blocked (suspended) waiting on a read, they are forced to return to the process that issued the read.

If either DTR Control Mode or RTS Control Mode are set to "Enabled" by the current DCB, then the appropriate signal(s) is turned off.

Finally, the IRQ is released back to the OS.

Device I/O Control (IOCtl) Commands

The following Generic IOCtl functions are supported by SIO.

         41h - Set Bit Rate
         42h - Set Line Characteristics
         43h - Extended Set Bit Rate
         44h - Transmit Byte Immediate
         45h - Set Break Off
         46h - Set Modem Control Signals
         47h - Hold Transmit
         48h - Start Transmit (after hold)
         4Bh - Set Break On
         53h - Write Device Control Block (DCB) 
         54h - Write Extended Mode Parameters
         61h - Query Current Bit Rate
         62h - Query Line Characteristics
         63h - Query Extended Bit Rate
         64h - Query SIO Status
         65h - Query Transmit Data Status
         66h - Query Current Modem Output Control Signals
         67h - Query Current Modem Input Signals
         68h - Query Number of Characters in Receive Buffer
         69h - Query Number of Characters in Transmit Buffer
         6Dh - Query SIO Error
         72h - Query SIO Event Information
         73h - Read Device Control Block (DCB)
         74h - Read Enhanced Mode Parameters

Each of these IOCtl commands are described in detail on the following pages.

All IOCtl functions are called with a uniform set of parameters. This uniform set of parameters is:

         DWORD     Address of a data packet.
         WORD      Size of the data packet in bytes.
         DWORD     Address of the parameter packet.
         WORD      Size of the parameter packet in bytes.
         WORD      Function code.
         WORD      Category code (01 for SIO).
         WORD      Handle for the opened device. 
    Each IOCtl function may or may not have a data or parameter packet. 
    If a given IOCtl function specifies a NULL parameter or data packet,
    the address of the appropriate item must be NULL (zero) in the
    parameters.  Passing an invalid data or parameter packet may result in
    a General Protection Fault.
    IOCtl Functions From a DOS Session
    Additionally the following describes how to call an IOCtl function in
    an OS/2 driver from a DOS session (VDM).  The "handle" is obtained
    simply by issuing a normal DOS open of the device, ie COM1.  The Data
    Packets (DataPkt below) and Parameter Packets (PrmPkt below) are
    different for each IOCtl function.  The structure and contents of
    these packets are described in the following pages.
            push    ds              ;save ds
            mov     bx,handle       ;file handle to bx
            mov     ch,1            ;category, 1=serial device control
            mov     cl,41h          ;func code, 41h=extended set bit rate
            mov     dx,Seg DataPkt  ;get segment of the data packet
            mov     ds,dx           ;load ds with seg of the data packet
            lea     dx,DataPkt      ;data packet address to ds:dx
            mov     si,Seg PrmPkt   ;segment of command packet to si
            lea     di,PrmPkt       ;offset of command packet to di
            mov     ax,440ch        ;IOCtl function call, handle based
            int     21h             ;issue IOCtl to OS/2 driver.
            pop     ds              ;restore ds
            jc      error           ;jump if error returned
                              41h - Set Bit Rate
    Parameter Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Bit/Baud Rate             │ WORD     │
         └───────────────────────────┴──────────┘
         "Bit Rate" is any number from 1 to 65535.  SIO does not attempt
         to validate a given bit rate.  The minimum bit rate accepted by
         SIO is 50, the maximum for this IOCtl function is 65535.
         Note that the hardware may not be able to generate the given bit
         rate.  For example, if a computer has a standard communications
         card installed and if a bit rate of 65500 is specified, the
         nearest bit rate that can be generated will be set, 57600 in this
         case.
         To set bit rates in excess of 57600, see IOCtl function 43h,
         Extended Set Bit Rate.
         Some commonly used bit rates that can be set with this function
         are:
         50        2000
         75        2400
         110       3600
         150       4800
         300       7200
         600       9600
         1200      19200
         1800      28800
         2000      38400
         2400      57600


    Data Packet - None


    12


                        42h - Set Line Characteristics
    Parameter Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Data Bits Per Character   │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Parity Setting            │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Stop Bit Setting          │ BYTE     │
         └───────────────────────────┴──────────┘
         Data Bits Per Character may be 5, 7, or 8, all other values are
         invalid.
         Parity Setting may have the following values, all other values
         are invalid:
              0 - No parity bit.
              1 - Odd parity.
              2 - Even parity.
              3 - Mark parity (1).
              4 - Space parity (0).


         Stop Bit Setting may have the following values, all other values
         are invalid:
              0 - 1 stop bit.
              1 - 1.5 stop bit (valid with 5 data bit characters).
              2 - 2 stop bits.


    Data Packet - None


                                                                        13


                          43h - Extended Set Bit Rate
    Parameter Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Bit/Baud Rate             │ DWORD    │
         ├───────────────────────────┼──────────┤
         │ Fraction                  │ BYTE     │
         └───────────────────────────┴──────────┘


         "Bit Rate" is the desired bit/baud rate.  The maximum bit rate
         supported by SIO is determined by the installed hardware. The
         IOCtl function "63h - Query Extended Bit Rate" can be used to
         determine the maximum bit rate allowed for the associated
         hardware.
         SIO does not attempt to validate a given bit rate.  This means
         that the hardware may not be able to generate the given bit rate. 
         For example, if a computer has a standard communications card
         installed and if a bit rate of 65500 is specified, the nearest
         bit rate that can be generated will be set, 57600 in this case.
         "Fraction" is used to set the fractional part of the bit rate
         when a high degree of accuracy is needed.  As of this writing,
         SIO does not support a "Fraction" other than zero.
         Some commonly used bit rates that can be set with this function
         are:
         110       3600      115200
         150       4800      230400
         300       7200      460800
         600       9600      921600
         1200      19200
         1800      28800
         2000      38400
         2400      57600
    Data Packet - None


    14


                         44h - Transmit Byte Immediate
    Parameter Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Character to Transmit     │ BYTE     │
         └───────────────────────────┴──────────┘
         "Character to Transmit" will be the next character loaded into
         the UART for transmission.  If a previous transmit immediate byte
         is still pending, it is lost.  This function does not wait for
         the character to transmit before returning.
         This function may be used by an application program as one of a
         set of functions to implement its own Xon/Xoff type handshaking.
    Data Packet - None


                              45h - Set Break Off
    Parameter Packet - None
         Look!! No Parameter Packet!
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ SIO Error Word            │ WORD     │
         └───────────────────────────┴──────────┘
         See function 6Dh "Query SIO Error" for a description of the SIO
         Error Word.  This function does not reset any bits in the SIO
         Error Word.


                                                                        15


                        46h - Set Modem Control Signals
    Parameter Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ DTR and RTS on mask       │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ DTR and RTS off mask      │ BYTE     │
         └───────────────────────────┴──────────┘
         Attempted changes in the the setting of signals that are used as
         flow control signals are ignored.  The current DCB determines if
         either or both RTS and DTR are used as a flow control signal.
         The setting of bit 0 affects DTR and the setting of bit 1 affects
         RTS.  Ones in the "on mask" specifies which of the two signals
         are to be turned on, and zeros in the "off mask" specify which of
         the two signals are to be turned off.
         Some examples are:
              On Mask        Off Mask
              01h            0FFh      Turn DTR on
              00h            0FEh      Turn DTR off
              02h            0FFh      Turn RTS on
              00h            0FDh      Turn RTS off
              03h            0FFh      Turn RTS and DTR on
              00h            0FCh      Turn RTS and DTR off
         If both on and off are specified for the setting of a signal, the
         result is undefined.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ SIO Error Word            │ WORD     │
         └───────────────────────────┴──────────┘
         See function 6Dh "Query SIO Error" for a description of the SIO
         Error Word.  This function does not reset any bits in the SIO
         Error Word.


    16


                              47h - Hold Transmit
    Parameter Packet - None
    Data Packet - None
         This function will disable the transmitter.  The transmitter will
         not be enabled again unless IOCtl function 48h is issued.  If the
         transmitter is already disabled, this call has no effect.
         This function can be used as part of a set that allows an
         application to implement its own Xon/Xoff type handshaking.
         Note that this is different from how COM.SYS implements this
         function.  COM.SYS will re-enable the transmitter if an Xon is
         received while SIO will not.


                             48h - Start Transmit
    Parameter Packet - None
    Data Packet - None
         This function will enable the transmitter, usually following an
         IOCtl function 47h.  If the transmitter is already enabled, this
         call has no effect.
         This function can be used as part of a set that allows an
         application to implement its own Xon/Xoff type handshaking.


                                                                        17


                              4Bh - Set Break on
    Parameter Packet - None
         Look!! No Parameter Packet!
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ SIO Error Word            │ WORD     │
         └───────────────────────────┴──────────┘
         See function 6Dh "Query SIO Error" for a description of the SIO
         Error Word.  This function does not reset any bits in the SIO
         Error Word.


                    53h - Write Device Control Block (DCB) 
    This is where the action is!  See also function 73h (Read DCB).
    Parameter Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Write Timeout             │ WORD     │
         ├───────────────────────────┼──────────┤
         │ Read Timeout              │ WORD     │
         ├───────────────────────────┼──────────┤
         │ Flags1                    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Flags2                    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Flags3                    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Error Replacement Byte    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Break Replacement Byte    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Xon Character             │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Xoff Character            │ BYTE     │
         └───────────────────────────┴──────────┘


         "Write Timeout"  The write timeout is given in hundredths of a
         second, relative to zero.  That is, 0 specifies 0.01 seconds and
         1 specifies 0.02 seconds.
         "Read Timeout" The read timeout is given in hundredths of a
         second, relative to zero.  That is, 0 specifies 0.01 seconds and


    18
         1 specifies 0.02 seconds.
         "Flags1" is a bit flag.  The bits in the flag specify the
         following:
         Bits 1 and 0        DTR Control Mode.
                   00 - Disable changing DTR during Open and Close.
                   01 - Enable Changing DTR during Open and Close.
                   10 - DTR used for input handshaking (not supported by
                        SIO).
                   11 - Invalid
         Bit 2,    Must be zero
         Bit 3,    Off to ignore CTS.  Forced on if RTS is handshake.
         Bit 4,    DSR is handshake.  Not supported by SIO.
         Bit 5,    DCD required to transmit.  Not supported by SIO.
         Bit 6,    DSR required to receive.
         Bit 7,    Must be zero.
    ─────────────────────────────────────
         "Flags2" is a bit flag.  The bits in the flag specify the
         following:
         Bit 0,    SIO is to honor Xon/XOff received from remote.
         Bit 1,    SIO may send Xon/Xoff to the remote.
         Bit 2,    Enable error replacement character.  Not supported by
                   SIO.
         Bit 3,    Enable null stripping.  Not supported by SIO.
         Bit 4,    Enable break replacement character.  Not supported by
                   SIO.
         Bit 5,    Enable Full duplex.  Half duplex is not supported by
                   SIO.
         Bits 7 and 6        RTS Control Mode
                   00 - Disable changing RTS during Open and Close.
                   01 - Enable Changing RTS during Open and Close.
                   10 - RTS used for input handshaking.
                   11 - Toggle on transmit (half duplex).  Not supported
                        by SIO.


                                                                        19


         "Flags3" is a bit flag.  The bits in the flag specify the
         following:
         Bit 0,    Enable infinite write timeout processing.  When set, a
                   write timeout will not occur.
         Bits 2 and 1        Read timeout processing
                   00 - Invalid.
                   01 - Normal read time out.
                   10 - Wait for something.  Reads will return before a
                        timeout occurs if something received.
                   11 - No Wait.  Reads will return immediately with any
                        available data.
         Bits 4 and 3        FIFO Control
                   00 - No change in FIFO state.
                   01 - Disable FIFO buffers.
                   10 - Enable FIFO buffers.
                   11 - Dynamic enabling and disabling of FIFOS.
              **Note, SIO forces bits 4 and 3 to "10".
         Bits 6 and 5        Set receive trigger level
                   00 - 1 character.
                   01 - 4 characters.
                   10 - 8 characters.
                   11 - 14 characters.
              **Note, SIO forces bits 6 and 5 to "10".
         Bit 7               Transmit load count
                   0 -  Load 1 character.
                   1 -  Load 16 characters.
              **Note, SIO forces bit 7 to "1".
         "Error Replacement Byte" Not supported by SIO.
         "Break Replacement Byte" Not supported by SIO
         "Xon Character" Any value, default is 11h.
         "Xoff Character" Any value, default is 13h.
    Data Packet - None


    20


                     54h - Write Extended Mode Parameters
    Parameter Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Enhanced Flags1           │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Reserved                  │ DWORD    │
         └───────────────────────────┴──────────┘
         This function is not supported (ignored) by SIO.  However the
         bits are defined here for completeness.
         Enhanced Flags1
         Bit 0 -   Enhanced mode hardware available (Query only 74h).
         Bit 1 -   Enable enhanced mode.
         Bits 3 and 2        DMA Receive Operation
              00 - Disable DMA receive capability.
              01 - Enable DMA receive capability.
              10 - Dedicate a DMA channel to receive.
              11 - Reserved.
         Bits 5 and 4        DMA Transmit Operation
              00 - Disable DMA transmit capability.
              01 - Enable DMA transmit capability
              10 - Dedicate a DMA channel to transmit.
              11 - Reserved.
         Bit 6 -   Receive in DMA mode (Query only, 74h).
         Bit 7 -   Transmit in DMA mode (Query only, 74h).
    Data Packet - None.


                                                                        21


                         61h - Query Current Bit Rate
    Parameter Packet - None.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Bit/Baud Rate             │ WORD     │
         └───────────────────────────┴──────────┘
         See function 41h for more information.


                       62h - Query Line Characteristics
    Parameter Packet - None.


    Data Packet


         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Data Bits Per Character   │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Parity Setting            │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Stop Bit Setting          │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Transmitting Break        │ BYTE     │
         └───────────────────────────┴──────────┘
         See function 42h for a definition of Data, Parity and Stop Bits.
         "Transmitting Break" is set to 1 if a break is being transmitted. 
         Otherwise, it is set to zero.


    22


                         63h - Query Extended Bit Rate
    Parameter Packet - None


    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Current Bit/Baud Rate     │ DWORD    │
         ├───────────────────────────┼──────────┤
         │ Fraction of Current       │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Minimum Bit/Baud Rate     │ DWORD    │
         ├───────────────────────────┼──────────┤
         │ Fraction of Minimum       │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Maximum Bit/Baud Rate     │ DWORD    │
         ├───────────────────────────┼──────────┤
         │ Fraction of Maximum       │ BYTE     │
         └───────────────────────────┴──────────┘
         "Current Bit/Baud Rate" is a 32 bit integer defining the current
         bit rate.
         "Fraction of Current" is always 0 when returned by SIO.
         "Minimum Bit/Baud Rate" is the minimum value that can be passed
         to SIO as a bit/baud rate.
         "Fraction of Minimum" is always 0 when returned by SIO. 
         "Maximum Bit/Baud Rate" is the maximum value that can be passed
         to SIO as a bit/baud rate.
         "Fraction of Maximum" is always 0 when returned by SIO.


                                                                        23


                            64h - Query SIO Status
    Parameter Packet - None
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ SIO Status Byte           │ BYTE     │
         └───────────────────────────┴──────────┘
         "SIO Status Byte" is a bit flag.  The bits have the following
         meanings:
         Bit 0 -   Set if SIO is waiting for CTS before transmitting.
         Bit 1 -   Set if SIO is waiting for DSR before transmitting.
         Bit 2 -   Set if SIO is waiting for DCD before transmitting.
         Bit 3 -   Set if SIO is waiting because Xoff received or
                   transmitter is disabled (see function 47h).
         Bit 4 -   Reserved, not used by SIO.
         Bit 5 -   Set if SIO is waiting because break is being sent.
         Bit 6 -   Set if a transmit immediate is pending.
         Bit 7 -   Reserved, not used by SIO.


    24


                       65h - Query Transmit Data Status
    Parameter Packet - None.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Transmit Status           │ BYTE     │
         └───────────────────────────┴──────────┘
         "SIO Status Byte" is a bit flag.  The bits have the following
         meanings:
         Bit 0 -   Set if write in progress or pending.
         Bit 1 -   Set if any data is in the transmit buffer.
         Bit 2 -   Set if UART is currently transmitting.
         Bit 3 -   Set if a transmit immediate is pending.
         Bit 4 -   Set if an Xon needs to be sent.
         Bit 5 -   Set if an Xoff needs to be sent.
         Bit 6 -   Reserved
         Bit 7 -   Reserved


                                                                        25


               66h - Query Current Modem Output Control Signals


    Parameter Packet - None.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Modem Output Signals      │ BYTE     │
         └───────────────────────────┴──────────┘
         "Modem Output Signals" is a bit flag.  The bits have the
         following meanings:
         Bit 0 -   Set if DTR is currently on.
         Bit 1 -   Set if RTS is currently on.
         Bits 2-7  Reserved, set to zero by SIO.


                    67h - Query Current Modem Input Signals
    Parameter Packet - None.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Modem Input Signals       │ BYTE     │
         └───────────────────────────┴──────────┘
         "Modem Output Signals" is a bit flag.  The bits have the
         following meanings:
         Bits 0-3  Reserved, set to zero by SIO.
         Bit 4 -   Clear To Send (CTS) is active.
         Bit 5 -   Data Set Ready (DSR) is active.
         Bit 6 -   Ring Indicator (RI), trailing edge detected.
         Bit 7 -   Data Carrier Detect (DCD) is active.


    26


              68h - Query Number of Characters in Receive Buffer
    Parameter Packet - None.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Number of Bytes Buffered  │ WORD     │
         ├───────────────────────────┼──────────┤
         │ Byte Size of the Buffer   │ WORD     │
         └───────────────────────────┴──────────┘
         Self explaining.


              69h - Query Number of Characters in Transmit Buffer
    Parameter Packet - None.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Number of Bytes Buffered  │ WORD     │
         ├───────────────────────────┼──────────┤
         │ Byte Size of the Buffer   │ WORD     │
         └───────────────────────────┴──────────┘
         Self explaining.


                                                                        27


                             6Dh - Query SIO Error


    Parameter Packet - None.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ SIO Error Word            │ WORD     │
         └───────────────────────────┴──────────┘
         "SIO Error Word" is a bit flag.  The bits have the following
         meaning:
         Bit 0 -   Set if a buffer overrun has occurred.
         Bit 1 -   Set if an overrun occurred at the UART.
         Bit 2 -   Set if the UART detected a parity error.
         Bit 3 -   Set if the UART detected a framing error.
         Bits 4-15 Reserved, set to zero by SIO.
         All bits of the SIO Error Word are reset prior to returning to
         the caller.


    28


                       72h - Query SIO Event Information
    Parameter Packet - None.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ SIO Event Word            │ WORD     │
         └───────────────────────────┴──────────┘
         "SIO Event Word" is a bit flag.  The bits have the following
         meaning:
         Bit 0 -   Set when a receive character is transferred from the
                   UART to the receive buffer.
         Bit 1 -   Set when a receive (Read) timeout occurs.
         Bit 2 -   Set if the transmit buffer is empty.
         Bit 3 -   Set if Clear To Send (CTS) has changed state.
         Bit 4 -   Set if Data Set Ready (DSR) has changed state.
         Bit 5 -   Set if Data Carrier Detect (DCD) has changed state.
         Bit 6 -   Set if a break has been detected.
         Bit 7 -   Set if a framing, parity, or overrun error has
                   occurred.
         Bit 8 -   Set if trailing edge of Ring Indicator (RI) has
                   occurred.
         Bits 9-15 Reserved, set to zero by SIO.
         All bits are reset prior to returning to the caller.


                                                                        29


                     73h - Read Device Control Block (DCB)
    Parameter Packet - None.


    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Write Timeout             │ WORD     │
         ├───────────────────────────┼──────────┤
         │ Read Timeout              │ WORD     │
         ├───────────────────────────┼──────────┤
         │ Flags1                    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Flags2                    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Flags3                    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Error Replacement Byte    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Break Replacement Byte    │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Xon Character             │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Xoff Character            │ BYTE     │
         └───────────────────────────┴──────────┘
         See function 53h for a description of the parameters.


                      74h - Read Enhanced Mode Parameters
    Parameter Packet - None.
    Data Packet
         ┌──────────────────────────────────────┐
         │         Item                Type     │
         ├───────────────────────────┬──────────┤
         │ Enhanced Flags1           │ BYTE     │
         ├───────────────────────────┼──────────┤
         │ Reserved                  │ DWORD    │
         └───────────────────────────┴──────────┘
         See function 54h for a description of the Flags1 Byte.


    30
                   Differences between SIO/VSIO and COM/VCOM
    What follows is some of the differences between SIO/VSIO and COM/VCOM
    that are known to the author.
    1 -  COM.SYS will automatically seek out and support four comm
         ports, COM1 through COM4.  SIO.SYS will automatically seek
         out and support only COM1 and COM2.  If SIO.SYS is to
         support more than two comm ports, they must be defined in
         the command line that loads SIO.SYS.  For example,
         DEVICE=SIO.SYS (COM3) (COM4) will cause SIO.SYS to support
         four comm ports.
    2 -  In the absence of overrides in the command line, COM.SYS
         will seek out four comm ports in the sequence of I/O ports
         3F8h, 2F8h, 3E8h and 2E8h.  The first port found in this
         sequence is given the logical name COM1, the second is COM2
         etc.  This means that the comm port at 2F8h could become
         COM1.  In the opinion of the author, this confuses some
         users to frustration.
         With no command line overrides, SIO.SYS uses a fixed I/O
         port concept.  That is, COM1 is always at I/O port 3F8h,
         COM2 is always at 2F8h, etc.  Another way of looking at this
         is;  if no comm port exists at port 3F8h, then you do not
         have a COM1.  SIO.SYS will not move another (found) comm
         port into the logical COM1 slot.
    3 -  VSIO opens a comm port for a DOS session with sharing allowed. 
         VCOM uses an "exclusive use" open for DOS sessions.
    4 -  SIO.SYS will accept any number from 50 up as a valid baud rate. 
         For example, if an application program attempts to set a baud
         rate of 9990 bps, COM.SYS will reject it and SIO.SYS will accept
         it.
    5 -  SIO/VSIO execute faster than COM/VCOM.  This means that some
         applications, that are timing dependent, may not work under
         SIO/VSIO that do work under COM/VCOM.
    6 -  SIO/VSIO provides more reliable active flow control for DOS
         sessions.  For example, if RTS/CTS handshaking is in use, VSIO
         virtualizes the RTS settings by the DOS program while SIO
         completely controls the real RTS signal.  COM/VCOM simply passes
         the RTS signal directly to the hardware.
    7 -  COM.SYS will block an application until a write completes.  SIO
         will return to the application program immediately if all of the
         write data will fit into SIO's internal buffers.


                                                                        31
                 Appendix A, Hardware Port and IRQ Assignments
    I have obtained the following information from various sources. 
    Accuracy is not guaranteed.  In fact, nothing in this document is
    guaranteed.
    Addresses and IRQs for COM1 and COM2
         COM1 uses 03F8h and IRQ4
         COM2 uses 02F8h and IRQ3
    Defacto standard for COM3 and COM4 for PC, XT and AT
         COM3 uses 03E8h and IRQ4
         COM4 uses 02E8h and IRQ3
    Addresses and IRQ for COM3 through COM8 on the PS/2
         COM3 uses 3220h and IRQ3
         COM4 uses 3228h and IRQ3
         COM5 uses 4220h and IRQ3
         COM6 uses 4228h and IRQ3
         COM7 uses 5220h and IRQ3
         COM8 uses 5228h and IRQ3
    All PCs (known to the author) prior to the PS/2 and EISA only use 10
    bits to address hardware I/0 ports.  Systems prior to the PS/2 and
    EISA can not address the standard PS/2 addresses for COM3 through
    COM8.  If an AT or below attempts to address COM3 through COM8 using
    the PS/2 addresses, only the low 10 bits of the address are used. 
    That is, the high 6 bits of the address will be ignored by the
    hardware.  This means that any reference to COM3 through COM8 will
    actually address ports 220h through 22Fh on older PCs.
    One should avoid expansion boards that use ports 220h through 22Fh
    unless the expansion board is a serial I/O board. 
    SIO will (attempt to) determine the hardware architecture (ISA/EISA
    and PS/2) that it is being executed on and use the appropriate
    hardware port addresses for COM1 thru COM4 as defaults.


    32
                             Appendix B, SIO Chips


    The following is a collection of comments developed from hearsay,
    random documents, experience, and technical specifications. So, take
    it with a grain of salt.  Order information from the various
    manufactures if you want accurate information.
    Very good reference books are available from National Semiconductor
    Corporation.  They cover most of the SIO devices (also called UARTs)
    that you will find in many PCs and clones.  I am not sure that books
    can be ordered directly from National Semiconductor. They may require
    that you get it from a distributor.  In any case, the address on the
    back of the one manual is:
         National Semiconductor Corporation
         2900 Semiconductor Drive
         P.O. Box 58090
         Santa Clara, CA 95052-8090
         Tel:(408)721-5000
         TWX:(910)339-9240
    8250
    As best I known, the 8250 was the first SIO chip (integrated circuit)
    that was used by the IBM PC and many clones.  In my opinion, it was a
    poor choice on the part of IBM.  I feel many superior devices, at
    comparable prices, were readily available.  At that time, I feel the
    8251A or the 8530 would have been better choices.  But, they used the
    8250 and therefore we must use it.
    From a hardware standpoint the 8250 is a relatively slow device.  It
    is advisable that programmers not perform successive inputs or outputs
    to this device.  It seems that software programs can load the various
    registers of the 8250 faster than it can process the information.  The
    8250 had a total of 7 registers.  The specifications state that 56kb
    is the maximum baud rate.
    8250A
    I believe the 8250A is the 8250 with some bug fixes.  I have no idea
    what the bugs may have been.  A quick glance at the specifications
    shows the speed of the 8250 and 8250A to be much the same.  The 8250A
    added an 8th register.  This additional register enables software to
    detect if an 8250 is installed.  The specifications state that 56kb is
    the maximum baud rate.


                                                                        33
    16450
    The 16450 seems to be a speeded up version of the 8250A.  There is no
    direct way (that I know of) for software to detect the difference
    between an 8250A and a 16450.  I believe the 16450 was developed to
    eliminate the need for software to insert delays between successive
    accesses to the device.  The specifications indicate the 16450 is a
    much faster device than its predecessors.  The additional speed is
    only the speed at which the processor can access the device.  The
    maximum baud rate for the 16450 is still stated at 56kb.  However, I
    have been told by some people that they have run the 16450
    successfully at much higher speeds. I do not believe there was ever a
    16450A.
    16C451
    The 16C451 is a CMOS version of the 16450.  CMOS is a term for the
    material and manufacturing process used to make the part.  CMOS
    typically uses less power than other technologies.  If you are not
    designing hardware, you should view the 16C451 as a 16450.
    16550 (Non A)
    It is hard to find a 16550 (Non A).  I was told by National
    Semiconductor that they did everything they could to get all 16550s
    back.  SIO will detect a 16550 and tell you if you have one.  I am
    told that the 16550 was installed in early PS/2 systems.
    The 16550 was the first shot at a FIFOed version of the 8250 family
    from National semiconductor.  However, I was told by National
    Semiconductor that the FIFOs of the 16550 are not reliable and they
    should not be enabled.  SIO will treat a 16550 like a 16450.  In this
    mode, they are reliable.  National Semiconductor would not provide me
    with a specification for the 16550.  However, I suspect its maximum
    baud rate is the same as the 16550A which is 256kb.
    16550A, 16550AF and 16550AFN
    In the manuals that I have, National Semiconductor does not explain
    the differences between the 16550A and the 16550AF.  I suspect the AF
    part may have a few bug fixes.  I believe the N in AFN describes
    packaging, ceramic versus plastic, DIP versus surface mount etc.
    In the opinion of the author, there is no substitute for the 16550A
    (and its successors) in the 8250 type series.  The 16550A is
    compatible with most software written for the entire family of 8250
    type devices.  Programs that are 16550A aware can provide much
    improved performance over previous devices.


    The maximum baud rate for the 16550A is specified at 256kb.  However,
    due to the hardware design of the PC et al, 115kb is the maximum baud
    rate that can be programmed by software.
    The 16550A can be plugged into the same socket that contains an 8250,


    34
    8250A or 16450.  If your SIO expansion board has the SIO chips in
    sockets, you can upgrade to the 16550A by simply removing the old
    chips and replacing them with 16550As.
    The key to the performance increase of the 16550A is its FIFOs.  It
    has 16 byte FIFOs for both transmit and receive data.
    16550s Made by Western Digital
    I have been told, but I have not verified for myself, that 16550s made
    by Western Digital have a problem with their FIFOs when working at
    2400 baud or below.
    16C551
    The 16C551 is a CMOS version of the 16550AF.  See the above
    description of the 16C451 for a discussion of CMOS.  Do not feel you
    need to upgrade from a 16550A or AF to a 16C551.  There is no gain
    from an existing users point of view.
    16C552
    The 16C552 is two 16C551s on a single chip.
    16C554
    The 16C554 is four 16C551s on a single chip.
    82510
    I believe Intel is the only company that manufactures the 82510.  The
    82510 is feature rich with several modes of operation.  Its default
    mode is to operate as a 16450.  The 82510 has a 4 byte FIFO for both
    transmit and receive data.  A 4 byte FIFO is sufficient to provide
    significant performance over a basic 16450.
    The 82510 is small in size.  Therefore, it is found in many lap tops.
    The 82510 is somewhat of a sleeper.  I believe it would be much more
    widely used if Intel had promoted it more.  However, given a choice
    between the 82510 and the 16550A, I would select the 16550A.


                                                                        35


    16650
    The 16650 by Startech seems a good logical step in the evolution of
    the 8250 type UARTs.  One of the many flavors of the 16650 is a
    package that is pin compatible with the 40 pin (DIP) 16550s.  This
    means that the 16650 can be placed into the same socket as any 16550,
    8250, etc.
    The 16650 has enhanced the FIFOs to 32 bytes, added on chip flow
    control ability, and increases the maximum bit rate to 460800.  The on
    chip flow control is a really nice feature.  On chip flow control
    practically eliminates the possibility of missed receive characters. 
    On chip flow control also means that some devices that require
    immediate attention when it sends Xoff, like the Seiko Label Printer,
    will work much better in environments like OS/2 and Windows.
    The 32 byte FIFOs of the 16650 is an improvement over the 16550, but
    is still a disappointment.  I feel that 1k, or larger, FIFOs would
    have been a much better choice.  When I quizzed Startech about this,
    they replied that the 32 byte FIFOs were chosen to keep the cost down. 
    They added that if the 16650 is successful, they plan a 16750 with
    larger FIFOs.
    The 16650 powers up in a 16550 compatible mode, meaning most software
    written for the 16550 should work.  I have discovered a serious
    incompatibility in the current (Oct 1994) versions of the 16650.  All
    previous UARTS in the 8250 family will issue only a single transmit
    interrupt if no additional data is sent to the UART.  The 16650 on the
    other hand, will issue continuous transmit interrupts making it
    necessary to mask the transmit interrupt.  I quizzed Startech about
    this and was told that a new revision of the chip would be available
    within a few months that would correct this incompatibility.
    16750 from Texas Instruments
    The 16750 from TI is an improvement over the 16550, but the omission
    of software flow control (Xon/Xoff) makes it inferior to the 16650, in
    the opinion of the author.  The 16750 has a 64 byte fifo and hardware
    flow control.  It is a good choice for a uart.


    36


    ComBic
    The ComBic is a very impressive UART used on the Hayes ESP card and
    some others.  The ComBic is basically a 16550 UART with on chip flow
    control and 1k FIFOs and a maximum bit rate of 921600.   Like the
    16650, on chip flow control practically eliminates the possibility of
    missed receive characters.  On chip flow control also means that some
    devices that require immediate attention when it sends Xoff, like the
    Seiko Label Printer, will work much better in environments like OS/2
    and Windows.
    The ComBic has two modes of operation, compatibility mode and enhanced
    mode.  In compatibility mode, the UART acts as a 16550.  However, when
    in compatibility mode, the 1k FIFOs and on chip flow control can still
    be used.  In enhanced mode, software gains the ability to look into
    the FIFOs and determine the exact number of bytes loaded.  This
    feature enables software the use block input and output to the ComBic
    which greatly reduces processor overhead.


                                                                        37


                    Appendix C, Multi-Port Serial I/0 Cards
    I have received many requests for information about multi-port serial
    I/O card.  I will describe the boards that I know about (or have heard
    about) here.  I will add to the list as I receive precise information
    from manufactures, users, or I develop it myself.
    Hayes ESP
    The original ESP (ESP V1) is not supported.  The SIO command line must
    contain the ESP's enhanced I/O port address.  For bit rates exceeding
    180000bps the IRQ MUST be specified for the master port. For lower
    data rates NONE may be specified for the IRQ.  SIO does not search for
    the ESP ports as the Hayes drivers do under DOS/Windows.  This
    searching can (will) break things under OS/2.  The SIO command line
    for an ESP at I/O port 180h using IRQ14 would be as follows:
    DEVICE=path\SIO.SYS (COM1,180,IRQ14) (COM2,188,NONE)
    If you would like the above reflected to DOS sessions as a standard
    COM1 and COM2, then use the following command line:
    DEVICE=path\SIO.SYS (COM1,180:3F8,IRQ14:IRQ4) (COM2,188:2F8,NONE:IRQ3)
    Note that the numbers after the colons ":" are the address and IRQ of
    where the port appears to DOS software running under OS/2.  
    Any IRQ specified for an ESP slave port is ignored.  For those that
    like to have a clean looking CONFIG.SYS, SIO will accept NONE as the
    IRQ specification of an ESP slave port.
    If you wish to use the ESP in Compatibility Mode, simply specify the
    base port address that the ESP's compatibility mode is configured for
    (ie 3F8 when configured for COM1).
    Telcor Tport
    The Telcor Tport is supported in both burst mode and 16450 emulation
    mode.  When in burst mode, the Tport's 16k FIFOs are enabled. 
    Additionally in burst mode, no IRQ is used by the Tport.  In burst
    mode, any IRQ specified for the Tport is ignored.  For those that like
    to have a clean looking CONFIG.SYS, SIO will accept NONE as the IRQ
    specification of a Tport.  An example command line defining a Tport
    card to be used in burst mode; and that is configured for COM1
    follows:
    DEVICE=path\SIO.SYS (COM1,Tport@3F8,NONE:IRQ4)
    The Tport's 16450 Emulation mode simply uses a normal SIO command
    line. eg:
    DEVICE=path\SIO.SYS (COM1,3F8,IRQ4:IRQ4)
    In both examples the "IRQ4" following the colon above, means that the
    virtual IRQs are to be sent to a VDM on IRQ4.


    38
    STB 4COM  (ISA bus)
    I am very impressed with the STB 4COM card, they have almost got it. 
    Yes, it's the same company that makes the VGA cards.  The 4COM card
    uses a 16554 which is the equivalent of four 16550A.  Comm ports can
    use IRQs 2/9, 3, 4, 5, 10, 11, 12, and 15.  Any or all ports can share
    IRQs, and they have done this right.  I tested SIO with all four ports
    of the 4COM on IRQ12 and they worked correctly.  The user can select
    any of the following I/O port address for a comm port: 3F8, 2F8, 3E8,
    2E8, 1A8, 1E8, 1F8, 2A8.
    The 4COM card is priced at $110 and I ordered mine from Prerapture
    Solutions which can be reached by telephone at 1-919-286-1502 for
    information and foreign callers.  In the US orders can be placed by
    calling 1-800-735-5266.  Their mailing address is:
         Prerapture Solutions
         1806 Albany St.
         Durham NC 27705-3135
    Gtek BBS550 (ISA bus)
    The Gtek BBS550 is a communications card with the equivalent of 8
    16550s (in reality two 16554s) UARTS.  Where were these cards 5 years
    ago!  Like the STB 4COM, I am very impressed with the BBS550.  It has
    a wide range of selectable I/O port address, some in the 16 bit range
    (above 3FFh) which can be very useful.  I tested the board with all 8
    ports installed using IRQ3 (although I never had more than 3 ports
    active at the same time).  The board performed well using shared IRQs.
    The BBS550 has 8 RJ-45 connectors (10 pin), 6 of which comes directly
    off the back of the card.  The other two RJ-45 connectors are internal
    and special cables with grommets are supplier for these 2 ports.  The
    cables supplied with the BBS550 expands the RJ-45 out to standard DB-
    25 connectors.
    Gtek is located in St. Louis Mississippi.  Their phone number is 800-
    282-4835 or 601-467-8048.


                                                                        39
    Neotech Quad Serial (MCA bus)
    Well, I just got my first PS/2 system (for testing of SIO) and I am
    using the NeoTech Quad Serial card in that system.  What can I say, it
    works beautifully and interrupt sharing has never been a problem on
    MCA systems.  This card (as shipped) came with four individual 16450
    chips installed, but they were in sockets so I installed 16550 UARTS
    before I installed the card.  The card has one large connector on the
    back of the card and the supplied cable expands that to 4 standard DB-
    25 connectors.  Those of you needing a 4 port card for an MCA system
    contact:
         Neotech Inc
         30295 Solon Industrial Pkwy.
         Solon, Ohio 44139
         Voice     (216) 248-4114
         FAX       (216) 248-5701


    40
                       Appendix D, Advanced SIO Options


    The following is provided for the experienced user.  Inexperienced
    users get themselves into a lot of trouble when trying to use these
    options and they cause an inordinate number of support problems.
    Therefore, the following are unsupported options.  If you use them,
    you must work out the problems yourself or ask a friend how they got
    it working.
    Protection
    Placing a dash "-" or a plus "+" in the fourth parameter position, eg
    (COM1,3F8,IRQ4,-),  causes SIO not to provide any protection for that
    port.  The OS may still protect the port somewhat.  When the dash
    parameter is used the port is wide open (like DOS) for any process to
    access it.  If the plus is used, then DOS/Windows sessions are
    inhibited from turning DTR off.
    Locked Baud Rate
    Placing a colon and a baud rate following the communications port, eg
    (COM1:38400,3F8,IRQ4), causes the port to be locked at that baud rate. 
    No program, OS2 or DOS, is allowed to change the baud rate.
    IRQ Reflection to DOS Process
    Normally the IRQ that is reflected to a DOS process is the same as the
    actual hardware IRQ.  Placing a colon and an additional IRQ following
    the normal IRQ, eg (COM1,3F8,IRQ12:IRQ4), causes SIO/VSIO to reflect
    the IRQ to a DOS process on IRQ4 (in this example) even though the
    real hardware IRQ is IRQ12.  If one wishes SIO to automatically locate
    the hardware IRQ, then the following construct can be used:
    (COM1,3F8,:IRQ4).  Notice that the hardware IRQ is simply NOT
    specified.
    I/O Port Mapping to DOS Process
    Some DOS programs, such as CompuServe's CIM, insist that COM1 be at
    I/O port 03F8, COM2 at 02F8, etc.  On OS/2 systems, comm ports can
    have any I/O port address as specified by a command line.  If COM1 is
    not at I/O port 03F8 for some DOS programs, then COM1 can not be used
    by the DOS program.  The SIO command line allows the I/O ports to be
    mapped to a different address for DOS sessions, eg
    (COM1,FF80:03F8,IRQ12:IRQ4).  In this example, the real COM1 is at
    FF80 on IRQ12, but is reflected to DOS processes on I/O ports 03F8 at
    IRQ4.  It is NOT necessary to map both the I/O port address and the
    IRQ.  When the I/O port is mapped to a DOS session, the UART will
    appear at BOTH I/O port addresses (FF80 and 03F8 in the given
    example).