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 How to Use CALL INTERRUPT to Diagnose COM "Device I/O" Error - Q67315
 
 This article contains a code example showing how to obtain and decode
 the information contained within the communications line-status and
 modem-control registers. The contents of the registers is found by
 calling ROM BIOS Interrupt 14h, with function 3. This information is
 helpful in pinpointing the cause of communications port (COM1 or
 COM2), errors such as "Device I/O" errors.
 
 More Information:
 
 Calling Interrupt 14h with function 3h is helpful in finding
 whether a "Device I/O" error stems from problems such as break,
 framing, or overrun problems during communications. If you are trying
 to pinpoint the underlying cause of a communications error, you should
 call the interrupt from within an ON ERROR GOTO handler in your Basic
 program. The code sample below demonstrates how the communications
 port status can be checked when a "Device I/O" error is generated.
 
 Information on Interrupt 14h with function 3h can be found in
 the following books:
 
    "The New Peter Norton Programmer's Guide to the IBM PC & PS/2; the
    Ultimate Reference Guide to the Entire Family of IBM Personal
    Computers" by Peter Norton and Richard Wilton, published by
    Microsoft Press (1985). See page 230.
 
    "The Programmer's PC Sourcebook" by Thom Hogan, published by
    Microsoft Press (1988). See page 220.
 
 Code Example
 ------------
 
 To try this example in VBDOS.EXE:
    1. From the File menu, choose New Project.
    2. Copy the code example to the Code window.
    3. Press F5 to run the program.
 
 ' To run this program in the environment, you must invoke the
 ' environment with the /L switch to load the default Quick library:
 ' VBDOS.EXE /L for Visual Basic 1.0 for MS-DOS
 '
 ' Use the following include file for Visual Basic 1.0 for MS-DOS:
 REM $INCLUDE: 'VBDOS.BI'
 DECLARE SUB GetPortStatus (MessageCount%)
 DIM SHARED InRegs AS RegType, OutRegs AS RegType
 DIM SHARED StatusMessage(15) AS STRING * 35
 CLS
 ON ERROR GOTO ErrorHandler:
 ERROR 57      ' This statement forces a "Device I/O" error.
 
 ' The following code (REMARKed out) is a sample routine that reads
 ' communications port input. Remove the ERROR 57 line above and the
 ' REMARKs in this section of code to run the sample when data is being
 ' sent from another device to COM1. It is unlikely that the following
 ' code segment will generate a "Device I/O" error. The code is merely
 ' included as an indication of where the communications routine is
 ' located relative to the rest of the code. It is recommended that you
 ' replace the following code segment with your own communications
 ' routine, especially if your code leads to "Device I/O" or other
 ' communications related errors.
 
 ' Remove the '_' character below - it is for readability purposes only:
 'OPEN "COM1:300,N,8,1,BIN,CD0,CS0,DS0,OP0,RS,TB2048,RB2048" FOR _
        RANDOM AS #1
 
 'WHILE LOC(1)=0
 'WEND
 
 'WHILE NOT EOF(1)
 '   a$ = INPUT$(LOC(1),1)
 'WEND
 
 Done:
 'CLOSE #1
 END
 
 ErrorHandler:
    IF ERR = 57 THEN
       CALLGetPortStatus(LastMessage%)
       FOR i% = 0 TO LastMessage% - 1
          PRINT StatusMessage(i%)
       NEXT i%
    END IF
    RESUME Done:
 
 REM $STATIC
 SUB GetPortStatus (MessageCount%)
    MessageCount% = 0
    InRegs.AX = &H300   ' Load the service number 3h in the
                        ' high byte of AX.
             ' Call ROM BIOS Interrupt 14h.
    CALL Interrupt(&H14, InRegs, OutRegs)
 
    ' Decode the upper 8 bits of AX containing line-status
    ' register information:
 
    IF OutRegs.AX AND 2 ^ 8 THEN
       StatusMessage(MessageCount%) = "Data ready"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 9 THEN
       StatusMessage(MessageCount%) = "Overrun error"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 10 THEN
       StatusMessage(MessageCount%) = "Parity error"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 11 THEN
       StatusMessage(MessageCount%) = "Framing error"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 12 THEN
       StatusMessage(MessageCount%) = "Break-detect error"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 13 THEN
       StatusMessage(MessageCount%) = _
          "Transfer holding register empty"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 14 THEN
       StatusMessage(MessageCount%) = "Transfer shift register empty"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 15 THEN
       StatusMessage(MessageCount%) = "Time-out error"
       MessageCount% = MessageCount% + 1
    END IF
 
    ' Decode the lower 8 bits of the AX register containing
    ' modem-status information:
 
    IF OutRegs.AX AND 2 ^ 0 THEN
       StatusMessage(MessageCount%) = "Delta clear-to-send"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 1 THEN
       StatusMessage(MessageCount%) = "Delta data-set-ready"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 2 THEN
       StatusMessage(MessageCount%) = "Trailing-edge ring detector"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 3 THEN
       StatusMessage(MessageCount%) = _
          "Delta receive line signal detect"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 4 THEN
       StatusMessage(MessageCount%) = "Clear-to-send"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 5 THEN
       StatusMessage(MessageCount%) = "Data-set-ready"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 6 THEN
       StatusMessage(MessageCount%) = "Ring indicator"
       MessageCount% = MessageCount% + 1
    END IF
    IF OutRegs.AX AND 2 ^ 7 THEN
       StatusMessage(MessageCount%) = "Received line signal detect"
       MessageCount% = MessageCount% + 1
    END IF
 END SUB