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Article Q67315
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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