How to tell if a serial port is "free"

[Robin England]

Hello all,

Is there a way in PBCC40 to tell if a serial port (COM port) is already in use by another application under Windows?

I would like to be able to tell not only if a serial port is available on a computer, but if an application is already using it.

I wrote some simple code to scan for available serial ports:

    STDOUT "The following COM ports are available:"
    STDOUT ""
    FOR x=1 TO 255
        a$="COM"+LTRIM$(STR$(x))
        COMM OPEN a$ AS #1
        ON ERROR GOTO Loop1
        PRINT "COM";x;" is available."
        COMM CLOSE #1
    Loop1:
    NEXT x

I have found that the COMM OPEN statement is reliable in determining the existence of the serial ports present on a machine, however, even if I open a connection to an available COM port using another application like HyperTerminal, COMM OPEN still indicates that the port is available.

Thanks!

[Michael Mattias]

> COMM OPEN still indicates that the port is available.
Are you sure?

....
   COMM OPEN a$ AS #1
   ON ERROR GOTO Loop1
....

You need to execute the ON ERROR GOTO before executing the statement(s) for which an error trap is to be actve.

You don't need to use ON ERROR, you can just test the system ERR variable.. AFAIK that will tell you if there was an error on the COMM OPEN regardless of the ON ERROR GOTO situation.

ie. I think something like this should work

    FOR x=1 TO 255
        ERRCLEAR               ' clear the ERR variable
        a$="COM"+LTRIM$(STR$(x))
        COMM OPEN a$ AS #1                 ' attempt OPEN
        IF ERR THEN                              ' OPEN failed 
           Print/Stdout USING$ ("Could not open COM # Err # &", X, ERR, ERROR$(ERR))
        ELSE   ' OPEN succeeds so we have to close the handle
           COMM CLOSE #1
       END IF
  NEXT

MCM

[Gary Barnes]

Michael your code wont work when you attempt to open a port above 9.

The code below will.

FOR x=1 TO 255
    ERRCLEAR               ' clear the ERR variable

    IF x > 9 THEN
       a$ = "\\.\COM" + FORMAT$(x)
    ELSE
       a$ = "COM" + FORMAT$(x)
    END IF

    COMM OPEN a$ AS #1                 ' attempt OPEN
    IF ERR THEN                              ' OPEN failed 
       Print/Stdout USING$ ("Could not open COM # Err # &", X, ERR, ERROR$(ERR))
    ELSE   ' OPEN succeeds so we have to close the handle
       COMM CLOSE #1
    END IF
NEXT

Windows com ports get all funny past 9.
Refer to the help regarding "OPEN COM"

The "COM" + ltrim$(str$(x)) is ok of course, but format$ is a little clearer.

I forgot to add that if you are cycling through the com ports in this manner, you really, really need to set the com timeouts via the api.
Otherwise, if you bump into a Bluetooth Virtual Com Port (Bt can use up to 8 com ports in a standard installation on a PC) your program will hang.
In fact, if you kill your app with Task Manager, it can still take some time to terminate and release the port.

Best Regards

[Michael Mattias]

I personally don't know squat about COMM port programming.

It looked to me like there was a misunderstanding re using "ON ERROR GOTO.." so I was addressing that.

MCM

[Roger Garstang]

I forgot to add that if you are cycling through the com ports in this manner, you really, really need to set the com timeouts via the api.
Otherwise, if you bump into a Bluetooth Virtual Com Port (Bt can use up to 8 com ports in a standard installation on a PC) your program will hang.
In fact, if you kill your app with Task Manager, it can still take some time to terminate and release the port.

Best Regards

That, Flushing, and a few other things work much nicer with API. I've suggested these before as possible enhancements to the existing language, so they are aware of them. As for checking for them being in use...I think I had asked this before too and someone gave me API to do it...other than that all you have is error checking after OPEN.

[Robin England]

Many thanks to all. Michael, clearly I misunderstood the correct usage of the ON ERROR statement - definitely a case of RTFM..

Thanks also for the additional information about scanning COM ports.

Roger - I am new to Windows programming and am unsure about how API calls are actually implemented in PBCC. Other than the PB website and manual, is there a good source of information on calling APIs from PBCC that you would recommend from personal experience?

Could you also point me in the right direction by advising which API call I would use in this specific case?

[Robin England]

I forgot to ask Gary; please could you explain why the following code in your example is necessary? I understand what it does but don't know why it is needed when COM port numbers greater than 9 are being specified.

    IF x > 9 THEN
       a$ = "\\.\COM" + FORMAT$(x)
    ELSE
       a$ = "COM" + FORMAT$(x)
    END IF

Thanks, Robin.

Please ignore this, I found the answer in the manual! Sorry.

[Mike Doty]

Have'nt done any port programming in a long time, but remember that to get caller id to work other conditions had to be met. Found this code that also tested other states before attempting to use a device.

#COMPILE EXE
#DIM ALL
#INCLUDE "win32api.inc"
GLOBAL ghComm AS LONG
FUNCTION PBMAIN () AS LONG
END FUNCTION
SUB CallerID
  LOCAL BufferBytes AS LONG
  BufferBytes& = 4096
  COMM SET #ghcomm, RXBUFFER = BufferBytes&
  IF ERR THEN
    ? "CAN'T SET RXBUFFER"
    EXIT SUB
  END IF
 
  COMM SET #ghcomm, BAUD = 1200
  IF ERR THEN
    ? "CAN'T SET BAUD"
    EXIT SUB
  END IF
  COMM  SET #ghcomm, PARITY = %FALSE
  IF ERR THEN
    ? "CAN'T SET PARITY"
    EXIT SUB
  END IF
  COMM SET #ghcomm, BYTE = 8
  IF ERR THEN
    ? "CAN'T SET DATA BYTES TO 8"
    EXIT SUB
  END IF
  COMM SET #ghcomm, STOP = 0   '0 = 1 stop bit
  IF ERR THEN
    ? "CAN'T SET STOP BIT"+STR$(ERR)
    EXIT SUB
  END IF
END SUB