I'm not sure, but I have code in place which works fine. See if this helps you at all (look at the oncomm event):

Re: MSComm Documentation Message #705364

>Dear All,
>
>I am writing a code to send and receive data on a machine connected on com port. By looking on the manual of machine I understood what i need to send (eg. U010000000) and what the machine will send back (eg. 010000001001).
>
>After using the MSCOMM32 control (object name on FORM is MSCOM1) I have written following code in the RECEIVE event (MSCOM1.ONCOMM()):
>cRetCode = cRetCode + MSCOM1.INPUT
>
>For sending the output I have following line of code:
>MSCOM1.OUTPUT = 'U010000000'
>
>The above statement send the correct instructions to machine and it starts working correctly.
>
>Now I have written following statement to verify the return value:
>IF cRetCode = "010000001001"
>MESSAGEBOX("Commands Successfull")
>ENDIF
>
>When program executes it does not show me the message box.
>
>But if I use a SUSPEND statement before checking the value of cRetCode then it gives me correct value.
>
>The RThreshold value for MSCOM1 is set to 1. As there are cases when the machine will send only 1 or two characters and I need to process that also.
>
>If anybody has done this type of work then please help me.
>
>Thanks,
>Rajeev