Serial Port VISA Resource

[daveh]

I have a Command/Status VI (Ver 6.1) that uses VISA serial port communications to continually read status or write commands when new commands are entered. I have added a selectable button to pause the command/status operation, open a sub-VI screen, and allow the user to enter unique commands to change a set of unique parameters. After the pop-up VI writes the commands and is closed, the VI continues normal Command/Status operation. Both the Command/Status VI and sub-VI close the serial port after each read or write, and work stand-alone. The Main VI and sub-VI also both use the same serial com port (and VISA resource).
The problem is that the Command/Status VI gets communications errors for several seconds after the sub-VI is closed befor
e working properly. Is the cause of the problem the common VISA resource? Will using re-entrant solve the problem? Any comments or suggestions would be appreciated?

Thanks,
Dave H.

[SteveChandler]

What type of communications errors are you getting? Does NI Spy show the serial port activity that you expect? I am not sure, but maybe it is a matter of flushing the buffers prior to closing the sub-VI.

[daveh]

It is my own error message rather than a NI error message. I have a time-out built into the read function that exits the read if there is no data available after 200 tries. The time-out message allows the user to either continue operation or exit the program. I get several of these in the Command/Status VI (after returning from the sub-VI) and then it seems to continue normal operation.

[Jeremy Braden]

You do not need to close the port after every read and write. If you open a session to the port, you can use the reference throughout your VIs. Although it is redundant, I prefer to use VISA open in the beginning, VISA read/write commands in the middle ,and VISA Close at the end. Passing the reference created in the main VI to the subVI and back out is fine. I suspect overhead from opening a session and closing it is why you get the error. Be sure to wire the error clusters so that you can see the error codes created.

I do not believe that setting the VI re-enterant will help. Re-enterant VIs just make a new copy of the VI in memory when they are run as opposed to using the same place in memory.

Jeremy Braden
National Instruments

[daveh]

Thanks!