Why can I read from my instruments in LabView only when NI Spy is open?

[JFoley]

This is a recurring problem, but I hope someone has an idea of what may be causing it. I have two older (c. 1992) instruments that we use for simple measurements. One is a Newport PMC-400 actuator driver/motion controller, the other is a EG&G/Princeton Applied Research 5302 lock-in amplifier. I am using Labview 6i with a PCI-GPIB card.

The instruments themselves work fine, and seem to run somewhat reliably if NI-Spy is open. However, if we close NI-Spy, the GPIB seems to "lock up" and time out, requiring resets of all the instruments. This seems to be something related to either command termination or timing.

Any thoughts/hints/solutions would be greatly appreciated.

[AaronK]

Hi JFoley,

The reason this happens is usually related to the instrument! Certain older instruments communicate with the GPIB board in such a way that they indicate that they are able to receive data before they really are. This was a nice trick back in the day when computers were slower, and it could get away with �lying� about its state as it would make the instrument look faster than it was! With the faster computers of today it throws off the bus timing and can cause problems.

You may be able to resolve the timing problems, either by introducing delays between GPIB commands, or by changing the GPIB bus timing to the slowest setting (in the GPIB Configuration Utility). The first option is usually successful, but the second option is less effective becau
se it only affects the time that the GPIB controller waits before valid data is on the bus and the DAV line is asserted (a step in the GPIB handshaking that occurs each time a byte of information is sent to the device).


Hope this helps out!


Best Regards,
Aaron K.
Application Engineer
National Instruments