Detecting GPIB Instruments when an HP 37204 Extender is involved

[Mike_B.]

Originally I had a simple VI that used the VISA FIND function to locate required instrumentation before stepping into the test application. This was a reliable method until the application was used in the test system which utilizes GPIB extenders. The Extenders force all 30 addresses to be handshaked as listeners before any code is sent programmatically.

I then used unique sessions for each instrument (mostly using *IDN?, ID?, ID, etc...whatever applies or status byte checking, and even simple visa reads and parsing to determine a remote status). There is a wide mix of instrumentation vintages. The VI now only functions well if most instruments can be located. The more units that
are missing from the expected communication, the more unstable the looping to establish communication becomes.

Is there a way, to reliably check remote status of the instruments, (not the address status of the card)? I dont expect to programmatically diagnose a situation in which a lockup is inevitable, like two instruments on a same GPIB address. I would like to diagnose any missing, powered down, or disconnected instruments in a looping fashion.

[rtamblin]

Mike,

There really isn't another good way to do this other than with the VISA Find function. My suggestion would be to use one of our extenders, a GPIB 140A cable, which does not cause the VISA Find function to lose functionality.

Ryan Tamblin
Applications Engineer
National Instruments

[Mike_B.]

Ryan,

I believe the original GPIB extenders were introduced to "expand" the instrument
load limit from 14 to a possible 28 on a single workstation.

I have been doing some further research on this issue and found the N.I. app. note 042 to be informative as well as the "Hardware Reference for GPIB-120A". Am I correct in stating that the GPIB-120A will expand our control capability without interfering with the Labview VISA-FIND results? N.I. uses the term "Protocol Transparency" to refer to their expander/extender performance. In which case, it sounds like I would get true listener results in my returned data.

Mike B.

[Mike_B.]

I have come to find out that the VISA find function CANNOT determine what
listeners are available on the GPIB bus when extenders or expanders are
introduced to the bus, regardless of manufacturer.

I have purchased 2 N.I. 120A expanders and they are NOT "transparent" when
it comes to the Labview VISA FIND function.