Older instruments do not respond to Meas and Auto Explorer Commands

[AnyOldAlias]

We use Measurement and Automation Explorer running on an XP PC to send simple commands to instruments via the NI GPIB-USB-B adaptor. Nothing fancy, just enough to confirm the instrument's GPIB interface is working. We repeatedly find that some older instruments (pre-SCPI) don't respond, even though 'Scan for Instruments' correctly finds them. If we send commands using a VERY OLD HP 85 computer, the instrument rsponds without a problem. Is there anything about the way NI software interacts over the GPIB that might cause this? Thanks.

[SynergyMike]

Try increasing the timeout.  I use a number of non-SCPI instruments here and found that LV software that works with the instruments on a slower PC wouldn't work on faster PCs (all with XP) until I added some delay (I had to go as long as 5S in one case).

Regards,

Michael Tracy

Synergy Microwave

[Dennis_Knutson]

If the scan for instruments finds them, then everything is ok. The only other thing that is done is that MAX sends each found instrument the *IDN? query and if your instrument does not support that command, then obviously the command will fail. Just ignore it. The other thing is that the GPIB defaults are set to IEEE.488-2 and that assumes the only thing required is the EOI line. Older, non-standard instruments often required an additional termination character. You can change this in the GPIB properties window.

All GPIB utilities will probably make the same assumptions. The standard has been in place for over 20 years.

[AnyOldAlias]

Hi All. Thanks for your brisk responses. I'm currently using a 10 second timeout, which times out! I tried messing around with various combinations of termination. Out of the blue, the unit started working over the GPIB. However, I couldn't find a consistent termination setting which worked every time.

The instrument reports a syntax error when it fails. I'm thinking that somewhere along the line, the message may be getting 'shifted' due to extra or missing characters (as expected by the unit), so the wrong sequence of characters is read by the instrument. None of this happens when using the HP 85.

I could almost walk away from this as a quirk of one unit, but I have seen this same issue many times, pretty much always with older Agilent (HP) instruments.

[Dennis_Knutson]

There is also a bus timing setting in MAX for your GPIB controller. Try setting it to the maximum.

If you have the instrument manuals, you should not have to guess at what additional termination characters you need besides EOI.

I've worked with numerous old Agilent instruments and the vast majority of problems have been fixed with bus timing and the use of correct termination. I have also found that using the GPIB functions instead of VISA works better. You also have the termination options (mode input) with the GPIB Read and GPIB Write that you don't have with VISA and you can use different settings for different instruments.

[RER]

Excellent advice, as always, from Dennis. I have also seen several GPIB issues which have been resolved by reconfiguring the GPIB timing. For more information on Dennis' suggestion, please peruse the following KB.

Changing the Bus Timing Speed of the GPIB Bus

http://digital.ni.com/public.nsf/allkb/736C655699AE2A938625709800737D9F?OpenDocument

I hope this was helpful,

Best wishes,