Why do I get some characters twice?

[dolphin]

I have two GPIB devices connected to my computer. When I want to communicate with them using LV, I get a good response from the first one (Agilent 4278A Capacitance Meter), but not from the second one ("Hewlett Packard EPM-441A" Power Meter). Here is the detail. I send a "*IDN?" command. First one responds by sending its ID String, "HEWLETTPACKARD,4278A,0000A00000,REV3.31". When it comes to the second one, it responds as "HEEWLLLEETTTT--PPAACCKKAARRDD,,443377BB,,,,22..00".
As you noticed, most of the characters are received twice. As I repeat the readings, the pattern changes a little bit, but again most of them are received twice.
So, my question is, do you have an idea why it happens? Is there a way to change/set communicati
on speed for LV's GPIB Read/Write objects?
I will appreciate your comments and helps. Thanks.
FYI, the VI is attached.

[Ray K]

Hello,
You can change the bus speed of the card in MAX by clicking on the card and going to Properties»Configure. You cannot change bus timing for individual reads or writes.

This however is likely not a problem with bus timing. GPIB uses handshaking so double characters is symptomatic of a possible short on the one of the lines or a faultly interface controller on the instrument. Try connecting to each of the devices individually and swapping GPIB cables. If the device still exhibits the behavior, send it to Hewlett Packard for repair.
Regards,
Ray K
NI Applications Engineer

[dolphin]

Hello Ray,
Thanks for your response.
Actually I did swap GPIB cables, and tried to access devices individualy. But nothing changed. The other point that I forgot to mention in my first inquiry is, if I use "CBIC32" application program which comes with the GPIB board, I don't have any problem about readings. Everything comes perfectly OK. Even if both devices are connected to the same board, I can get correct readings from each of them. That's why my focus is more on LV's GPIB Read/Write objects.
Thanks anyway, I appreciate it.

[Ray K]

Ahhh, very interesting. It sounds like you have two 488.2 drivers installed (except for the fact that both instruments don't respond with double characters even though they are connect to the same board, which is very strange)

I'd test if it happens when you use VISA and if NI-Spy shows the duplicate characters.

In any case, it's likely that something is indeed wrong with your installation. I'd bet that you have LabVIEW 6.0.2 or earlier, did a "complete" installation of LabVIEW, then later installed NI-488.2 separately.

Reinstallation Instructions:
*Remove All NI software through start -> settings -> control panel -> add/remove programs
*Open Windows Explorer and go to c:\program files, if there is a National instruments folder delete it and all of it's c
ontents
*Shut down the computer and remove the cards
*Install LabVIEW, do a custom install and cross out the NI-DAQ, VISA, and 488.2 drivers so they won't be installed
*Install 488.2 software
*Shut down and put in the GPIB card

All drivers can be found here

Regards,
Ray K

[Ray K]

Acutally, what is this "CBIC32 application program which comes with the GPIB board". Is this a National Instruments board? Did you mean ibic.exe? I somehow missed the phrase "which comes with the GPIB board" when I initially read your response.

Ray K

[ywilliamli]

I ran into this problem before and I posted the solution to my own question. 🙂 Please refer to the thread titled "duplicate output".

I hope that solves your problem.

--
Regards,
-William

Columbia Integrated Systems Laboratory
Columbia University, NY 10027