[volt-nuts] GPIB Issue with 3458A

Dr. Frank frank.stellmach at freenet.de
Sun Dec 31 09:00:02 EST 2017


I doubt, that your hp unit is defective, as it might work under certain 
circumstances, and also the FW version 8.2 vs. 9.2 will not make any 
difference, as only a few pair of bytes are different. All versions of 
the A5 board are designed to be completely interchangeable over all 
versions of 3458A instruments.

I have another theory, which deals with the hardware version difference 
between the agilent and the hp unit.

Your agilent unit might already have the 66547, full SMD board for A5, 
whereas the hp will for sure have one of the old 66505/515, through- 
hole boards, Rev A, B, or C. Illya has all of them pictured in his first 
3458A repair blog.

All the  old versions (A, B, C) have GPIB bus transceivers, TI 75ALS160 
for the data bus, and NS 75161 for the handshake signals.
The new SMD board has TI 75ALS160 / TI 75ALS161, instead.

So, the handshake bus uses a different logic family for the new board, 
and therefore, the logic levels / immunity might be slightly different.

My hp3458A has got the REV C A5 board, and it's from 2000.
Since 2009, I run a similar datalogging program for capturing LTZ1000 
output versus time, each 4 seconds, on an old PC with WIN98, Turbo 
Pascal,  an XT bus GPIB card from CEC, with the NEC chip.

This program and hardware ran properly for years, (decades), until more 
and more errors during the 24h acquisition runs occured, vaguely 
reminding me of your trouble.
The output values were always correct, but frequently, 10sec time-outs 
ocurred, giving zero readings in the datalog. This time-out is obviously 
related to the handshaking mechanism.
Finally, I found out, that the old ATX PSU was failing, especially its 
input snubber network was gone defect, which probably created big EMC 
After replacing the PSU, all these errors vanished completely.

My conclusion was, that the GPIB bus is susceptible to such EMC 
disturbances, although it's solid TTL logic.
As obviously the handshake signals were affected only, their higher 
susceptibility to disturbances may be caused by the difference in the 
logic family of the transceiver IC. The agilent 3458A might be more 
tolerant to that, having the ALS161 version, instead.

So you might also look for EMC problems in your lab, like SMPS, LED 
lamps, or a defective PC power supply.

Maybe, these Prologix adaptors (clones?) have critical logic levels, 
especially on the handshake bus, or an improper ground.
You might search for these signal problems directly on the GPIB cable.


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