Valhalla 2701C GPIB (IEEE) upgrade

[Le_Bassiste]

continuing my work on improving an early model Valhalla 2701 from here https://www.eevblog.com/forum/repair/valhalla-2701c-(early-models)-level-shifter-upgrade/, this post is about a GPIB upgrade which is almost identical to the original Valhalla IEEE/IRP option and can be used as a direct replacement. if you have a 2701C on your shelf that you would like to program via GPIB, this might be for you.

central element is the MC68488 GPIA, which is rare, but still not too hard to get. the output stage to the GPIB bus consists of the notorious 75ALS160/162 combo, doing away with the MC3447P drivers of the original design. i threw in two MMAD1103 for protection, but frankly, i don't use them on my interface board, although you might sleep better when having them populated as well.
other than that, there was only the challenge of getting a physical connector for the GPIB cable, so i went for
 
1 pc. NorComp Inc. / 111-024-213L001 / 24pin vertical SCSI receptacle  (the connector)
1 pc. TE Connectivity AMP Connectors / 5552633-3 / KIT, HARDWARE STANDOFF STUD MT (kit of two M3.5 stand-off screws w/ washers and nuts)
2 pcs. Würth Elektronik / 963040174 / ROUND BRASS SPACER L:4,0 MM;ID:4 (these go between pcb and connector flanges to improve rigidity. yeah, ID too large, but i couldn't care less)
 
the lock-spring holders on the connector's flanges were stripped off (easy) and the mounting holes in the flanges were drilled out to 4.5 mm (difficult) to fully accommodate for the studs of the M3.5 screws. when using a 1.6 mm thick pcb, the threads of the M3.5 screws protrude just far enough to fasten them with a washer and a nut. some Loctite 243 prevents them from loosening.

as stand-off beween pcb and rear chassis wall, i used
5 pcs. Würth Elektronik / 963060042 / ROUND BRASS SPACER L:6,0 MM;ID:3

connection between main board and GPIB interface is done via 20-conductor ribbon cable (approx. 1 meter) that should ideally be shielded in order to not add to the already pretty high RF noise level of the 2701C. afaik, the orignally equipped GPIB interface came with unshielded cable, so you may be ok with doing the same, though.

i've verified on my 2701C that the interface board works as expected in conjunction with an agilent USB-GPIB interface and can confirm that the firmware of the 2701C does contain the GPIB command set. don't expect too much of the GPIB implementation though, as it seems to be a bit rudely implemented. it is for example lacking the IDN? command which i would expect (correct me if i am wrong) to be some sorts of a "standard" on GPIB instrumentation.

if you are interested in brewing your own 2701C GPIB interface, you can drop me an email (no PM please). i have 4 unpopulated pcbs left over from my JLCPCB order that i will part with for 10 EUR each. (EU only)
otherwise, feel free to roll your own from the attached files, as long as you are not using it for commercial purposes. as always, use at your own risk.

[coromonadalix]

Nicely done 

[Electrole]

Hi Le_Bassiste
Do you have any GPIB PCBs left, or are they all gone?
I was not able to send an e-mail, hence the post.
Best regards
Ole

[Le_Bassiste]

I wonder why you chose the name Valhalla. I associate this word with Scandinavian mythology.

i have no clue why the original manufacturer chose that name.

[Le_Bassiste]

Hi Le_Bassiste
Do you have any GPIB PCBs left, or are they all gone?
I was not able to send an e-mail, hence the post.
Best regards
Ole

PM sent.