Tek TDS 784A GPIB problem - Page 1
Forum section: electronics / repair

author: mssbid
author_rank: Contributor
timestamp: June 27, 2022, 03:06:03 pm
content: Hello all,got this wonderful 784A but GPIB doesn't work. It isn't found on the bus (another 744A is foundand I enabled the 1M option so we can consider the setup to be OK).I tried the "normal" way when the scope is running and it should be discovered on whateveraddress had been set in the menu. I also tried it with the protection switch set to "unprotected"when, after powering on, it normally listens on address 29.Does anyone have any idea what could be going on here? I assume that s.o. has "zapped"the GPIO Bus by static discharge or similar.I have searched the whole :-) Internet regarding schmatics but didn't find anything. Doesanyone have these (or at least the stuff around the GPIB)?Is any of the two PLCCs in the attached pic responsible for the GPIB (I didn't follow the tracesyet)? There is not much information about these MM9522 or MM9521...Thanks!

author: m k
author_rank: Super Contributor
timestamp: June 28, 2022, 08:21:58 am
content: Did you have both machines connected simultaneously?Check NDAC.When 744A is accepting data it will address NDAC and you can verify that.784A should do the same.If you had only one machine connected check NRFD, it's up when bus is free.It should be up also when 784A is connected.

author: serg-el
author_rank: Regular Contributor
timestamp: June 28, 2022, 09:06:45 am
content: U 60 and U 59 have little to do with GPIB.Check your adapter.Information from 070971003_TDS520B_Mod_CM_Digitizing_Oscilloscope_Component_Service_Manual.pdf

author: mssbid
author_rank: Contributor
timestamp: June 30, 2022, 05:53:51 am
content: Quote from: serg-el on June 28, 2022, 09:06:45 amYes, you are right. As I said, I didn't follow the traces. And the schematics of the GPIBpart of the 520 seem to be similar (if not equal) to the one of the 784. At least I havefound a NAT9914APL and the two bus drivers. And I followed the data lines as a firststep.QuoteYou mean the GPIB interface on the PC end? This can be considered OK. As I wroteI talked to a 744A with it (enabling the 1M option). And we regularly use it with otherinstruments.

author: mssbid
author_rank: Contributor
timestamp: June 30, 2022, 05:57:40 am
content: Quote from: m k on June 28, 2022, 08:21:58 amNo. I first connected the 744A (enabling the 1M option) and thenreplaced it with the 784A.QuoteI will check the signals as soon as time permits. It's been a while (25 years) thatI played with GPIB hardware :-). Maybe someone zapped the bus driver(s) andit might be easier to just replace them...

author: mssbid
author_rank: Contributor
timestamp: July 08, 2022, 05:08:46 am
content: While scanning the bus and triggering on the first ATN transition, I noticedthat pin 6 of the 75160 (GPIB data 5) appears that it wants to get high atsome point but apparently doesn't succeed. I retried this with the working744A and here it was OK.So I inspected the 8 data lines on the GPIB side of the 75160 with a simpleDMM against GND and found pins 6 and 8 to be at around 5 Ohms whileall other have some reasonable higher values (kOhms). So I suspect the75160 to be damaged and will see if I can get my hands of one of those...

author: mssbid
author_rank: Contributor
timestamp: July 11, 2022, 04:31:48 am
content: Changed the 75160 and GPIB is working again...