Tek TDS 784A GPIB problem

[mssbid]

Hello all,

got this wonderful 784A but GPIB doesn't work. It isn't found on the bus (another 744A is found
and 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 whatever
address 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. Does
anyone 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 traces
yet)? There is not much information about these MM9522 or MM9521...

Thanks!

[m k]

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.

[serg-el]

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

[mssbid]

U 60 and U 59 have little to do with GPIB.

Yes, you are right. As I said, I didn't follow the traces. And the schematics of the GPIB
part of the 520 seem to be similar (if not equal) to the one of the 784. At least I have
found a NAT9914APL and the two bus drivers. And I followed the data lines as a first
step.

Check your adapter.

You mean the GPIB interface on the PC end? This can be considered OK. As I wrote
I talked to a 744A with it (enabling the 1M option). And we regularly use it with other
instruments.

[mssbid]

Did you have both machines connected simultaneously?

No. I first connected the 744A (enabling the 1M option) and then
replaced it with the 784A.

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.

I will check the signals as soon as time permits. It's been a while (25 years) that
I played with GPIB hardware :-). Maybe someone zapped the bus driver(s) and
it might be easier to just replace them...

[mssbid]

While scanning the bus and triggering on the first ATN transition, I noticed
that pin 6 of the 75160 (GPIB data 5) appears that it wants to get high at
some point but apparently doesn't succeed. I retried this with the working
744A and here it was OK.

So I inspected the 8 data lines on the GPIB side of the 75160 with a simple
DMM against GND and found pins 6 and 8 to be at around 5 Ohms while
all other have some reasonable higher values (kOhms). So I suspect the
75160 to be damaged and will see if I can get my hands of one of those...

[mssbid]

Changed the 75160 and GPIB is working again...