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.