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The DAV signal is also used as the J clock signal on the analyzer pod.

The combination of ATN and EOI comes out on a separate pin on the device. This should be connected to the K clock of the next pod. The analyzer clocks should be J down OR K down.

Example trace

I connected my HP 5385A frequency counter to my HP 9825A calculator and wrote a simple loop to read the ID of the counter:

...

After the poll it UNLISTENs and UNTALKs; the other commands I do not yet know

Another try delivered this:

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More info on HP-IB:

• http://www.interfacebus.com/Design_Connector_GPIB.html

• https://gist.github.com/asanza/4e2719744fb0a41c8828e21dc2b7f012 (hpib commands)