Hi! Krisz.
I watched.
This module has two common buses.
1- digital
2- analog
When you take measurements take it into account !!!
You should first of all make measurements of the signal levels at the input U7 / U6 / U5.
This way you can check the low and high levels of signals and compare with the table.
U7 / U6 / U5 create a signal level for the analog part
and send it to the keys Q2 / Q3 / Q4, Q10 / Q11 / Q12 / Q13 / Q14.
A high and a low level is better measured by an oscillograph. In DC open mode.!!!
Make a table for yourself.!
On your question:
Extender boards buy for yourself convenient.
If you want, you can wait for me next week, I can send
you need a file to make it.
This mine, but it is not high. There will be another, adapted for HP4275A
Check D-GND! So on the board?
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3g57