HP 59401A


HP 59401A HP-IB Bus Analyzer

The HP 59401A is an early, specialized logic analyzer for the HP-IB bus. Although it is limited in its memory depth to 32 transactions, it is delightful to use because the whole control of the analyzer is brought bit by bit to dedicated switches and lights. No need to go into a menu, this is completely intuitive. It is also a generator, you can make 32-bytes sequences of HP-IB commands with the paddles, which I demonstrate in the video while commanding an HP 7970E tape drive to rewind. It is the best trainer ever for understanding amd experimenting with the HP-IB bus.


My HP 59401A came with broken switches, which is a common affliction in these instruments. The paddle switches stick out on instruments without the optional side handles, and recylclers and eBay shippers who are notoriously very rough and unrespectful of the instruments in their possession will often break them. Here you can see the two broken switches on the left.

But it's very well constructed and easy to open up. The view below shows the beautiful 1970's electronics inside. There are two large boards on top of each other, plus a board for the front panel.

The front panel very easily disconnects as a unit as a whole, it just plugs out.

You can then remove the witch and board assembly from the front decorative panel.

And then get to the pretty board itself.

The switches are C&K, and still made today.

I had both a 3 position paddle switch and a 2 position paddle switch broken. I had a replacement of the 3 position in my bin of parts, but not the two position. However it was available from Digikey:

C&K DPDT switch, part number 7201P3DZQE

Digikey part number: CK9731-ND

They are easily desoldered and replaced. See the repair in the video.



HP 59401A repair and demo