HP 37204 HP-IB Extender - Serial Protocol?

[TaylorD93]

Recently I couldnt resist dropping a bid on an Ebay listing for a HP 37204 HP-IB Extender.

On its own, i think its pretty useless as typically you need two, however i wondered, as it converts GPIB to Serial for two Coaxial Connections (or Fiber Optic) being TX and RX, could it be possible to interface this to another serial protocol, say RS232/485 etc, ignoring ofcourse the difference in voltage levels (I think i could easily resolve that).

Is anyone familiar with the actual Serial protocol these HP-IB Extenders use?

I dont have any USB to GPIB devices, but have a fair few instruments with GPIB, so wondered if it could be possible to "upcycle" these HPIB extenders to something more useful as a low cost USB to GPIB device.

thanks

[alm]

I assume you've read the service manual? It is surprisingly sparse for a HP product, I guess during this time they were already starting to put less effort in it. It does not contain a full protocol description, but it does mention it's using an alternate mark inversion encoding (page 8-17), and that the master sends a frame every 1.6 ms when no other device is connected (page 8-18). Possibly some sort of keep alive or handshaking signal. Seems like to use it in the way you intend, you'd have to reverse-engineer the serial protocol by sending it a bunch of GPIB signals using a controller, and observing the response with a scope or logic analyser.

I'm not convinced that this is less effort than just using the transceivers and using the GPIB protocol documentation to build your own GPIB interface. All the 37204 is going to give you is media conversion, after all, if won't implement any of the control logic like addressing, handshaking, etc.