HEPAX internal EPROM coding. (Maybe too) Techie post... ;-)

[Diego Diaz]

Hi all,

While waiting for the next episode of the Steen's HEPAX Treasure Chest, I decided to go a bit deeper inside my HEPAX module; namely inside its EPROM content.

It uses a standard UV erasable 27C256 chip. Later models use the OTP PLCC version.

The ceramic PCB has pads on its sides that allow reading and even in-circuit programming so I built a "dirty" fixture in order to access the EPROM directly.

   

(Probably unnecessary, but just in case: Do not try this at home! :-D)

I knew HEPAX only use 5 bits (b0-b4) of every byte to build every HP-41 10 bit word.

The way it handles this arrangement was however a bit surprising.

Most EPROM boxes place the MSB on the even byte of the EPROM and the LSB on the odd byte; much like the .ROM file format.

HEPAX on the contrary places all (5) LSbs of a given page on the lower 4K of this page while the upper 4K hold the (5) MSbs.

An example of that may help to clarify.

   

The four banks of the HEPAX code are sequentially stored in the 32K of the EPROM.

Best wishes from the Canary Islands and enjoy your weekend.

Diego.

P.S.: The HEPAX module in the picture above survived the procedure and is comfortably sitting in my CX port 1. ;-)