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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.

[attachment=8787]

(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.

[attachment=8788]

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. ;-)

Hi Diego,

great - you found one of the features we wanted in the HEPAX module from a warranty point of view. It was a design feature to be able to re-program HEPAX module in case of firmware updates (yes, we still needed to replace the plastic case), however there was an issue (I do not remember what it was - it was hardware related) which made it impossible for us. This is why we changed to OTP in the second batch of modules.

Regards,
Steen

Hi Steen,

I see a potential source of problems to reprogram the EPROM directly on-board.

Since the EPROM power (Vcc) is not separated for the RAM & ASIC chips, chances are that some spurious Address/Data signals conflicts with those comming from the programmer. But that's just a guess.

The EPROM reading however has worked as expected for me.

I've even written a short program to extract the 4 .ROM files from the EPROM dump .BIN and compared ok with currently available ones.

Best wishes.

Diego.

[Edit] Correction: After checking the 27C256 datasheet it becomes evident that the EPROM cannot be programmed on-board since it has CE/ and OE/ tied together.