+- HP Forums (https://www.hpmuseum.org/forum)
+-- Forum: HP Calculators (and very old HP Computers) (/forum-3.html)
+--- Forum: General Forum (/forum-4.html)
+--- Thread: HEPAX internal EPROM coding. (Maybe too) Techie post... ;-) (/thread-15707.html)
HEPAX internal EPROM coding. (Maybe too) Techie post... ;-) - Diego Diaz - 10-10-2020 09:35 AM
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. ;-)
RE: HEPAX internal EPROM coding. (Maybe too) Techie post... ;-) - SteenPetersen - 10-10-2020 03:53 PM
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
RE: HEPAX internal EPROM coding. (Maybe too) Techie post... ;-) - Diego Diaz - 10-10-2020 04:46 PM
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.
