Got my HP71B HHP 32k EPROM working ... finally!

[ThomasF]

Hi!

Last thing a bought to my HP-71B back in the 80's, was a HHP 32k RAM/EPROM device.
I used the RAM, but I never attempted to get the EPROM working.

Some days ago I thought of fixing that, but my first obstacle was to get a 32k EPROM, with programmer etc, so I went another road instead - by getting myself some 28C256 (32k EEPROM electrical erasable) which is pin-compatible (almost ... ) with the 27256 EPROM.

First of all, I needed a programmer, that was done by using 2 shift-registers and a Arduino-board.
There are many projects for simple EEPROM programmers, here is a easy project that I looked at: https://medium.com/@erikvanzijst/diy-eep...fd11a486ea
I just build it on a test-board, and programming the EEPROM was done in a flash ...
   

Secondly, and most annoying was to get the content right.
* First I just flashed the module I wanted (the JPC was on top of my list), but didn't work ...
* I check the forum and found information about that the content needed both nibble-swap and bit inverting - but didn't work either ...
* Realized that the nibble-swapping was valid when dumping ROM from the 71B, but I already got the dump - so only bit inverting was needed!

And, since the 28C256 is almost - but not quite - pin compatible with the 27256 EPROM, I had to fiddle with 2 pins - swapping the write pin (*WE/VPP, pin 27) and A14 (pin 1). This was quite easy since the pins involved are provided on the jumper pins on the HHP board. I also disabled the transistor (so the memory is always on),
   
(The green wires are for the easy serial interface ("HP-71 Fifty Cent Serial I/O" - CHHU V2N7P57).)

So now I can finally use and program new modules for my HP71B ...
   

After this success I will try to replace the EPROMs in my MLDL for the 41 to the EEPROMs to be able to update the ROM much more easily now that I have a programmer!

Cheers,
Thomas