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Eprom Modules for the HP 75
06-04-2016, 12:38 PM
Post: #9
HP 75 schematics - help needed!
After a long while I'd like to start a new attempt to find a way to produce HP 75 custom modules.

Fortunately Dominique Berget from Rennes/France, a very competent electronics expert, is willing to undertake the task. Some of you may have heard of him, he built a BASIC language DIO board for the 200/300 series computers, for example.
Dominique is not yet familiar with the HP 75; I sent him pdfs of the manuals, especially of the service manual I found on the museum's site. But this manual seems to be incomplete with regard to the schematics. Here comes a quotation from his email:

"I have ordered some connectors I hope to be usable for the HP75. They have what I hope to be the right pitch. They have too much contacts, but I should be able to cut them at the right size. The most important is the pitch which say if there is something to develop on the 75C. The other components are easy to source (new eeproms in plcc package).

I have a question not that clear for me after reading the books : There are 3 available slots to plug add-ons. Having a look on the schematics, the address decoding are specific for each slot. That means one module must be located in a specific slot ... Or the internal program scan the 8k segment available with something in it, and relocate the program in rom ... A bit strange for me because in the '80, the code was not easily relocatable. At a specific level of one specific slot, it's not a problem, because only one block of 8k is addressable by the processor, and there is a two segment selection lines to address one of the four 8k blocks of the module. It's a classic way to expand memory on a 16 address's memory map.
Would you have a schematic more complete of the mother board the the one at the end of the Service Manual ? A part is missing, especially the block decoding. "

Can anyone help and provide the complete schematics, and/or answer Dominique's questions, please?
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HP 75 schematics - help needed! - Michael Fehlhammer - 06-04-2016 12:38 PM

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