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Eprom Modules for the HP 75
10-05-2014, 09:44 PM
Post: #1
Eprom Modules for the HP 75
Hello to all HP 75 aficionados,
I recently opened a thread at the Classifieds - section, because I am trying to find an eprom module:
HP 75 Eprom Modules
This thread started to become of general interest; especially Sylvain Cote contributed many interesting ideas how to build a specialized eprom burner and eprom modules for that machine.
The final goal could be: providing a set of tools ( lex files ) in one module, like the JPC rom does for the HP 71b.
Especially since the HP 75's ram can't be enhanced beyond 24 kBytes, an eprom extension would help a lot.
Please read the mentioned thread and add your comments here.
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10-06-2014, 02:52 AM
Post: #2
RE: Eprom Modules for the HP 75
(10-05-2014 07:15 PM)Sylvain Cote Wrote:  
(10-05-2014 05:10 PM)Michael Fehlhammer Wrote:  Should we move, or should I post a reference to that thread in the general forum?
That is a good idea, it will be a better place than here.

(10-05-2014 05:10 PM)Michael Fehlhammer Wrote:  I dream of a collection of HP -75 tool set like the JPC rom for the HP 71b.
Same here. 8-)

(10-05-2014 05:10 PM)Michael Fehlhammer Wrote:  Even if we had a programmer adapter or specialized eprom programmer, we still lack a source of eprom modules, they seem to be extremely rare.
Or can you imagine a way of producing them, including the module housings ?
Could rom modules be modified to become eprom modules?
( But since even the most common HP 75 roms are relatively rare and expensive, that wouldn't help that much ).
This means that I would have to create something like Diego did with it's Clonix-D/NoV-64 modules for HP-41.

If we go that route, then we need to do the following ...
- Choose a microcontroller to do the 8/16/24/32KB module simulation
- Make a prototype, test the electrical consumption and the impact it has on the HP-75 OS.
- Find a connector that is compatible with the motherboard socket
- Design/produce/assemble the PCB and it's components
- Come up with something to help push/pull the module inside and keep it in place.
--> a traditional mould for the housing would be way too much expensive.
--> a 3D print module housing (I would need to experiment on the plastic push/pull resistance of such a module)

Such a project is feasible, but currently I do not have the time to do it.

I have put on dropbox the following zip file for the PMS

82713A Plug-In Module Simulator [manual+soft] (sc).zip
https://db.tt/jtqt6ivN

The 65MB zip file contains ...
82713A Plug-In Module Simulator [manual 150dpi ledger] (sc).pdf
82713A Plug-In Module Simulator [manual 150dpi letter] (sc).pdf
82713A Plug-In Module Simulator [manual 300dpi ledger] (sc).pdf
82713A Plug-In Module Simulator [manual 300dpi letter] (sc).pdf
82713A Plug-In Module Simulator [soft cards] (sc).pdf
82713A Plug-In Module Simulator [soft catalog] (sc).txt
82713A Plug-In Module Simulator [soft tape] (sc).dat

Best regards,

Sylvain

Finding a compatible connector will be a big issue, the pitch for the connector is very odd and I have never seen any like it. A gentleman I correspond with has a tool for measuring connectors and he says the pitch is 0.109 inches, it is just far enough off of the common 0.1 inch pitch be be a problem. The other issue is the boards in the 75 are made of quite thin fibreglass board stock. I just ran into this trying to find an edge connector to plug one of the 8K modules onto (it is 0.1 inch pitch) but any boards I had here where two thick, so i ended up hacking the connector off of a junk 75 board I have here that from a 75 that had suffered the worst battery leak I have ever seen. This will be my source of edge connectors to make my connection to the EPROM as well.

Later this week I should get around to trying to see if I can talk to the EPROM outside of the 75, I finally figured out how the get data in and out of the DIO cards in my 9000-310 and I have it set up right now to test a memory module, when I am done with that I will give the EPROM a try.

In one of your posts you mention a bed of nails connection to the EPROM, well the one I have the pads on the bottom next to the connector and on the outside edge are very tiny and would make getting a reliable connection difficult. There should be a pad on the bottom that corresponds to each active pin on the EPROM.

My ideas for enhancement to 75 are:
  1. Build a non-volatile PMS, this should be pretty easy just replace the SRAM memory with FRAM. FRAM is non-volatile and you can read and write it just like RAM and is plenty fast enough to keep up with a 75 and also replace the discrete logic with a CPLD and fit the works into an expansion POD.
  2. Convert the RAM disk in the expansion POD to use non-volatile FRAM memory so contents would be preserved even if the the batteries go dead, it may be even possible to expand the size of it but that would require some coding.


I have considered making things that will plug into the front but that odd pitched connector makes it difficult.
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10-09-2014, 12:53 AM
Post: #3
RE: Eprom Modules for the HP 75
(10-06-2014 02:52 AM)Paul Berger (Canada) Wrote:  Finding a compatible connector will be a big issue, the pitch for the connector is very odd and I have never seen any like it. A gentleman I correspond with has a tool for measuring connectors and he says the pitch is 0.109 inches, it is just far enough off of the common 0.1 inch pitch be be a problem.

Bonjour Paul,

I found this edge connector on DigiKey that could be used for a front port connector
http://www.digikey.ca/product-detail/en/...ND/2310840
It is too deep but at first look the other specs seems ok.
I will open up a broken 75 that I have this weekend, take the exact mesurements for my connector research and then order some promising connectors for testing.

Best regards,

Sylvain
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10-09-2014, 01:45 AM (This post was last modified: 10-09-2014 02:27 AM by Paul Berger (Canada).)
Post: #4
RE: Eprom Modules for the HP 75
Sorry that connector is 0.1 pitch that will not work, the pin spacing is slightly larger that that as I mentioned in a previous post and it is just enough that they will not work. I sure you have some connector there that is 0.1 pitch, as they are very common, that you can compare to. The connector for the 8K memory expansion is 0.1 pitch.
Take a look at the picture I just uploaded I think this will be pretty clear, Pictures
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10-10-2014, 02:43 AM
Post: #5
RE: Eprom Modules for the HP 75
(10-09-2014 01:45 AM)Paul Berger (Canada) Wrote:  Sorry that connector is 0.1 pitch that will not work, the pin spacing is slightly larger that that as I mentioned in a previous post and it is just enough that they will not work. I sure you have some connector there that is 0.1 pitch, as they are very common, that you can compare to. The connector for the 8K memory expansion is 0.1 pitch.
Take a look at the picture I just uploaded I think this will be pretty clear, Pictures

Yep! Very clear. Thank you 8-)

Sylvain
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10-10-2014, 01:53 PM
Post: #6
RE: Eprom Modules for the HP 75
(10-09-2014 01:45 AM)Paul Berger (Canada) Wrote:  Sorry that connector is 0.1 pitch that will not work, the pin spacing is slightly larger that that as I mentioned in a previous post and it is just enough that they will not work. I sure you have some connector there that is 0.1 pitch, as they are very common, that you can compare to. The connector for the 8K memory expansion is 0.1 pitch.
Take a look at the picture I just uploaded I think this will be pretty clear, Pictures

my guess is a 0.112" (2.845 mm) pitch. a rare beast indeed!

(btw. paul, you have mail)

hans
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10-10-2014, 03:49 PM
Post: #7
RE: Eprom Modules for the HP 75
(10-10-2014 01:53 PM)Hans Brueggemann Wrote:  my guess is a 0.112" (2.845 mm) pitch. a rare beast indeed!
It might be possible to use a connector with a higher pitch and connect just the pins that mate correctly with a trace on the board.

Marcus von Cube
Wehrheim, Germany
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10-10-2014, 05:23 PM
Post: #8
RE: Eprom Modules for the HP 75
yup, a 0.039" or 0.050" might do the trick. after all, it's nyquist.
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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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06-04-2016, 02:18 PM (This post was last modified: 06-04-2016 02:19 PM by Paul Berger (Canada).)
Post: #10
RE: Eprom Modules for the HP 75
All of the ROMs plugged into the front as well as ROMs contained in the expansion pod are bank selected in and out of the same 8K window in the address space. There are also two internal ROMs in the 75 that share this space. The 75's bank selecting scheme using specific addresses to select and deselect a ROM. It is up to the ROM to monitor the appropriate addresses to know when to turn on and turn off. There are pins on the front ports that the plugin ROM uses to identify which port it is in and which addresses it should monitor. The ROM section of the service guide shows a map of the addresses used.

I would be interested in knowing what connector Dominique plans to use.
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06-04-2016, 05:22 PM
Post: #11
RE: Eprom Modules for the HP 75
Thank you Paul for your quick response!
I'll forward your hints to Dominique, and I will post here when there is any progress - or when there are further questions.
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08-02-2016, 01:21 PM
Post: #12
RE: Eprom Modules for the HP 75
Meanwhile Dominique went on to produce a modern version of the ROM simulator, in order to better understand all the intricacies of the ROM addressing and bank switching schemes, and before attempting to pack the logic into the small housing of an EPROM module, which is the final goal, of course.
He has got the NoMas listings of the HP 75 operating system, but they are huge, of course, and he is not (yet) familiar with them.
Can someone (Paul?) help him to locate the part (subroutine) where the bank switching (ROM addressing) is managed; literally Dominique says:

"My objective is to locate in the source file the sub's managing the paging process depending of the number of 8k pages used by external modules. The explanation in the Simulator Manual explains only the management of one (A + B double x 8ko" banks)."

Please help us; I am convinced that, with some help, Dominique finally will succeed in producing extension modules for the HP 75, and this would be a great achievement and would lift the usefulness of this calculator to a new level, wouldn't it?

By the way: Yes, Dominique has found a solution to the connector problem, but he hasn't revealed it yet. :-)
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08-02-2016, 01:54 PM
Post: #13
RE: Eprom Modules for the HP 75
(06-04-2016 12:38 PM)Michael Fehlhammer Wrote:  Can anyone help and provide the complete schematics, and/or answer Dominique's questions, please?

It seems I missed the June part of this thread.
You may already know this schematic, from Paul :
http://users.eastlink.ca/~phberger/

Quote:Can someone (Paul?) help him to locate the part (subroutine) where the bank switching (ROM addressing) is managed
My knowledge of the HP-75 is not as good as for the HP-71B, but I may help to investigate the source code.

Quote:By the way: Yes, Dominique has found a solution to the connector problem, but he hasn't revealed it yet. :-)
I'm definitively interested !

J-F
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08-02-2016, 04:50 PM (This post was last modified: 08-02-2016 05:03 PM by Sylvain Cote.)
Post: #14
RE: Eprom Modules for the HP 75
HP-75 NoMaS Vol.2 P.166 to 168 - The Kangaroo ROM Switching Guide
Contains the explication of the HP-75 ROM enable/disable scheme

Notes ...

The internal routines uses the ROM ID to determine which ROM is active.
A ROM can only contains 1 LEX file and it must be the first file of the ROM.
From comments in the document, it seems that the internal code uses the LEX ID as ROM ID.
ROM ID & LEX ID had to be unique were provided by HP at the time.

Sylvain

edit: added the notes section
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08-02-2016, 07:30 PM
Post: #15
RE: Eprom Modules for the HP 75
(08-02-2016 01:21 PM)Michael Fehlhammer Wrote:  Meanwhile Dominique went on to produce a modern version of the ROM simulator, in order to better understand all the intricacies of the ROM addressing and bank switching schemes, and before attempting to pack the logic into the small housing of an EPROM module, which is the final goal, of course.
He has got the NoMas listings of the HP 75 operating system, but they are huge, of course, and he is not (yet) familiar with them.
Can someone (Paul?) help him to locate the part (subroutine) where the bank switching (ROM addressing) is managed; literally Dominique says:

"My objective is to locate in the source file the sub's managing the paging process depending of the number of 8k pages used by external modules. The explanation in the Simulator Manual explains only the management of one (A + B double x 8ko" banks)."

Please help us; I am convinced that, with some help, Dominique finally will succeed in producing extension modules for the HP 75, and this would be a great achievement and would lift the usefulness of this calculator to a new level, wouldn't it?

By the way: Yes, Dominique has found a solution to the connector problem, but he hasn't revealed it yet. :-)

On page 2-45 of the service guide it states that up to 144K of extra ROM may be added, however on page 2-47 where there is a map of the I/O addresses which included the addresses used by the bank switching scheme there are 16 pairs enable/disable addresses listed and two of those are occupied by internal ROMs ALT an MEL which contain part of the operating system. Which does not add up to 144K. It would seem to me that a ROM emulator could use any of these other than the ones used by the two system ROMs, however you should keep in mind that the expansion POD also has a couple ROMs built into it.

The mechanism to do the bank switching seems to be that you turn off the active ROM by writing to its disable address and enable the desired ROM by writing to its enable address. The ROMs are obliged to monitor these addresses to decide when to turn on and off. There are a couple signal lines on the front slots that allow the plugin ROMs to know what slot they are in and from there it knows what address it should monitor.

There is better documentation in the NOMAS, specifically "Joey's Big Book of ROMs" if the NOMAS that Dominique is working from is the scanned copy on the MoHPC documentation set, I wish him luck, the four up page scans are not of great quality, and it has been OCRed however because the scan is not really good the OCR results are even worse you need to be really creative when searching. This documentation seems to be more complete and up to date than the service guide. It also mention the use of addresses FF50 to FF5F for bank selection, with the exception of FF55 which is used for testing. It also mentions that there was a second version of the ROMs that uses a slightly different and improved scheme for bank selecting ROMs when the ROM is turned on as usual by writing to even enable address, but the ROM is automatically deselected when any other address in the FF30-FF5F range is written to, however The ROM routines may still use the original method of using even enable addresses and odd disable addresses for ROM that are in the FF40-FF4F range that the original version of ROM supported. I just looked at the board for a production level 75C and all the on board ROMs are the original part number, two of which would be the bank selected ALT and MEL ROMs, and the enable/disable addresses for these ROMs is the FF40-FF4F range.
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