WTB: Eprom Modules for the HP 75

[Michael Fehlhammer]

I just have acquired the HP 75 module simulator and now I am looking for eprom modules for that machine. I don't have a special eprom burner, but since I've read that CMT used standard eproms (27C256) I might be able to burn an eprom outside the module, using a standard eprom programmer.
Help on that topic would be appreciated very much!

[Mark Hardman]

I have two of my HP-75Cs that were purchased as military surplus. Both came with what is definitely an EPROM module. They both have the clear erasure window over the chip.

If I can locate these units, I'll forward one of the modules to you. Since the modules were of no practical value, they are currently in "storage." It might be a few days.

[Sylvain Cote]

(10-04-2014 02:35 PM)Michael Fehlhammer Wrote:  I don't have a special eprom burner, but since I've read that CMT used standard eproms (27C256) I might be able to burn an eprom outside the module, using a standard eprom programmer.

Hello Michael

This link point to the CMT flyer for their EPROM modules
https://db.tt/UdfstGeg

The following pictures shows the CMT-75-32KE modules complete and taken apart
https://db.tt/KKisinJB
https://db.tt/rTW1HBjL
https://db.tt/yuNOnyVy
https://db.tt/0Z8PnzpW

This link point to the Microchip technical Reference Document for the 27C256 EPROM.
https://db.tt/WSVBPjUw

The CMT module uses the 27C256 EPROM with the PLCC package format directly soldered on the PCB.

Two solutions exist to reprogram the chip ...
1) unsoldered the chip from the board and put it into a an standard EPROM programmer with the PLCC adapter on it.
2) create a special PCB EPROM programmer adapter that map the connector and the three additional pins

In both case we also need to understand the PMS file structure to extract the 32K image needed for burning.
I have a PMS with the softwares and manual for it.

I have started 2 years ago a project to do the special PCB (option 2) and the extraction software, but never got around to finish it.
At some point I will finish that project, maybe in 2015.

Best regards,

Sylvain

[Paul Berger (Canada)]

(10-05-2014 02:32 AM)Sylvain Cote Wrote:  

(10-04-2014 02:35 PM)Michael Fehlhammer Wrote:  I don't have a special eprom burner, but since I've read that CMT used standard eproms (27C256) I might be able to burn an eprom outside the module, using a standard eprom programmer.

Hello Michael

This link point to the CMT flyer for their EPROM modules
https://db.tt/UdfstGeg

The following pictures shows the CMT-75-32KE modules complete and taken apart
https://db.tt/KKisinJB
https://db.tt/rTW1HBjL
https://db.tt/yuNOnyVy
https://db.tt/0Z8PnzpW

This link point to the Microchip technical Reference Document for the 27C256 EPROM.
https://db.tt/WSVBPjUw

The CMT module uses the 27C256 EPROM with the PLCC package format directly soldered on the PCB.

Two solutions exist to reprogram the chip ...
1) unsoldered the chip from the board and put it into a an standard EPROM programmer with the PLCC adapter on it.
2) create a special PCB EPROM programmer adapter that map the connector and the three additional pins

In both case we also need to understand the PMS file structure to extract the 32K image needed for burning.
I have a PMS with the softwares and manual for it.

I have started 2 years ago a project to do the special PCB (option 2) and the extraction software, but never got around to finish it.
At some point I will finish that project, maybe in 2015.

Best regards,

Sylvain

For option 2 do you mean the connector that plugs into the 75? or the pads under the EPROM? If you mean the the connector that plugs into the 75 you would need to emulate the 75 bus to be able to present address and data to the EPROM. Fortunately the 75 bus is not complicated but address and data are clocked out in 3 sequential bytes so that would rule out just mapping over the pins. The one I have does not have the 3 pin connector or the diode on the top side but there are a couple solder bridge connection that pull up the Vpp line that would need to be cleared.

[Sylvain Cote]

(10-05-2014 02:58 AM)Paul Berger (Canada) Wrote:  For option 2 do you mean the connector that plugs into the 75? or the pads under the EPROM? If you mean the the connector that plugs into the 75 you would need to emulate the 75 bus to be able to present address and data to the EPROM. Fortunately the 75 bus is not complicated but address and data are clocked out in 3 sequential bytes so that would rule out just mapping over the pins. The one I have does not have the 3 pin connector or the diode on the top side but there are a couple solder bridge connection that pull up the Vpp line that would need to be cleared.

Hello Paul,

You are right, there is two way of doing this, directly to the EPROM of through the 75 bus.

Option A: direct EPROM connection through a bed of nail
Advantage: easy to map it to a standard programmer
Disadvantages: not an easy task for a typical user, unglue/reglue the module, manage the connected components.

Like you said, I would have to manage the diode, the resistor and possible the hidden component on the side 2 of the PCB.
Since the PCB is a two layer, unsoldering the EPROM should give me access to the EPROM mapped pins in order to come up with the correct bed of nail. (PCB side 2)

Option B: through the 75 bus connector
Advantage: easy to reprogram for a typical user
Disadvantage: none really, except money and time for me ;-)

I would have to build a complete EPROM programmer (HW & SW) for the CMT-75-32KE.
I could take a arduino board with a AVR or ARM microcontroller (software: USB communication, HP-75 bus logic plus the GPIO lines)
connected to a custom daugther board that will handle the correct voltage and the lines mapping.

In both cases, I still need to convert the PMS generated file to an Intel Hex format for the EPROM programming.

If you have any suggestion or alternative please let me known, you are the expert here, not me.

Best regards,

Sylvain

[Michael Fehlhammer]

Now I regret that I didn't post at the general forum instead of classifieds.
Fortunately a few of our HP 75 experts found my question all the same, Sylvain and Paul, thank you for your hints and ideas!
Should we move, or should I post a reference to that thread in the general forum?
@ Mark: Thank you for your offer, Mark, I am looking forward to your eprom module!
@ Sylvain: I also got the PMS ( at least I bought it, it should be on its way across the pond now ), that's why I thought of the eprom modules.
I dream of a collection of HP -75 tool set like the JPC rom for the HP 71b.
Since I don't know much about electronics, my only idea was your option one, but of course it would be great to have the special eprom programmer adapter!
Especially Option B sounds like a great project that could greatly resuscitate interest in the HP 75. I really would like to help, but I'm afraid the only thing I can do is to offer to accept the costs of the necessary hardware, what I am offering gladly hereby.
About the file structure within PMS:
I assumed the binary is identical with that residing in an eprom. I thought the HP 75 operating system runs software residing in the PMS the same way as if it were located in a rom or eprom. Then one could just read (PEEK) the PMS contents and transfer it to the eprom programmer ... obviously it isn't that simple ?
Another problem comes to my mind:
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 ).

[Sylvain Cote]

(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

[Michael Fehlhammer]

(10-05-2014 07:15 PM)Sylvain Cote Wrote:  I have put on dropbox the following zip file for the PMS

Thank you Sylvain, now I can read and learn about the PMS even before it has arrived; besides I am not sure if it comes with the magnetic cards in working condition. Now I don't have to worry about that!

I opened a new thread in the general forum, please let's continue there!

HP 75 Eprom Modules