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reading ROMs from compaq portable, commodore 64, etc

jafir

jafir

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I recently bought a cheap USB EEPROM writer, a TL866II, and I've been playing around reading and writing various ROMs. I figured for any board I have that I'm not too concerned about damaging or if I have a spare, I'd pull the ROM and try to back it up, in case anyone (including me) ever needs it.

So one of the things I tried was reading a BIOS from my compaq portable. I had removed this one for an upgrade to version C so that I could have 640K of RAM and an XT-IDE. Anyway, the original compaq chip is a Hitachi 61364PC21, which I'm assuming (maybe wrongly so) is the same as an HN61364P. Anyway, when I try to read it I get a "pin detect" error, which is kind of expected because this particular ROM isn't in the database, but I used the datasheet and found that all of the address pins, Vcc, and Vss line up with a 2764 and other common EPROMS, which makes sense because I was able to use a W27E257 as a replacement. If I disable PIN detect and ID, when it reads the ROM I only get FF.

Am I making bad assumptions and doing something wrong?

The next thing I tried was reading a ROM from a C64 and also a 1541. I pretty much stopped right away, because I couldn't find anything "modern" that has a similar pinout, and this programmer doesn't seem to allow for custom configurations. I'm assuming that I'll need to use an adapter socket, kind of the inverse of this: https://github.com/SukkoPera/Open2327RomAdapter to even attempt reading.

Or do you think I could just use the in circuit programming harness and just connect directly to the pins I need?

Thanks!
 
Chuck(G) said:
The Hitachi chip here uses a JEDEC pinout and says that it's pinout-compatible with EPROM. So you're good there.
Click to expand...

I guess that's why I'm confused. This was a chip I pulled out of my working portable, I only replaced it to upgrade to a later version of BIOS that supports more RAM and option roms. But when I attempt to read it, I just get FF.
 
No, it is not exactly the same as a 2764. The linked PDF shows two extra Output Enable pins in place of the Program and a NC pin. Extra Output Enable pins on ROMs can be used to avoid the need for additional decoding logic when there are multiple ROMs.

Basically you will need to pull/isolate Output Enable pins 27 and 26 and either tie them high or to ground.

You could check the motherboard wiring to see how these pins are set on that socket. Otherwise 1 out of 4 guesses should get one that works.

It is possible the first OE and CS could also differ, but if a 2764 compatible could replace this chip, then these pins should not differ.
 
jafir said:
...The next thing I tried was reading a ROM from a C64 and also a 1541. I pretty much stopped right away, because I couldn't find anything "modern" that has a similar pinout, and this programmer doesn't seem to allow for custom configurations. I'm assuming that I'll need to use an adapter socket, kind of the inverse of this: https://github.com/SukkoPera/Open2327RomAdapter to even attempt reading.
Click to expand...
If your programmer knows about the Motorola 68764 or 68766 you should be able to read them; I believe there are also compatible Hitachi and Cypress 24-pin 64Kb EPROM versions but I can't locate the numbers at the moment.

However, note that especially in Commodore disk drives some of the ROMs have positive logic chip enables.
 
SomeGuy said:
Basically you will need to pull/isolate Output Enable pins 27 and 26 and either tie them high or to ground.

You could check the motherboard wiring to see how these pins are set on that socket. Otherwise 1 out of 4 guesses should get one that works.
Click to expand...

Thanks! I’ll give that a try. Since there are OE pins where my replacement EEPROM has a couple of extra address inputs could that cause an issue? Or does it not matter since the old chip is only 25% the capacity of the new chip? Maybe it doesn’t use the address inputs? Obviously I’m way out of my depth currently :)
 
MikeS said:
If your programmer knows about the Motorola 68764 or 68766 you should be able to read them
Click to expand...

It doesn’t look like it does. Which isn’t really an issue. None of this tuff is critical, I’m just playing around and trying to learn some stuff.

Thanks!
 
jafir said:
Thanks! I’ll give that a try. Since there are OE pins where my replacement EEPROM has a couple of extra address inputs could that cause an issue? Or does it not matter since the old chip is only 25% the capacity of the new chip? Maybe it doesn’t use the address inputs? Obviously I’m way out of my depth currently :)
Click to expand...
If the content is repeated so it fills the entire chip, then it should not matter. It will simply pick one of the copies that corresponds with however the pins are wired.
 
I'd be a little more cautious. OEx enables the output drivers on the chip (otherwise they're tristate-floating). If one of the OEx pins is actually used to enable/disable the ROM output, then you've got a bus contention issue and things will not be gleefully happy. :) Buzz out the extra OE pins and try to find where they go.
 
I can't remember what I did exactly to read my Compaq ROMs, but it did require something a lil extra.

I believe I lifted a pin or two from the programmer and connected to GND or VCC.

I know I used info from here, since my chip was OKI M3864 which is another "almost-2764" chip like your Hitachi.

http://www.primrosebank.net/computers/mtx/components/memory/roms/roms.htm
Scroll down to the part about OKI M3864 8KB

Ahh I remember what I did now.
See where the extra OE / CS pins are? Check the Compaq Socket and see if they are tied to GND or VCC.
 
nztdm said:
Ahh I remember what I did now.
See where the extra OE / CS pins are? Check the Compaq Socket and see if they are tied to GND or VCC.
Click to expand...

I finally got around to messing with this. What you suggested worked fine, of course. Thanks! It seems that a lot of the Compaq ROMs are set the same way, so I went ahead and dumped the ROM for a VDU and HDD controller too, since I don't see those archived anywhere yet. If I get some time this week, I'll get down my deskpro and try dumping the ROMs from it too.

eeprom burner.jpg
 
jafir said:
I finally got around to messing with this. What you suggested worked fine, of course. Thanks! It seems that a lot of the Compaq ROMs are set the same way, so I went ahead and dumped the ROM for a VDU and HDD controller too, since I don't see those archived anywhere yet. If I get some time this week, I'll get down my deskpro and try dumping the ROMs from it too.

View attachment 57498
Click to expand...

Excellent!

I also dumped the VDU ROM too, but I don't have a hard drive controller.
I should also dump the two bipolar PROMs on the VDU, but they are soldered; for another day.
 
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