A807 EPROMs - backing them up

[jpmorris]

I don't know if anyone here has done this, but I'd appreciate some guidance.
Basically, EPROMS are supposed to have a storage life of about 10-15 years. The program in my A807 is 22, and the possibility of the deck going senile does not make me very happy.

I now have the equipment necessary to read and write EPROMS, and after doing some simple tests, I cloned IC15 from its 27128-3 chip into a 27256-2.
Although the ROM apparently verified OK, the deck went apeshit when the new ROM was installed. Theoretically a 27256 should act just like a 27128, right? Or is it activating the unused high address line instead of leaving it be? If so, would writing the program into the ROM twice work?

One thing I have not been able to determine is the significance of the -3 and -2. If this is addressing speed and the chip I've used is too slow, I can imagine the processor getting kind of confused.

Any ideas? Suggestions?

 

[jpmorris]

Okay, in case anyone else wants to do this, here's what I've found.

After some advice from my father and an inspection of the controller schematics, it looks like pin 27 is being held logic high by the Studer. On a 27128 as was originally present in the machine, this is necessary, as it being set low is part of the mechanism for putting the chip into Programming mode, which we Do Not Want.

On the 27256 chip I replaced it with, this is address line 14, which means that if it is strapped high, the Studer will be reading the (blank!) top half of the address space, not the bottom half.
As a quick fix, I simply replicated the program into both halves of the cloned ROM and the machine booted up happily.
I'm going to clone the other one now.

 

[cjacek]

My hat's off to you for understanding all of this (I have no clue ). In layman's terms, is your A807 deck computer controlled to the extent that you can't use it if the memory is non-functional or is lacking is some way?

 

[jpmorris]

My hat's off to you for understanding all of this (I have no clue ). In layman's terms, is your A807 deck computer controlled to the extent that you can't use it if the memory is non-functional or is lacking is some way?

Yes. The entire deck is controlled by a program which is split between two ROMs, IC14 and IC15. If these get corrupted or erased, the deck is basically dead unless you can replace them or design your own control logic to replace it all.

There are two ways of making a ROM, you can either get the thing masked at the factory, which is more permanent but also more expensive, especially for limited runs. What most manufacturers do is to use a programmable ROM (EPROM), which is cheaper but has a design life of about 10-15 years before the contents evaporate, since they are effectively a big capacitor bank which gradually loses its charge.

With the right equipment you can read the contents of a ROM and also program your own. There's a guy in Sweden who makes a custom program for the Cheetah MS6 synthesizer, for example. Adds bank switching and various other goodies which didn't exist in the original software.

There have been cases of some machines like the APR-5000 losing their EPROM contents and freaking out - I believe the 'cheap' Studer A810 on ebay UK at the moment may be is one of those.

 

[cjacek]

Man, that's pretty interesting and freaky too! I'm sure you must have pondered, though, the thought why Studer chose this seemingly crippled design, for the lack of a better term. Anyway, I hope you get your machine running asap.

 

[Beck]

Okay, in case anyone else wants to do this, here's what I've found.

After some advice from my father and an inspection of the controller schematics, it looks like pin 27 is being held logic high by the Studer. On a 27128 as was originally present in the machine, this is necessary, as it being set low is part of the mechanism for putting the chip into Programming mode, which we Do Not Want.

On the 27256 chip I replaced it with, this is address line 14, which means that if it is strapped high, the Studer will be reading the (blank!) top half of the address space, not the bottom half.
As a quick fix, I simply replicated the program into both halves of the cloned ROM and the machine booted up happily.
I'm going to clone the other one now.

Nice work!

 

[evm1024]

now what

So now you have a custom deck.... I'm not badmouthing just stating the obvious. Are 27128 hard to source? I might have some lying around from when I worked at intel. We did low yield analysis on 2716 up to 27256.

EPROM have a floating gate that is isolated by oxide on all sides. UV erases by giving any electrons trapped in the gate enough energy to tunnel through the isolation oxide. During programming electrons are forced to jump the oxide and put a net negative charge on the floating gate. There they stay. Some migrate through the oxide and others are knocked out by cosmic rays and other radiation (alpha from the package and such). This is the cause of the speced lifetime of the programming.

What this all means is that you can erase the device and reprogram it for another 10+ years.

I'll assume that you have burned a copy of the program onto a cdrom. It would be useful to post it and share it around the net. Assuming that the copyright has expired of course.

Regards, Ethan

 

[jpmorris]

So now you have a custom deck.... I'm not badmouthing just stating the obvious. Are 27128 hard to source? I might have some lying around from when I worked at intel. We did low yield analysis on 2716 up to 27256.

I couldn't get any at all when I started the project. The ones I can find now are very expensive. The 27256s were simply more readily available so I used them instead.

What this all means is that you can erase the device and reprogram it for another 10+ years.
I'll assume that you have burned a copy of the program onto a cdrom. It would be useful to post it and share it around the net. Assuming that the copyright has expired of course.
Regards, Ethan

It's not likely to expire in my lifetime. But yes, the main objective was to ensure that I had a backup copy of the firmware on a PC rather than modding it for its own sake. I'm currently running the copied ROMs for the main purpose of ensuring they work - since I'm not particularly good at removing the chips, I'm likely to stick with the copies simply for the sake of convenience. The originals have been put in a safe place.