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Digital Group 1702A EPROM Board

falter

falter

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MyUS 'unlost' a batch of items I bought a while ago including some digital group gear. Among that stuff was this EPROM board that uses 1702As.

https://drive.google.com/file/d/1YBV...ew?usp=sharing
https://drive.google.com/file/d/1Y2g...ew?usp=sharing

Ideally I'd love to use this to, for example, read the contents of the EPROM in my Microkit machine. However I am a little cautious about just plugging it in as I worry about blowing something up. There is no documentation that I've found for it, even on Bryan Blackburn's site.

As far as I can tell, the board conforms to the same pinout as one of the digital group memory boards. The voltages all seem to land at the same pins, although on the actual RAM boards several like the -12V aren't used. On this one, it looks like -12V is fed into a regulator that reduces it to the -9V needed for the 1702As. I'm thinking I *should* be able to just plug it in place of one of those boards. Reading the labels on the EPROMs, it seems like it is set up to operate around F000-FFFF. I don't know how all this jives with the original digital group monitor ROM.. if it has to be disabled. I think I read somewhere the original dg ROM resided at $0000, but I figure these EPROMs must be to allow the machine to boot right up into CP/M?

Anyone have any tips/precautions before I try this? Obviously I could pull all the ICs and check voltages. I'd also look for shorts.

Could I potentially take the 1702 marked FC00, plug in a different one and then look for the contents at that address?
 
There are two issues here:

One is blowing up the card itself.

The second is blowing up the system you are plugging the card into.

I see all of the devices on the EPROM card are in sockets - so I would take a high-resolution photograph of the front of the EPROM board and note down which ICs are plugged in where and their orientation. Remember our previous problem with the orientation of a device!

I would then remove all of the socketed ICs from the EPROM board.

First, test out the system you are going to plug the EPROM card into to make sure it works OK. Power OFF.

Next, I would plug the 'unloaded' EPROM board into your system and perform some rudimentary tests with a multimeter (with the power OFF):

1. Make sure that the voltage rails (e.g. 0V, +5V and -12V) are correctly connected through from the system to the EPROM card.

2. Make sure that the voltage rails are not accidentally shorted out by the presence of the EPROM card.

If that looks OK - power up (but without the chips in the EPROM card). Hopefully, your system should power up OK and still work without letting out the magic smoke. Power OFF.

Next, I would install the TTL chips into the EPROM board (but not the EPROMs at this point) and test the board again in the system. It is just possible at this point that that the EPROM board prevents the system from working due to an address clash. We can worry about this if it happens...

Dave
 
I would add one thing to Dave's suggestions: Get thy EPROMs to a Prom-Programmer (and read/save/archive the contents) - unless you know for certain that the contents of the ROMs is already archived somewhere. If it were me, I'd do that before ever applying power to the board (when the EPROMs are installed). Bad power can erase those EPROMs.
 
Many thanks. Yes Dave and I are on the same wavelength. I am particularly nervous because the digital group motherboard requires a lot of point to point connections based on which cards are installed and I didn't trust that I could just plug this in. I also wondered if the power draw here would be too much for my original dg power supply. I have a second digital group system in parts I got from Marty. With the other system I can rig up a modern switching power supply and if something blows I won't be crying too hard.

I would love to read these EPROMs before powering up with them installed, but 1702as have been the bane of my existence that way. I have two vintage EPROM readers now.. one is a homebrew deal I don't think I can decipher. Another is an Midwest Scientific unit which I have all the hardware for but not the software. Finally I bought the PCB for a modern 1702A reader but am still trying to get it working. I had hoped that this digital group board would break the impasse and give me a means of reading EPROMs.
 
Good point - although (if memory serves me correctly) that is falter's problem to start with in that he hasn't got an EPROM reader/programmer that can 'do' 1702 devices. I may be wrong though.

In my post I was not planning to put any of the original 1702's back into the board until we had checked out the voltage on the sockets themselves. Even then, I am not sure what value putting an unknown EPROM into an unknown functioning board would be.

Falter, do you have a way of programming 1702(A) EPROMs? If so, you could program a 'dummy' device with some known data patters in it. If not, perhaps someone could do that for you as a test?

Dave
 
daver2 said:
Good point - although (if memory serves me correctly) that is falter's problem to start with in that he hasn't got an EPROM reader/programmer that can 'do' 1702 devices. I may be wrong though.

In my post I was not planning to put any of the original 1702's back into the board until we had checked out the voltage on the sockets themselves. Even then, I am not sure what value putting an unknown EPROM into an unknown functioning board would be.

Falter, do you have a way of programming 1702(A) EPROMs? If so, you could program a 'dummy' device with some known data patters in it. If not, perhaps someone could do that for you as a test?

Dave
Click to expand...

I've no way to program or read anything at the moment. I do have several known good digital group EPROMs, so I could use one of those to test, assuming once the board is checked out, that it's just a matter of plugging it into the socket where, say, that EPROM marked FC00 is and then going looking for data there.
 
That would be my 'plan'.

First, make sure you can plug the board in OK without disturbing the operation of the system itself.

Then check the power supply rails on the logic and EPROM socket(s) of the new board.

Next, plug the TTL chips in and ensure that the system still works and we haven't got a memory conflict error preventing the system from starting up.

Then, plug a single EPROM in (preferably with known contents) and see if we can find it in the memory space of the system.

Build the system up in small steps and make sure each small step works. If not, we can backout knowing where the procedure failed.

Dave
 
It's a bit surprising to see a Digital Group board made for 1702A EPROMs exclusively. The 2704/2708 came out in 1975 and was infinitely easier to program and held more than 1702. So this board must be for a very eary DG system.

You might want to look up Greg Peterson and see if he has any comments on your board.
 
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t's a bit surprising to see a Digital Group board made for 1702A EPROMs exclusively. The 2704/2708 came out in 1975 and was infinitely easier to program and held more than 1702. So this board must be for a very eary DG system.
Click to expand...

It was being advertised in flyers as late as 1977. Given digital group's ever shaky situation who knows.. maybe 1702As were getting cheap then and they wanted to offer a cheap ROM board option? I think Solid State Music had an S100 board around the same time, also 1702A based.

The flyer says it holds 4K of 1702As, ROM and a portion of RAM may overlap with ROM taking precedence. Jumperable to any 4K boundary.

I've not been able to find any documentation for it anywhere, even Bryan Blackburn doesn't have anything, and he seems to have everything else. It would be really handy to understand how the jumpering works.

It and the 4K color graphics board they made have seem to be rare. I would kill for the latter.
 
Well, D-G had fairly shaky financials, so anything off the beaten path is going to be rare. Add to that, not S100, but proprietary bus structure definitely put them at a disadvantage.
 
So I've removed the EPROMs and done a bit of continuity testing looking for shorts. I also did diode tests with my DMM.

The diodes seem to come out fine. There is very high resistance between the -12V rail and the +5V (1M OHM), however after the regulator on the -9V there is 'only' 1.5k ohm between that and the +5V. I'm not sure if that's a problem - it's not a dead short.. but I'm not sure if it should be higher or not.
 
Try your meter probes both ways round between the two measurement points and see if the reading changes. What you may be reading is a ‘sneak current path’ as a result of the internal voltage regulators etc. 1.5k is Ok in my book.

Dave
 
Seems to be same resistance with probes either way. I've seen lower resistance ratings between power rails before, I just wasn't sure because the -9V and +5V rails have a much higher resistance on the CPU board.
 
With the EPROMs removed, I'd power it up. If you have a linear supply on your machine, you can use a variac, to bring things up slowly. The -9V regulator may be open or it may just be the voltage of your meter is not enough to turn it on.
I can send you a 1702A with some data on it if you want.
Dwight
 
After looking at the board some, I see there is no +5V regulation. It would require an external +5V, Possibly borrowed from some other board?? Is it compatible with a S100?
Dwight
 
+5V is provided directly off the motherboard. I think the regulation for that is in one of the power supplies (there are a couple). The board has connectors for +5v, -5v, +12v and -12v.
 
I am assuming you have looked up the TDG Bus in the S-100 and Other Micro Buses book. It's on the Internet Archive.
 
There will be a ‘sneak path’ from the +5V rail, through the on-board TTL logic devices to the 0V rail and from there to the negative regulator and back to the -12V rail.

I would expect to see something between the 0V rail and the -12V rail in this case.

Did you remove the socketed TTL devices before performing the continuity test or not?

Dave
 
daver2 said:
There will be a ‘sneak path’ from the +5V rail, through the on-board TTL logic devices to the 0V rail and from there to the negative regulator and back to the -12V rail.

I would expect to see something between the 0V rail and the -12V rail in this case.

Did you remove the socketed TTL devices before performing the continuity test or not?

Dave
Click to expand...

I removed all the socketed devices, but there is that one IC that has been glued to the board and has wires soldered to its pins that I haven't disconnected.

Resistance between -12V and 0V is quite high - 5.5M. Resistance between +5V and 0V is 1.5k OHM.
 
I am surprised at the resistance from +5V to 0V. I suspect the upside down IC is a 7404 or 7406 inverter. Only one gate appears to be used.

There has to be something that is creating this. I don’t think it is a problem though...

Dave
 
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