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Digital Group Z80

Thanks Marty.

I'm interested. I'm just realistic about my abilities. :) I'll take a look at this and see if I can figure out how to do it. I'm wondering if this computer already has a halt/step switch already -- the keyboard has a switch marked 'stop', and then two mysterious momentary switches, one on either side that I'm not sure the function of.
 
Just as an aside.. until I get a front panel built, I decided there was no harm continuing RAM swapping. I took the board that had the newest 2102 chips on it, and swapped three at a time from it into the first bank (IC0-IC7). Around the sixth or seventh, suddenly we had action. Instead of a static display of characters that only changed with reset, it was dynamically working and changing them. Still random but.. it was trying to do something. And then it came back to the original garbage screen I'd had before and stayed there and would not change for reset or anything else no matter how many times I tried to change it. I swapped two more out, and suddenly the screen was different.. almost all /s.. but responding again to RESET. Fascinating. I'm going to carry on swapping RAMs in the second bank and see what happens. Thank goodness I have so many to work with, although I kind of wish I had another working machine that used 2102s that I could test against.
 
Kept swapping RAMs around but seem to be going in circles. On one swap, it started 'running' on its own, gradually changing all the random characters into ? marks, then started changing them to random again. Another swap brings a random screen, however, if I keep hitting reset, gradually more and more of the characters become / marks. Interesting.
 
Either you've got bad RAM, and at this point swapping them around won't really help, or, it's a coincidence, and there's something else wrong.
 
Hi All;

Falter, That sounds like some good Progress, Also, had or have You switched out the Ram chips in/on the Video Card ??

"" I'm interested. I'm just realistic about my abilities. I'll take a look at this and see if I can figure out how to do it. ""
My I suggest that if You cannot find a dual 22 pin card, that any tenth inch Printed Circuit card will do, and You just wire it to the appropriate pins on the Bottom of the Memory card slots..
"" I'm wondering if this computer already has a halt/step switch already -- the keyboard has a switch marked 'stop', and then two mysterious momentary switches, one on either side that I'm not sure the function of. ""
The only way to find out is to follow the wire(s) from the switches to where ever they are attached to, and then look up on the sheets I took pictures of as to where they are connected to find the signal name..

I am also wondering why You have not gotten the Initialize message, I am not sure at this point if it is the CPU Card or the Video Card.. And of course it could be Your Eprom is only slightly Corrupted..

THANK YOU Marty
 
I did not think to check the video RAMs. I'll go there next.

I'm wondering too about differences in RAMs. The RAMs from the newest RAM board were 2102AN-4L -- when I had swapped them all over, all the system would do when turned on was produce a screen full of , marks. I've checked all the 8T97s (put them all in my Apple IIs and they run fine). So I think those are okay.

I switched the board back to the older AMD 2102 RAMs.. this is what it does now:

https://www.youtube.com/watch?v=cV2VVmIe5h4

Note when I press the mystery button on the side of the keyboard.. the screen freezes in place.

Thanks for all of your help thus far Marty.. greatly appreciated!!!
 
falter said:
I did not think to check the video RAMs. I'll go there next.

I'm wondering too about differences in RAMs. The RAMs from the newest RAM board were 2102AN-4L -- when I had swapped them all over, all the system would do when turned on was produce a screen full of , marks. I've checked all the 8T97s (put them all in my Apple IIs and they run fine). So I think those are okay.

I switched the board back to the older AMD 2102 RAMs.. this is what it does now:

https://www.youtube.com/watch?v=cV2VVmIe5h4

Note when I press the mystery button on the side of the keyboard.. the screen freezes in place.

Thanks for all of your help thus far Marty.. greatly appreciated!!!
Click to expand...


At least you know the uP is running. It's probably not running what you'd expect it to. Have you rung out all the data and address lines?
 
Hi All;

Falter, I know Your are making some Progress, BUT, with No Equipment like a Scope or another Running Digital Group Machine, Your fighting a very Steep Uphill fight !!
Not only do You need the 8T97's to be good, which I think are from what You have said are most likely good from Your Apple II testing of them..
But, You also have two 74125's that go to the Eprom and the 2102's and the 7403's that go to the I/O board that has 7401's that then go to the Video Board..
All of these Ic's need to work properly for You to get something Sense-able from the TVT-32 Screen..
And to top all of that off, If some other IC on the Z-80 CPU board isn't functioning then You can see Why I am saying it's an uphill battle..
I have when I have had a Bad CPU Board, I have had to Swap out One IC at a Time from a Good CPU Board, Test it swap it back test it again and then go to the next Ic and do the same thing.. SomeTimes it was one IC sometimes it was more than One IC..
And You have Multiple Boards that All need to be Checked out.. Same thing when I was Testing either of my TVT (Video Boards) was the same way.. But I will have to say that the TVT-64 was the more difficult Board to make work right..
Let me see about making my System Work again.. And when I get to that point I will let You Know..
Then once I have a for sure working System, I will make the Offer of Asking You to send me Your Four Boards, or five Boards.. (Memory Boards, Z-80 CPU, I/O Board and Video Board).. I would get them Working and then Send them Back to You, Only thing I would need is Money to Ship them Back to You.. Based on the time it took You to get this machine, I would guess it would be gone about a Month..
THINK About it and once I get my Machine going, then You can give me Your Answer.. For now You can mull it over in Your Mind..

THANK YOU Marty
 
If it were me, as general debugging techniques, I'd build a little debug board for one of those 22/44 I/O connectors (the boards are cheap and available, you can get NOS Vector boards on eBay for cheap or from Douglas Electronics) and try to put together a diagnostic ROM. For a debug board, you should be able to put an 8-bit I/O port on a board very easily, just an 8-bit latch, 8-bit buffer, some LEDs, a DIP switch, and the random logic to tie it all together. My first step is usually to see if I can HALT the processor, just burn a ROM full of 0x76 and inspect the appropriate line on the CPU board (doesn't look like it's brought out to an edge connector). Then see if you can write a bit pattern to your diagnostic display and HALT. Then read the switch register and write its contents to the diagnostic display. At that point you can work up a memory test which gives you the high byte of the first error it finds on the diagnostic display. This is how I got going with my first S-100 system, with nothing other than a Cromemco ZPU, backplane, and a Vector prototype card.

I don't have a Digital Group machine, but I could probably build you a debug board if you don't have the resources to do so yourself. I've got a load of 22/44 prototype boards in my box of perfboard!
 
Sooooo close:

20160324_094158.jpg

I swapped the video rams with the newest 2102s I had from the RAM boards. Started to get a bit of life but not quite. I decided to look at the IO board and noticed one of the 74100s had a pin bent out. I bent it back and reinserted.. voila.. i didnt try actually loading anything.. i assume it wouldnt work.. but.. kind of gives me hope the eprom is somewhat ok and that what we are likely dealing with now is mostly ram. Problem is.. i have tons of chips but no known good to start from.

Note: if I hit Reset multiple times, more of the 'Initialize Cassette' message reveals itself.. almost got a full line 'Z80 Initialize Cassette'. On reset though the video slowly changes itself.. sometimes to good, sometimes to bad.

Btw thank you for your offer Marty and I may have to take you up on it.. unless I get really lucky this weekend.

BTW.. these newer RAMs I'm pulling from that memory board.. they are the L or low power variety.. would those have an issue running on the CPU board?
 
Thanks Glitch. Yeah, I really need to learn how to do this stuff. I didn't do so well back in highschool when we did circuit design.. hehe. Some of the concepts went right over my head. But it'd be useful to have this ability, especially if I aspire to build a TVT. If you're interested in building a debugger I'd be happy to pay for that. Having a concrete example in my hands to observe and learn from sometimes helps a lot.

glitch said:
If it were me, as general debugging techniques, I'd build a little debug board for one of those 22/44 I/O connectors (the boards are cheap and available, you can get NOS Vector boards on eBay for cheap or from Douglas Electronics) and try to put together a diagnostic ROM. For a debug board, you should be able to put an 8-bit I/O port on a board very easily, just an 8-bit latch, 8-bit buffer, some LEDs, a DIP switch, and the random logic to tie it all together. My first step is usually to see if I can HALT the processor, just burn a ROM full of 0x76 and inspect the appropriate line on the CPU board (doesn't look like it's brought out to an edge connector). Then see if you can write a bit pattern to your diagnostic display and HALT. Then read the switch register and write its contents to the diagnostic display. At that point you can work up a memory test which gives you the high byte of the first error it finds on the diagnostic display. This is how I got going with my first S-100 system, with nothing other than a Cromemco ZPU, backplane, and a Vector prototype card.

I don't have a Digital Group machine, but I could probably build you a debug board if you don't have the resources to do so yourself. I've got a load of 22/44 prototype boards in my box of perfboard!
Click to expand...
 
OK! I'll probably set myself up with a test fixture for it (probably a S-100 adapter board) so I can at least exercise it. I'll set it up to decode the low 8 bits of the address only, since my bus book says that's all that's present for the 8080 and Z80 Digital Group systems. I'll set it up for separate assign-able addresses too -- one trick I use with my S-100 debug board is to map the output port over an existing port to monitor its state or contents when debugging.
 
Hi All;

Falter, Wonderful News, with the Picture !!
This Indicates that much of the System is working..
There might be still a Ram issue somewhere, but this looks and Sounds Promising..

"" BTW.. these newer RAMs I'm pulling from that memory board.. they are the L or low power variety.. would those have an issue running on the CPU board? ""
No, basically, it's a speed issue, that the Data is Available, during the window of time When the CPU is looking for that Data..
If Glitch can't, I could Build the Board as well..
I have a spare 22/44 pin card, that all I need to do is clear off what is on it and build it..
I do have a Board that fits in the I/O Slot, which is already built that I can Send You if you think You would need it..
That is the first Picture, the Second is a memory Board that I Don't Remember for sure what all it did, I do know it has provisions for a 1702 and either a 2708 or a 2716, and possibly provisions for Data and Address Led's as well.. I will have to follow the wires and see what all is there..

001.jpg 002.jpg

THANK YOU Marty
 
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That doesn't look too bad Marty. I have to work on my soldering skills but I would think I should be able to muster something like that.

Here's a video of the machine in action. I really wish I knew which of my 2102s were good to start. I once again borrowed all the newer RAMs from the third RAM board and the system went back to garbage screen. So then I changed half out with the older and got back to where I was. At times, it will actually display the correct message up top left, but it isn't stable and changes. I'm thinking that's a video problem, probably still video RAM, but not sure.

https://www.youtube.com/watch?v=58uFALQf2is
 
Tada!

20160324_144144.jpg

My instinct was right! It was the video RAM. I carefully swapped them out one at a time and at last got a perfect screen. Unbelievable! 40 years old and this thing still can work! Take that, Acer/Dell/HP/etc!

With this working, I think I will start testing 2102s with the video card as test bed and start eliminating the bad ones. Then I can reconstitute what I have left onto the RAM boards and try enabling those. I can also check the RAMs on the CPU board in case any of those have problems that aren't presenting up front.

I guess immediate next step is put in the OS tape and see what happens?
 

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falter said:
My instinct was right! It was the video RAM. I carefully swapped them out one at a time and at last got a perfect screen. Unbelievable! 40 years old and this thing still can work! Take that, Acer/Dell/HP/etc!
Click to expand...

Congratulations. I was concerned that you were only concentrating on the memory chips and the failure might be in some to the other chips somewhere.

I'm happy to see this system working. I had seen the auction and I was tempted, but since I do not have any prior experience wit Digital Group stuff, I let it pass. I'm very happy to see that a good owner now has it, and is making progress toward getting it all up and running.

smp
 
Hi All;

Falter, Way to Go !!!
Congratulations !!!

I Think that getting at least One Ram Board going Would be in Your next Big thing to tackle..
After that, then I would Work on getting it to Load from the Cassette.. Which, may be Just as Difficult, there is a Write up on this as well..
I'LL see what I can find..
One of the things that I did, was get a short Tape, like 5 to15 minutes and on one side (assuming that the Tones are the Correct value) and Record a Continuous tone of all One's and on the other side of the Tape a Continuous tone of Zero's.. Which would Help getting the system aligned..

THANK YOU Marty
 
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Thanks guys!

smp: Yeah, I almost didn't do this, but, I thought having the first computer designed around a Z80 and that whole homebrew thing too much to pass up on. I'm glad to see it's (somewhat) alive now and I can put RAM issues to bed.

I've tested about 30 2102s so far and found 5 bad ones. Decent ratio.

I decided to give the cassette load a try. Per the instructions, I fired up the Z80, then rewound the tape. You're supposed to have a special set up so that you can hear where the tone begins, but I used the counter to know there that was. As the tone began, I hit RESET and let it play. No reaction from the machine unfortunately, and I supposed that is to be expected. I read in one manual (CPU) that when you put in the Z80 Initialize tape in (I'm assuming that's the OS?? I have a TVC 64 OS cassette here), it's supposed to fill the screen with 1s. But mine doesn't react at all.

I'm not sure which port is the correct one.. he has Ear and Mic marked on the back, but you know how those could mean the opposite of what we think they mean. I'll have to figure out which is C1 and C2 and follow from there.
 
Okay, so it's the EAR port. I tried loading again but still no dice. Following the manual, it suggested looking at the 7403 on the CPU or 7401s on the IO card. The 7403 looked to be newer and I didn't have a replacement for it, so I went for the 7401. As it turned out, I had a tube of 5 brand new ones intended for my TVT project, so I plugged em all in to the first five IO ports. Voila.. we have action:

https://www.youtube.com/watch?v=QkF09vG0Yro

According to the manual, we should see 256 1s, then 256 2s, etc.. up into 7s as it loads. The manual says each number represents the 'page of the current data byte being read'. If I understand correctly, I *think* from what I can see, something is going wrong in one of the RAMs.. around Page 4. I think... don't quite understand how it works. Thoughts? Maybe some of the RAM on the CPU board isn't cherry?
 
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Hi All;

Falter, Great !!!!!!!!!!!!
Ok, First thing to understand is that it starts to Load at Page 1, So, My math is not quite there right now..
But, I think that by Page 4 You have Run out of Ram !!!
So I would Try and get at Least one (I assume 8K Memory Board) going..
Up Your CPU jumpers that You addressed at '0000, and Re-address them for 9K.. Go back to Posting #57, and Set the Jumpers accordingly to the 8-10K Position..
And then try to Re-load Your Tape..

You mention in Your Other Posting about 2102's that You do not want to turn it on and off to much..

I have gone thru about 6 to 8 of the +5 Volt Power Supplies, and So I would Recommend either getting A PC Switcher Power Supply or getting a Modern +5 volt Switcher Power Supply..
But funny thing is I have never had a problem with the smaller -5 and +/-12 volt supply..
THANK YOU Marty
 
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