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Digital Group Z80

Hi All;

Falter, two things, I noticed, first, It looks You have a TVT-64 and not a TVT-32, because of the two columns it shows..
One on the left and one in the middle, that can (if I remember correctly) can be changed, but I don't remember how at the Present..
Secondly, that actually is good that You have gotten that far with it..
I think it might be a 7404 that is bad driving that last row or two of memory, But it could be a Bad Ram chip as well..
The only way to tell, is to pull that last row of Ram, and try only one ram chip at a time in it and see if it messes up the screen..
Once that has been accomplished, and You have tried those same rams in the Video card one at a time, and if they work, Then it has to be either a Bad IC like a 7404 or a Trace short..
I have never had a Cassette Break like that on me, what I suspect is the Tapes were stored in the Sunlight or You Tape player has too much Tension on it on the Take up reel..
Good work Overall though..
"" Anyway, I started re-adding RAMs line by line to the board and powering up to test. The first three rows of 8 on the left side = no problem. As soon as I add the last row on the bottom left side (IC 70-77), that's when it hangs. I did a look around at the soldering and traces but I don't see any shorts, etc. ""
Use a Magnifying Glass !!! And if need be a meter..
""Can the board still be accessed with that one line out? I know you mentioned the boards could work with some rows out, but I wasn't sure if certain rows (like the bottom or top) were special in some way. ""
No they are all the same..
I also think that You still have some Rams that are not quite right, there used to be, but I don't know if You have it, but on the basic Operating System Tape, (Not BASIC) a Memory Testing Program, See if You can find it, that would be alot of Help to You..
Also, check all of the IC's and Rams for black like Corrosion on the pins and if it is there use an Ink Erasure on the pins to clean them some.. There could be the same thing in the Sockets, but the only fix for that is to replace the socket.. And unless You are really good at that I wouldn't attempt it..
I Just had a thought, and so I need to ask, is Row Ic's 70 to 77 Fully Populated ??
If it is not, then Possibly there is part of Your problem, Try this as well, Take out 8 rams from the CPU card, going up and Down, not side to side, the column closest to the 1702 Eprom and put them in that last row (Ic's 70 - 77) of the 8K Memory Board, If You can get them to work in there then You will have 17K of memory instead of 15K of memory and a 1K space and then 2 more K of memory from the CPU..
And that 17K will be as much memory as You can have until You get some 2102 Replacements for Your Bad ones..

THANK YOU Marty
 
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If you hit a wall with the 2102 testing, I can test them for you if you want to ship them to me. I've got a large bin of tested 2102s as well, so if you need replacements I've got plenty.
 
IC row 70-77 are the 8 ICs on the bottom left of the RAM card that I've left out. At least.. thats how DG has them numbered in the diagram. I swapped all the 7404s, 7442 and 7420 from the working board and it didn't make any difference. But I did not try adding RAMs to that row one at a time, as I think you mentioned before needing to have complete rows of 8 when we were talking about the last card.

Apparently MAXI Basic requires at least 18k of RAM.. so that could be my problem right there with that one. That one I can get to a READY prompt and even type a bit before it locks up. And you're right Marty.. I do recall seeing a 64 char mode switcher on one of the other tapes. Hmm.

So if I cannot figure that last row out for now on that card, does that mean I cannot populate my third card with the last RAMs I have and add that in also?

I'll try what you've suggested and see how much further I can get.

Glitch thank you for your offer. I may have to take you up on that. All of the RAMs I'm currently using were put on the video card for testing and did not produce anything strange.. but maybe that isn't a robust enough test.
 
Hi All;

Falter, I did Not know You had a third Memory Card..
Yes, Try to get that last Row of memory Working, and up the 2K to 25-26K position, and take out the Rams from that last card for Replacement..
And hopefully, You can get 24 to 25K of Memory Working.. And then Try to Load Some of Your Programs..
And then Hopefully, They will come up completely..

THANK YOU Marty
 
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Okay.. so I think we still have some memory problems. First, I successfully got the second RAM Board running. I left the 2102s I'd pulled from IC 70-77 out, and subbed in replacements from my 'good' static bag. One by one I inserted and powered up.. now it works fine with everything in. So, mystery solved.

Now we get to loading programs. On a hunch, I counted out how many passes of 0s through 7s happened during the load before we got what I think is corrupted data during the load process. I did this for both Tiny and MAXI BASIC. As suspected, they both experience trouble at *exactly* the same pages. For both, the system will go 0s through 7s twice with no problems, but on the third run of 0s through 7s, the 5s and the 7s produce those periods. The rest loads fine and the program comes up, however corrupted and you can't actually run anything you program without it crashing.

So I think, if I understand how DG's page indicator system works, I have a problem somewhere on the first board with bad RAM. I'm just not sure how to figure out which page or bank or row the number corresponds to. Am I understanding how that works Marty?
 
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Hi All;

Falter, Yes I think So.. My math is not the Greatest, But, each digit listed is 256 bits, (Correct me If I am wrong)..
And there are eight sets of numbers before it, repeats itself.. Which Translates into 2K..
Ok, You have an 8K Board with eight Rows, so each Row is 1K of Ram..
"" For both, the system will go 0s through 7s twice with no problems, but on the third run of 0s through 7s, the 5s and the 7s produce those periods. ""
So, two rows of 0s and 7s is four K, and 0 thru 3 on the third try, is another 1K, and the 4 thru 7 is 6K, which is where the Trouble is,
So, Take all Eight Rams from Row 50 to 57, put them aside, and take out eight rams from Memory Board Three, and Replace the ones You took out from 50 to 57 and Retest, and see if my Math is Correct..
If it is, then Retest the eight You took out, and see which ones show bad in the Video Card, Put the Bad ones aside, and the good Ones back into the Last card up to a full row of eight..

THANK YOU Marty
 
Thanks Marty.. I kind of had a hunch it was the same thing.. but after reading this I tried and sure enough.. something in Row 50-57 was bad. After changing those out, all was good and MAXI BASIC now works properly. Tiny BASIC sort of works but I think it's just messed up because of my 64 character card.

The strange thing is, those RAMs passed my 'video card test'. So now I'm wondering how reliable that method is. Perhaps what I should do is sub these suspect chips into the first row, and then keep subbing chips around until I find the bad ones?
 
Hi All;

Falter, Good !!!!
Do You have the Third Memory Board in place ??
And have You upped the CPU Ram to the 25-26K Position ??
I also, took a look at one of Your Video's that I hadn't looked at before, which had a scan of Your Cards and the Box, and So, Now I can say, that Yes it is the TVT-64 and Not the TVT-32 card.. They look enough different that even with a partial view I could tell..

"" Perhaps what I should do is sub these suspect chips into the first row, and then keep subbing chips around until I find the bad ones? ""
What I would suggest is put all of You Bad 2102's in the Last couple of Rows of the Third Memory Board, and wait until You can get or Write a good Memory Test Program to test then.. Or if You have less than two rows of Bad Rams, put them in the CPU Ram Sockets and with it at the 25 to 26K section, and they can be tested later..

THANK YOU Marty
 
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Not yet on the memory board. I am wading through the remaining chips I haven't tested. I am putting them 4 at a time on IC 50-57 on the first memory board and then seeing how BASIC loads. If there's an issue now I know what to look for. Once I have the last batch sorted I'll reconfig jumpers, throw in as many banks as I can get on the last board and then voila!

I wish I could fix the broken tapes. In the old days I'd unscrew the case and then reattach the tape to the spool. The tapes that broke all broke at the beginning so they might be salvageable. These tapes are all sealed.. can't disassemble them easily.

I'm not sure how to load the game tapes either.. assuming they are for this machine. Ie. If I have to load BASIC first and then load them in somehow, or if they have their own bootstrap.

Other thing is.. I can't figure out how to backspace on this keyboard. I've tried every non letter and number key on it but nothing deletes the previous character.. I'm assuming backspace functionality existed by the time this computer came out..
 
Hi All'

Falter, "" I'm not sure how to load the game tapes either.. assuming they are for this machine. Ie. If I have to load BASIC first and then load them in somehow, or if they have their own bootstrap. ""
For some Reason the Word 'Load' sticks in my mind.. But, Since I never did any of the Games, I am not much Help.. But, yes, those Game tapes were offered by Digital Group, so there must have been a way..
I would try to Load up a Game tape and see if or what appears on the Screen.. Couldn't hurt..
"" Other thing is.. I can't figure out how to backspace on this keyboard. I've tried every non letter and number key on it but nothing deletes the previous character.. I'm assuming backspace functionality existed by the time this computer came out.. ""
Backspace existed, but I can't say for sure if it did on this Video System.. I would suggest holding the Control down and then trying the different Keys..
"" I wish I could fix the broken tapes. In the old days I'd unscrew the case and then reattach the tape to the spool. The tapes that broke all broke at the beginning so they might be salvageable. These tapes are all sealed.. can't disassemble them easily. ""
A Razor Blade works wonders, but be careful not to Cut Yourself in the process, and then Tape the case back together..
I usually use Scotch tape on the on head side of the tape (shiney side) and cut it to cassette width with shearers.. About 1/2 Long or so..

And then as soon as You can, get a NEW Cassette or an OLD Music Cassette and first Erase the whole Tape Both sides, and Copy the tape to the new Cassette..
The best way is to modify the Z-80 operating System Tape AFTER You have Read it in, so that It will Load the next tape starting at Page '011 and the Ending address at some ridiculously high address and Read in the Broken/fixed tape so it loads at page 011 instead of page 001..
This was How I made my Copy Hi Tape, and I could load it in and all I had to do was change the ending address after Loading in what I needed to copy..
Note the last page that it loads to and change the ending address to that page..
And then Record at least three copies to the new tape, (Hopefully) Your Recording frequencies are OK on Your Video Card, they should be as the signals are Divided and not a Voltage Controlled Oscillator..
Of Course You could do a Straight Cassette copy from one tape machine to another Tape machine, but the Quality is not the same, too much noise..

THANK YOU Marty
 
Well, I did it. I got the third board up and running. As it turned out, exactly 24 2102 RAMs were bad, so I put the rest on the third card starting at bank 0, leaving the last three empty. I jumpered the CPU correctly and everything came up fine.

I also started subbing back in the 7401s I took out on suspicion of being bad. So far got three back in and everything works.

I did try booting straight off my game tapes with no luck. So then I loaded up MAXI BASIC and just issued a LOAD command (just LOAD by itself), and threw in the Star Trek tape. Sure enough it loaded.. it typed RUN and there it is:

20160327_185156.jpg

I powered off (the NEW command just causes an error and pressing reset doesn't clear the program) and loaded MAXI again. This time I went for Quest III. It loaded ok and you can list it.. but it complains there isn't enough memory to run. Guess I'll have to get some more 2102s!

20160327_191524.jpg
 
Hi All;

Falter, Great News !!!
You could take the 2K from the CPU card and put them in the two of the three remaining rows and gain Yourself 2K more of memory for Quest III.. I don't know if that Would be enough or not..
Also, You could try Loading Quest III from Tiny Basic and see if that helps it fit into the memory you currently have, along with Star Trek..
When You took Your Pictures, the Camera was far enough away, that I could not Read the Script.. But, I am Glad that You have things working..
Hopefully, SomeOne can sell You another 24 2102's for a descent price.. If I were You, I would Try to get about 30 of them, Just in case some more show up to be Bad..
Also Could You try to bring up the MICROPRODUCTS ASSEMBLER MP8101-1 2000.2FFFR , I am curious about it ?? Thank You..

THANK YOU Marty
 
Super! Great effort. You've put in a lot of work and you are getting the payoff.

I'm glad to see this relatively rare machine coming back to life.

smp
 
It doesn't like the very first statement in the program - 10 DIM I(25,26) -- soon as it attempts that first line it says out of memory. I'll try pulling from the CPU card (have been busy at another project). It can operate with no RAM there?

Also I tried the Assembler. No dice. The tape has a note that says to play it on the reverse side, which I did. And there is some kind of tone there, but the Z80 just goes through 0-7s a bunch of times and then nothing.
 
I've lost track of how much memory you have working now...

Let's assume you have like 20K, as an example. You read in your Tiny BASIC. It takes what? 13K or something like that, IIRC. Then you read in your program, and I don't know but maybe it takes up another 4K maybe. That's 17K used out of your 20K. BASIC will have a variety of storage needs, too. Then your program says DIM I(25,26). 25x26 elements is 650 elements. How many bytes are needed per element? Maybe 3-4 bytes per element to hold floating point numbers, maybe. 650 elements times 3 bytes each would be 1950 more bytes used. 13K + 4K +2K = 19K plus the BASIC internal reserved memory stuff... and you're out of memory.

Now I'm just talking off the top of my head here, with just for example numbers, but I can see that you could still be short of memory in your system pretty easily.

smp

Edit: I went back to find that you said Tiny BASIC was 10K, so maybe my example is not as good as I originally thought. Are you using the Tiny BASIC or the Maxi BASIC?
 
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Hi All;

Falter, Remember, just like the System with Two memory Boards and then the Memory of the CPU Board was above the Second Memory Board..

So, this is basically the same kind of thing..
You have three 8K Boards, which is 24K if they were all full of Memory, and then 2K of memory from the CPU Board for a Grand Total of 26K of Memory..
But, since You have 3K taken out of the Last memory Card, You have 24K minus 3K which leaves 21K of memory And You 2K of Memory from the CPU card sitting at 25 to 26K, which means You have memory from 00 to 21K and from 25 to 26K, So if You take the CPU memory out and move it to 22 and 23K on the last 8K memory Board, You will have 00 to 23K of continuous Memory..

"" Also I tried the Assembler. No dice. The tape has a note that says to play it on the reverse side, which I did. And there is some kind of tone there, but the Z80 just goes through 0-7s a bunch of times and then nothing. ""

OK !!

THANK YOU Marty
 
Okay that makes sense. Thanks Marty.

I'm curious about this Assembler program.. when I try to load, it does a bunch of 1s sort of sporadically at the beginning, and then settles into reading at a much slower pace than say, MAXI Basic. ie. the numbers move really slowly. Does that tell us anything?
 
Ah.. I've figured something out. With these tapes, you have to get them to exactly the right position before getting the machine to read them. Otherwise, it does the 'slow numbers', because any out of the ordinary noise can cause the system to read it as data and mess everything up.
 
Sorry I missed your message there.

Thank you for explaining that. My memory is a bit hazy on DIM statements, etc. Been a long time since I programmed in BASIC. I think as it stands I have 2k (CPU), + 16k (two memory cards) + 5k (not fully populated memory card) = 23K. But I guess as Marty explains that 2K on the CPU card is sort of off limits? Still not clear on how that works. Someone in another thread suggested my OSI 500 board might use 2102s.. the part numbers on them don't look like it but I'll check that out. Or I'll do what Marty said and try stealing from the CPU card. Or just cough up and buy 2102s.

Marty -- I understand these systems could be upgraded beyond 3 8k memory boards -- was that just a matter of bigger chips or did it require different boards?

smp said:
I've lost track of how much memory you have working now...

Let's assume you have like 20K, as an example. You read in your Tiny BASIC. It takes what? 13K or something like that, IIRC. Then you read in your program, and I don't know but maybe it takes up another 4K maybe. That's 17K used out of your 20K. BASIC will have a variety of storage needs, too. Then your program says DIM I(25,26). 25x26 elements is 650 elements. How many bytes are needed per element? Maybe 3-4 bytes per element to hold floating point numbers, maybe. 650 elements times 3 bytes each would be 1950 more bytes used. 13K + 4K +2K = 19K plus the BASIC internal reserved memory stuff... and you're out of memory.

Now I'm just talking off the top of my head here, with just for example numbers, but I can see that you could still be short of memory in your system pretty easily.

smp

Edit: I went back to find that you said Tiny BASIC was 10K, so maybe my example is not as good as I originally thought. Are you using the Tiny BASIC or the Maxi BASIC?
Click to expand...
 
Last note of the evening.. sure enough.. my 500 board had 2102s.. and since it isn't operable at the moment, I borrowed enough to complete my third RAM board. And now Quest III runs! Although I wonder what happened to Quest I and II..
 
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