Digital Group Z80

Hi All;

Falter, The Board that You are waiting for goes in between the CPU and the Video Card, Just as (If I Remember Correctly) You Stated in an earlier Posting..
Once, I hope You have the I/O and installed it might if everything is working Correctly come up with the Initialization Which has "Read Z80 INITIALIZE Cassette", Even though I think it should have said 'INITIALIZE Z80 Read Cassette'..
Once You have that on the TV/Monitor Screen, Then You can be sure that it is either Working or Mostly Working..
Then Without Plugging any Tape Cassette Player in, If there is Only the Read Z80 etc..
Then after Plugging in the Tape Cassette Player then You should see a "1" or a couple of Ones..
After a Reset, The Blue Button.. It should only display the Initialize Z80, But, I You see a String of 1's and the 2's up to 7's, then the Cassette ""PLAY"" Interface is out of Adjustment, And would need to be Adjusted, Before You can use it..
So, Please Let me Know When You get this Far and What You see..

THANK YOU Marty

I/O card arrived today!

And it turns out the I/O slot was labelled. So I plugged in the I/O card and fired up annnnnd... same thing as before. Solid screen of blocks.

I pulled all the extra RAM boards out and just tried with the three cards. Same thing. I pulled the CPU card just to see if the system reacted to it's not being there. Still the same screen. I pulled and tested the Z80 CPU in another machine.. it's ok. So now I'm worried about that EPROM. However, on flipping the CPU card over.. there is a resistor that has broken off at one end. It appears to attach to one pin of the 2.5000 MHz crystal (think that's what it is.. silver can).. and goes to pin 9 of the 7400 IC next to it. Maybe that?

Also plugged in the keyboard to the port I thought it belonged in. When the computer is turned on, all the little LEDs on that little board attached to the computer light up, and then flash or change according to what keys you press.

Hi All;

Falter, Congratulations on getting the I/O Card..
That Resistor is Added, where does the other end of the Resistor go to ?? (+5, pin 14 of that Ic, maybe)..
What do You have for Test Equipment ??
Scope, Logic probe, Meter ??
It looks like You have the Keyboard working, which is Good..
"" I pulled all the extra RAM boards out and just tried with the three cards. Same thing. I pulled the CPU card just to see if the system reacted to it's not being there. Still the same screen. I pulled and tested the Z80 CPU in another machine.. ""
With the CPU pulled it would give only the Blocks, One thing You can try, Very carefully, is Ground with a wire some of the Input data lines to the Video card, and You should see something other than the Blocks, hopefully some letters.. This would say that its not in the video card, if You can do this..
Do You have the Assembly Manual for the this machine ??
I will look to see what I have left anymore..

Also, while I think of it, that IC the 7400 Next to the crystal, HAS To to be a 74S00 or a 74AS00.. Not an 74LS00 or Plain 7400..

THANK YOU Marty

So I pulled the crystal off to check it out.. and as I move it I hear a faint metalling 'clinking' noise, like when a lightbulb burns out and the filament is just bouncing around in there. Thinking the crystal may be toast.

Hi All;

Falter, do You have a Crystal that You can borrow from something else ?? Just to check out its operation, even if it is not a 2.5 MegaHertz Crystal..

That Resistor could be what took out the Crystal..

Also, here is a Picture of what the Jumpers should be on the I/O Card..



The page I have taken a picture of is from the TVT-64 and not the TVT-32 manual, but what is says would be the same in either case..
The text says under #17 grounding pins 2-8, but it would be Grounding one pin at a time..
This would be one method of Checking out the Video Card, without the CPU in the System, in fact when doing this I would Pull the CPU and the I/O card, just in case..

THANK YOU Marty

I played around a bit more. First, I noticed in your pick the rightmost 74100 socket is empty. So took the one from the left most on mine and then put it into the rightmost. I did find one crystal, a 2.48 or something mhz, not 2.5.. but plugged it in. When I fired up, this time instead of blocks I got a random sequence of text characters. No prompt. A few more tries at powering on and off, and it went back to blocks. I rotated the 74100s around, and again got text.. this time little commas. Not sure what's causing the changes. Hoping I'm not just blowing chips as I rotate those 74100s around.

After getting blocks through 3 power ups.. I took the cpu board out and checked the 2.4mhz crystal.. i had to pull it off an old broken video board. For all I know its dead or partly dead. Anyway the legs on it were short because they had been trimmed to fit the component they were attached to. I subbed in some pins to help them make contact. Again.. without touching the io board.. i got commas. But when i shut down and powered up one more time.. back to blocks.

Feeling like the crystal is where its at.. trying to find replacements.. would something like this work: http://ca.mouser.com/Search/m_Produ...=/ha2pyFaduhECNg//ZYuDDefmBK/zVpaIKslhy/Ll7w=

Rechecked the 2.4mhz crystals connection to the socket.. and rechecked the resistor connection on the back.. made sure they were secure.. now I consistently get this screen on every power up:

A bit more playing around.

If the CPU board is in, I get the screen above. If I remove it, I get blocks. I tried changing all the RAMs out but no difference - always a random pattern.

If I yank the 1702 EPROM, I get a field of commas again. If I put it back, I get that garbage.

Now, I don't know what effect running the wrong crystal has. The crystal I've subbed in for testing is 2.4576 MHz vs the 2.500 that was in there. Is that enough of a difference to mess it up? Or is it more likely I'm hitting an EPROM problem? I would assume if the other ICs were dead, I wouldn't get as far as I have..

I could be an EPROM problem, it could be a RAM problem, it could be a data path problem. It's likely not due to having the wrong crystal.

Don't think it's RAM.. I've got TONS of working 2102s and so far cannot make this thing go.

But interestingly enough, after carefully resoldering that resistor on, now with the 'broken' 2.5mhz crystal in, that same random character pattern is up. I thought for sure the other crystal was dead, with something loose banging around in there.. but.. yeah.. now it's behaving the same as it did with the temporary 2.4. I'll keep swapping RAM and see what happens, and any other chips I have spares of. All I can do at this point I guess.

Hi All;

Falter, You have made some Good Progress, Your Picture, shows that the Video Card is either working or Mostly Working..
Remember that On the Video Card there are 8 2102's Rams chips, Which are what You see on the Screen..
On the I/O Card the 74100's need to be in at least the 2 left Sockets, but all four would be fine as well..
Using the 2.4 Crystal would work as far a it being able to show the beginning text, but it might not work for the the Cassette interface, I just don't know on that, Since it is a software interface..
Let me relook at Your other postings and I can make sure I have commented on Everything..
All For Now that You need to get the Text is the two Rows of Ram Chips on the CPU card, You could take out the other Memory Cards, until You get the CPU and the I/O and the Video Card Working..
"" If I yank the 1702 EPROM, I get a field of commas again. ""
This might Indicate that one or more of the Data Drivers is Bad, Look at what the Ascii for the comma is Octal 254 10 101 100 and the block is 377 11 111 111..
The Data Drivers are either 74367's or 8T97's, The 8T97 work better, You could swap some of those around and see if You get a different Character than a comma with the Eprom take out..

If I can get feeling better later Today, I should get out my Digital Group Chassis and see about connecting +5 to it and Powering it up and then I can (hopefully, If I can get my system to work) tell You Exactly What would be on the Screen (TV/Monitor) when the Eprom is Pulled..

THANK YOU Marty

Thanks Marty. Ill give that a try. Mine has the 8t97s.. and I have lots to test with.

What do you make of the crystal situation? I thought for sure my original 2.5mhz was toast.. but now with it back in I get the same result I had with the borrowed 2.4. If I pull it and have no crystal attached, I get blocks again.. so I'm assuming that means I have life in it..

Hi All;

Falter, Yes, for now, until proven OtherWise, let's assume that the Crystal is OK..
You, never did tell me What You have as Far as Debugging Equipment..
Do or can You build anything with Wire-wrap ?? If You have a Dual 22 pin Card, You can build a set of Led's for Showing Data and Address Status..






THANK YOU Marty

Unfortunately I don't have much.. just a digital multimeter with frequency meter. I keep meaning to buy a scope but keep buying computers instead. Maybe I'll have to give in. I'm not at all sure how to build that testing device, though it looks like it might be a fun project.

I've swapped around all the 8T97s, borrowing some from my Apple IIs. What I get now when I pull the EPROM is not commas but just the blocks again. With the EPROM back in, back to random characters. Well, actually, they look random, but I've verified it's the exact same screen every time.. ie. same characters in same positions.

I thought I had read somewhere that it was possible to disable/bypass the EPROM. But I might have read that incorrectly. I have a strong suspicion, given age and that the window on it wasn't covered for eons, that we are dealing with a corrupted EPROM here. I regret there isn't a readily available 1702a programmer out there, I'd be inclined to buy one and replace/reprogram this chip along with some Ohio Scientific stuff I need to change.

When Digital Group refers to 'EROM', is that the same thing as EPROM/1702?

Another note -- if I try to put any/all of the RAM expansion boards in, the screen reverts to all blocks again.

Hi All;

Falter, You have given me some of the Information that I needed..

Ok, on Equipment.. You should try and get a Scope, it would Help alot all around..

"" Another note -- if I try to put any/all of the RAM expansion boards in, the screen reverts to all blocks again. ""
What this means is that the Ram on the CPU Board is Set to a higher Address than '0000..
So, for Testing Purposes, it needs to be changed to '0000..
In just a few minutes, Let me find the Relevant Pages.. I will get back to You, on what You need..

Here are a couple of Pictures of the Z-80 Board, with the Jumpers for On Board Ram Set for '0000 in the First Picture and the Normal setting for the other two jumpers settings.. When You have the Ram for the on Board Ram set to '0000 the other Memory Boards need to Pulled out of the System.. Unless, You re-jumper the Memory Boards as well, But let's keep it simple for now.. It could be one or more of Your Memory Boards is not working correctly and it could be where the problem Exists, So taking them Out would make for less to worry about initially..



THANK YOU Marty

Okay I'll change the jumpers appropriately and see what happens.

Wish I had the means to test the EPROM... really suspicious of that.

Okay.. I set the jumpers to the 2k setting and voila, we have a tiny bit of action.

When I power up, I now get a different field of random characters. If I hit the Reset button, whereas before nothing happened, now the pattern changes.

The pattern is mostly / marks, with a smattering of random letters and numbers. Pressing reset replaces some slashes with letters or numbers or vice versa. Random.

Hi All;

Falter, "" Wish I had the means to test the EPROM... really suspicious of that. ""

I gave You a way, But, You didn't sound like you were that much interested..
Look at the first Picture in Post #54, that Would Tell You what is on the Data Lines.. Granted You would need the Full Front Panel, so to set the Address for the Eprom most likely, not so according to the Information in the Pictures below..
I will Look thru my Stuff and see If I still have the Full Schematic for a Front Panel.. I Looked for the Full Front Panel, and I found two partial sheets, but very light, If I took a picture, all You would see is a white sheet of Paper..
But, using the Picture from Post #54 and building it will show You what You need to know as far as the Eprom Contents with the Instructions pictured below.. That will tell You if You have a valid Eprom..



"" Okay I'll change the jumpers appropriately and see what happens. ""
This should tell You if it is the Eprom or Not the Eprom..

THANK YOU Marty