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Datamedia DT80, and Telex 078 restoration

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Jan 9, 2022
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Alright folks. I have here a Datamedia DT80 that I found in *near* a creek; an area that sees frequent flooding. With it I also found some other stuff such as some 8" floppy drive, one still having it's disk in it, a Telex 078 that I also can't find much info on, and remnants of a Tandy and it's keyboard which was still intact. I saved as much as I could, but this stuff has been there for so long that it's pretty rotted out. IC chips literally falling off the 8" drives.

The DT80 and Telex 078 are in surprisingly "good" condition, but only compared to the other stuff I found as they were both full of mud and silt and corroded to all hell. I've done the bulk cleaning of both of them and they are looking pretty good. I started a yt series on them if you want to follow along.


Now my question to you all; Does anyone have any info on these computer terminals? Detailed pictures of the main board, ROM dumps, schematics? I'm currently working on the DT80, which I've already repaired the power supply and got the CRT board *mostly* working as seen here in this image album:
Any help is appreciated!
 
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Jan 9, 2022
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I've got more of the circuit working.


I found the two missing crystals are somewhere around 16mhz and the other is around 32mhz. I wrote my own ROM that allows you to read/write to memory and IO and it should be compatible with almost any 8085 based systems. https://github.com/RingingResonance/8085debugger

At the moment, I'm trying to debug a problem with the video side of the circuit. It only ever wants to show one line of text.

Here is some pictures and other info I have gathered. https://github.com/RingingResonance/DT80-Stuff
 
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Jan 9, 2022
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I've managed to correct the character ROM using gimp by putting a *.PBM header at the beginning of the file. Still haven't found any original copies of the firmware ROMs so I'm SOL there unless I can find someone else who has one of these terminals. Therefore, my goal is to reverse engineer this thing and write my own firmware, and maybe turn it into something a little more useful than a museum piece such as a fully functioning micro-computer, or just a clone of a clone of a VT-100.


Text is much clearer now, but I still haven't figured out how to make it show more than one line of text at a time, and my current way of getting it to show any text is hacky at best as I did it through trial and error and took notes on the steps to get there.

It has two registers that can be loaded with a start address, and appears to have some kind of line termination. One of the registers sets can be modified by CPU IO, and the other set appears to also be the same way, but accessible from a wide range of IO address (0x10 - 0x17 for the first one, and 0x18 - 0x1F for the second one) which doesn't make sense. I'm thinking that the second one is loaded with data during DMA via the video side when it reaches the beginning or the end of a line of text similar to how the VT100 works. This is a VT100 clone after all. It has all the same features + a bit more, but it seems to do it all in a different way as to avoid lawsuits.

Now, the second register getting it's data from a DMA request would make sense, except I haven't found any way for the video side to write to it! No RD or WR lines going to the CPU side of the circuit so it's not accessing them through those. There appears to be absolutely no way for those registers to be written to by anything except the CPU. That would be fine if the CPU would periodically get IRQs to update them, but the CPU only gets IRQs from vertical sync, and the RS232 ports.

One thing I did find was a weird circuit up at the top right of my pictures. I'm going to tall it the "terminator" circuit as that's what it appears to be for though I haven't confirmed that yet. I reverse engineered that circuit with a friend, and we determined it's looking for specific ranges of characters being loaded into the char latches. Still not sure what it does, BUT it's close to the same ranges as the second start register IO address ranges. Weird.

Truth of that circuit is as such:

Hex 0x10 - 0x13, and 0x18 - 0x1F, and then again at 0x90 - 0x93, 0x98 - 0x9F

X00100XX = 1
X00110XX = 1
X00111XX = 1

...and the address ranges for the two 8 bit registers are 0x10 - 0x17, and 0x18 - 0x1F

Another thing I found was that it uses two character spaces for each character, one above the other when displayed, in order to do underlining and for letters such as 'g' where the lower part goes lower than the rest of the text. This sorta coincides with the upper address ranges of the "terminator" circuit from above (0x90 - 0x93, and 0x98 - 0x9F) as it seems to use the last bit of the char ROM to access this lower part of the characters, and the terminator circuit ignores the last bit all together. Then again, it all could be a coincidence that the "terminator" circuit and the second start registers have similar ranges.
 
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I wrote a little program in C this afternoon that attempts to 'un-stick' a bad bit in a data file such as from a failing ROM. So far it unfortunately hasn't proven useful to me like I hoped it would, but maybe it will be useful to someone else. I did manage to make it correct some deliberatly corrupted text, but other than that there isn't enough information to make a proper correction even through brute forcing less using pattern recognition. I might try something like that later.

 

Al Kossow

Documentation Wizard
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It's a shame you lost out on the keyboard that was on ebay to some keydingo

Would be nice if people would keep an eye out for another DT80 to try to get you the firmware.
 
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It's a shame you lost out on the keyboard that was on ebay to some keydingo

Would be nice if people would keep an eye out for another DT80 to try to get you the firmware.
Thanks for letting me know about it, even if I wasn't able to get it.

I would have outbid them if I had the budget atm, but part of the problem was the seller had set a reserve on that item. The first time he had it posted I was the highest bidder, but reserve wasn't met. Oh well, I'm sure another one will pop up at some point or another. I'm pretty sure I could also use a VT100 keyboard as according to Dave's Old computers it's such a well made clone that the operator's manuals are nearly interchangeable for most features so maybe the keyboards are as well. Keyboard is important though, it houses the speaker, LEDs, and the NVmem used to store settings. Interesting choice that they would put the NVmem in the keyboard as it means if you need to move to another workstation you could just unplug the keyboard and move it instead of the entire terminal.

I also contacted the seller and asked them if they would reach out to the winner and ask if they could post their roms on the off chance that they actually have a DT80 and aren't just some collector or dumb dumb looking for that *unique mechanical keyboard* that they probably wont even be able to use unless they build a converter for it, but I'm not holding my breath on it.

ATM I'm in the process of debugging the serial ports. They're pretty simple and generic, but nearly all the chips in that section are bad, so it makes me think that in it's previous life it may have been hit by lightning and that's why it was decommissioned and ended up where it was once it was time to clean house. Could be why all the RAM and ROMs are bad or corrupted as well. Oddly enough, the only parts on the CRT board that were bad are the parts that were weathered. Makes me think that being buried in a flood zone is much more survivable than previously thought for most CMOS and TTL chips. Maybe it was sold off and then the buyer couldn't get it to work and didn't want to pay for disposal fees and that's why it ended up in an illegal landfill. Forensics is fun!

Once I have those working, I can tell my debugger program to use them instead of the 8085's software serial ports. That way the video circuit's DMA won't interfere with the timing and I can do a lot more brute fore trial and error to figure out more about the video circuit.

Anyways, I've got flight classes coming up this summer so I may not be able to put as much time into this and is also the reason for the strict budget.
 
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Dave reached out to me and says he found the DT80 he thought he cleared out years ago. Unfortunately he lives in Ontario Canada and I live all the way in Texas. I offered to pay for shipping but I'm also on a strict budget at the moment so it's not a high priority for me. If anyone else is interested feel free to make an offer and grab it from him as I'm probably not going to be able to afford that kind of shipping + compensation for something large, heavy, and delicate. I'm not a fan of shipping CRT's anyways if it can be avoided. You can find his contact info on his website. https://dunfield.themindfactory.com/dave.htm
 
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Oh that's awesome! I had just texted a friend of mine about it as well as I was sure no one here would want it. I guess I was wrong. I let him know that you might want it for your museum, and I'll go ahead and email Dave that you are interested in it as he has a strict spam filter that might catch your email.
 

Al Kossow

Documentation Wizard
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From Dave..
Is there anyone that can help out with this?
" I thought I might have sent it to electronic recycling (I can easily do that by dropping it at a shop in town), but I just found it under a pile of stuff in a corner of my "discard" room - so I let Jarred know.
It's located near Ottawa, Ontario Canada, and I will happily give it to anyone that wants to "pick it up" but I just don't have the time or energy to ship it across the country."
 
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From Dave..
Is there anyone that can help out with this?
" I thought I might have sent it to electronic recycling (I can easily do that by dropping it at a shop in town), but I just found it under a pile of stuff in a corner of my "discard" room - so I let Jarred know.
It's located near Ottawa, Ontario Canada, and I will happily give it to anyone that wants to "pick it up" but I just don't have the time or energy to ship it across the country."
Well, it's a bit risky damage wise, but he could drop it off at the local UPS/Fedex and have them box/crate it up. Whoever wants it can pay shipping which can be pre-calculated by approximating the size of the box and it's weight (which I have, it's about 20lbs). If he can't do even just that then IDK what to say other than someone local coming to pick it up.
 
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For reasons I don't want to expound on, he needs someone to pick it up from him.
I don't blame him, especially if he is in retirement and simply doesn't want to mess with it. I'm still asking around to see if anyone I know lives in Ontario and is willing to pick it up.
 

mfriethm

New Member
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Jul 10, 2014
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As mentioned in the post on Reddit about this, I would be willing to pick it up and dump the ROMs, if no one else steps up. It’s a 9 hr round trip from where I live in upstate NY. It’s easiest for me, schedule-wise, if I do it in mid-July. Dave seems OK with that. Where it goes after dumping ROMs doesn’t matter much to me, other than I don’t want to see it destroyed.
 
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UPDATE: Dave emailed me again this morning and said he has someone who is coming to pickup some equipment including the terminal in a few days. He says he will happily dump the ROMs for us. If that falls through then we can resort back to our previous plan. Thanks again for all of your help.
 
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