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Datamedia DT80, and Telex 078 restoration

any luck getting the firmware?
also, is there any chance the dt80 manual could be loaned to me to do a proper scan?
 
I got the tracking number for the ROMs. They will be here some time next week. They gotta go through customs. I just hope that if they do x-ray the package it doesn't damage the data on them.

Yes I can loan the manual to you. It's in mint condition. PM a mailing address.
 
Update: I got the ROMs in the mail. Here is my best attempts at reading them.


ROM1 didn't want to play nice at first, but I finally got a consistent read after some cleaning and using another ZIF socket between my TL-866 and the ROM.

I'm trying them out in my DT80 but it doesn't seem to work. For all I know it could be that the data was corrupted during shipping/reading, or there is a hardware difference I don't know about. But at least there is a copy of them now.

I also don't know if it's because I'm missing a keyboard and it just refuses to 'boot' without one. So that could be my problem.

After comparing my original ROMs to the 'good' ones, I found that my original ROM2 matches, but none of the others match except for ROM1 but only a byte here and there. Interesting failure mode. At least I'm almost certain that I have a good read now.
 
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Found another keyboard on ebay that might work. Don't know if I'll bid on it yet. Money is really tight right now and I can't justify something that won't immediately make a ROI.

 
I understand it's for a VT-100, but based on the VT-100's service manual, and the way the circuit looks on the DT80, there is a chance they are electrically compatible. It's also the cheapest VT-100 keyboard I have ever seen, and it looks to use the same or similar keys to the early TRS-80's which I have a spare and failing keyboard of one of those I might try using keys from.

I also have this kit someone sent me that emulates a VT-100 keyboard using some simple circuitry and a keyboard. I might assemble it and try it on the DT80. I just don't have all the parts needed as of yet.
 
A recent update. Someone reached out to me on HAD mailed me ALL of the schematics to the DT80! This inspired me to try and get the original firmware working again rather than just re-writing my own which I had gotten pretty far on. After about an hour of video recording clips and minor trouble shooting, I managed to find that two chips I had previously replaced weren't up to spec for the DT80's high-speed (25mhz) 132char mode clock crystal. After replacing those two chips, 132 char mode started working and the original firmware was able to display an error code. Code '6' to be exact which means two things by what the manual says. Keyboard error, because I still don't have a keyboard for it yet, and NVM error. So it's working! I just need to get a keyboard solution now. I also ordered some new RS-232 level shifter chips to replace the ones that had been zapped. Apparently they are still being manufactured.

I ordered some parts for the VT100 keyboard adapter. Maybe it will work, or at least I can modify it as it appears to be electrically similar to the DT80, which I can now confirm whether or not that's true because I have full schematics now along with theory of operations for all the circuits!
 
The DT80 and VT100 keyboard circuits are so close in design that I would be surprised if the VT100 keyboard didn't work on the DT80, though not surprised at all if the DT80's keyboard didn't work on the VT100. Both of their send/receive circuits are different, but perform the same function at approximately the same voltages and appear to be electrically compatible. However!...

The VT100 derives it's keyboard clock from the horizontal circuits and passes it through the data/clock line for the keyboard and the keyboard separates the two signals. It's UART is designed to be able to do that. It's data rate is approximately 7867 baud based on the keyboard emulator's code that I'm working with. I think this uses a TR1865 UART based on a few things on the internet, but I have yet to find full schematics of the VT-100 for some reason. I've only found a few partial schematics and block diagrams.

The DT80's keyboard uses an internal clock of 7.3728mhz, the same that's used in the terminal for all of it's UARTs, and it's communication is asynchronous. This clock get divided down to 92.16khz before being sent to the HD-6402 UART where it is further divided by 16 for an exact 5760 baud rate (or is it half this? The terminal side appears to have this clock further divided by two?). The HD-6402 UART is used on both sides of the keyboard interface with almost mirrored circuits. The HD-6402 states that it is compatible with nearly all industry standard UARTs, and looks to be pin compatible with the TR1865.

This makes me wonder with the VT100's keyboard would be happy with the signals from a DT80, or if the engineers of the DT80 wanted to design it close enough that with a few simple mods to the DT80 or VT100 keyboard that it could use a VT100 keyboard. Maybe they liked the design so much they decided to mimic it but without copyright infringement, and also wanted to make sure that neither keyboard or terminal would be damaged if one were plugged into the other.

I'm going to continue experimenting with this keyboard adapter/emulator and see if I can't modify it to work. So far it looks like all I need to do is omit a capacitor, and change some code.


As for schematics and additional documentation, I plan to scan and upload it all as soon as I can.
 
ah, thank you! I should have guessed to look there since I had been working with Al on preserving documentation of this stuff.

I did some more tests and I think my HD-6402 is bad. It doesn't appear to be sending any data on it's output even though it's getting a clock signal and RD/WR signals. Apparently they still make these chips, but digikey only sells in bulk so I will have to order elsewhere. I found a pair on ebay that looks decent. I will give those a try.
 
I got the emulator to work (without it's USB host adapter, I disabled that code for now until one arrives in the mail). Here's what I had to do.

First off, after some probing, I had accidentally grounded the signal line in my connector during assembly so I wasn't getting any data through. After I fixed this and installed the new HD-6402 UART chip, the adapter beeped happily as the DT80 went through it's POST, but still resulted in error '6' (keyboard AND NVM error) on the screen. I went ahead and swapped the original UART chip back in and the adapter LEDs acted erratic or wouldn't do anything at all after resetting a few times. Dead HD-6402 UART confirmed. So on I went with looking at the code and schematics for the emulator/adapter and keyboards.

I studied the DT80's keyboard schematic and found that it's UART is configured slightly differently from the VT-100's. This confirms that neither keyboard will work for either terminal without modifying one of them first, though the modifications wouldn't be very complicated. The DT80's keyboard sends back data after every receive, while the VT-100's only does a scan and data send back when it receives an enable bit (0x40). The DT80's keyboard UART is also configured to use ODD parity. After changing both of these things in the emulator's/adapter's firmware, the DT80 was satisfied with the result and displayed error '2' which is the error code for NVM and the emulator/adapter would go through the DT80's LED test and beep happily once more.

Now to see if I actually get text when I type with a keyboard once I get the USB host adapter in the mail. Hopefully after setting the terminals settings, the error '2' will also go away, but I half expect the NVM circuits to be damaged in some way with how this has been going. If anything I aught to be able to retrofit this circuit with a single PIC or something.
 
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I've gotten the USB host adapter for the arduino in the mail and got it all working. I had to do some more tinkering with the code to make the DT80 happy with input. One of the things I had to change was the key mapping. It's completely different! Afterwards though, I got everything except CapsLock to function.

The NVM in the DT80 worked at first after I repaired it's programming circuit that takes 48v input, but then it got flakey before failing entirely. On removing the NVM chip, it crumbled into a few pieces. So new chip went in and now saving settings works reliably.

I've had to do a few more repairs on it lately such as replacing a couple more resistors and diodes on the video board that were getting flakey. Thermal cycling on this thing is reaping havoc on some of these compromised parts! The 5V section of the power supply had to be rebuilt because the single buck converter I used began overheating and it's voltage would drop. I replaced it with two of them and wired the power supply as it originally was with two separate 5V busses. The 15V supply is still noisy after adding additional filter capacitors so the screen is still a little unstable, but there isn't much I can do about that without going back to a linear regulator which is something I want to do later but it's not a high priority.

Other than that, it's working somewhat reliably as a terminal now. Fun thing is I can plug in a wireless keyboard to it and use it's video out on an AUX monitor across the room. :D

Still though, wish I had an original keyboard. Hey Al, I can re-staple the binding of the manual if you send it back and it's not too damaged. If not, you got a really good scan of it so I'm not heart broken. The copy of schematics I aquired came with a copy of the operators manual, though it's not the best copy.
 
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