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Thread: CIT-101 terminal

  1. #1
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    Default CIT-101 terminal

    A while ago, Jack Rubin gave me a CIT-101 video monitor, with out a keyboard. I have not had any luck finding a keyboard for this monitor. In fact I'm not even sure the monitor works. After doing some reading, I have found that the CIT-101 is a copy of the DEC VT100. I have found some user manuals for the CIT-101, but no schematics. I did find the schematics for the VT100. My question here, Is the CIT-101 the exact same as the VT100? Can I use the VT100 schematic for my missing CIT-101 keyboard? Thanks Mike

  2. #2

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    Quote Originally Posted by Mike_Z View Post
    A while ago, Jack Rubin gave me a CIT-101 video monitor, with out a keyboard. I have not had any luck finding a keyboard for this monitor. In fact I'm not even sure the monitor works. After doing some reading, I have found that the CIT-101 is a copy of the DEC VT100. I have found some user manuals for the CIT-101, but no schematics. I did find the schematics for the VT100. My question here, Is the CIT-101 the exact same as the VT100? Can I use the VT100 schematic for my missing CIT-101 keyboard? Thanks Mike
    I'm no expert, but I think I remember an issue with keyboards of the VT100 and it's clones. So I don't think I'd try it without some kind of authoritive check.

    Vince

  3. #3
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    I am not an expert of CIT-101 either, but I examined the schematic of VT100 keyboard for my project last year.

    The VT100 keyboard port is a 1/4 inch stereo jack with these three lines:

    Tip: +12V
    Ring: Bidirectional serial line
    Sleeve: GND

    On the VT100 Basic Video board, the keyboard serial line (Ring) is connected to an LM393 and a 7416, via a complex RC network.

    My suggestion is to check the traces from the keyboard port on the CIT-101 main board (it also looks like a stereo jack) against VT100 Basic Video board schematic. If you find an equivalent circuit, it is likely that VT100 keyboard is compatible with CIT-101 main board, or at least electrically safe to connect.

    There may be other differences, say in UART clock speed or in the status bytes, but there is little chance that these differences cause hardware failure (please post a caution if I am wrong - I don't want to ruin anybody's unit).

    By the way looking at several photos of CIT-101 here, l found that the CIT-101 keyboard has two additional keys, CLR/HOME and REPT, at the right end of the main keyboard. They are not included in the original VT100 keyboard. I am not sure if these keys are essential for operation of CIT-101.

    Ryu

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    Ryu, I have looked at your project and tried to contact you via the comment section of the blog. I have fired up the CIT monitor and found that the 1/4" jack is as you show, has the +12, signal and ground. I've had the unit on before, but didn't notice the error message on the screen. It is rather dim, but says 'keyboard error'. I think that the monitor, while scanning the key array is looking for the last key 7FH which is always closed within a certain period of time. If that code is not received on time, I think, the keyboard error is displayed. I'm hoping that this week I can use an Arduino to send this end of array code at the 7867 baud rate to see if that will clear the monitor's keyboard error. If that works I want to try and send the setup codes to see if I can access the set up menus. I am optimistic that this may work. The fact that the keyboard error shows up means that the monitor CPU is working. I have also copied your software from your keyboard project. I plan on using an old parallel keyboard I have, rather than a USB keyboard, so I probably will not need all of the code in your project. I'm a novice at C programming, but am learning. My approach is to assume the CIT-101 is similar enough to the VT100 that I can use the VT-100 prints. We will see. I have already used an Arduino to connect my RCA parallel keyboard to my ASR33 at 110 baud. So I think now it is just a matter of making a look up table to convert the ASCII character codes to the VT100 key addresses. Thanks for the help Mike.

  5. #5
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    The VT-100 and the CIT-101 are totally different animals inside. I've got both. The keyboards are definitely *not* compatible, though I think I've swapped them by accident with no harm done.

    Your method of using a generic keyboard/key matrix and an Arduino should be fine, that's all that really goes on inside the stock keyboard. I think someone had used a Tandy Model 100 as a keyboard for one or the other at some point. A little bit of a strange solution!

    FWIW, I've found the CIT-101 to be functionally superior to the VT-100. It will handle much higher bitrates than the VT100 without dropping characters. It's not so much a clone as a completely different implementation in a very similar looking case.

  6. #6
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    We used CIT 101e terminals with our Vax. Very nice terminals that made the VT100 seem clunky. Cheaper than the VT100 also.

    The queen of the clones was the Tab 132--15" screen, rock-solid keyboard--and expensive. But I really liked it.

  7. #7
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    Last night a wrote a short Arduino program that would send the end of array signal 07FH to my CIT monitor. Reading the VT100, CIT 101 and RYU's project notes. I thought that I'd try a baud rate of 7867 one start bit, one stop bit and even parity. I made the end of scan signal to loop every one mSecond. This morning I found an old stereo 1/4" jack with three connections and wired it up to the Arduino. Connected it all up, started the program and monitor. The keyboard error message came up again. I thought that maybe once displayed the monitor had to be recycled on and off. No soap. I noticed that my 1/4" jack was not inserting all the way into the monitor. Seems the plastic case has a smaller hole that would not allow my jack to push in all the way. Apparently the old keyboard must have had a special jack. So, it is just a matter of removing the case and trying again. Well there are some goofy devices holding the upper cover onto the monitor. There is a screw slot in them that you can turn, but that doesn't loosen anything up. Back to the manuals. The CIT was no help. The VT100 shows that these plastic jobs have a pin hole in them that you pry up. My luck with prying plastic nearly always results in broken pieces, but this time all four of these goofy tabs, popped up and the upper cover came off. Quite dirty inside, maybe I'll clean it up later. But with the upper cover off it was easy to see that the jack was not seating, by about 1/4". The plug socket was removed which allowed the jack to fit properly. Fired it all up again and this time the 'keyboard error' message was gone and a rectangular cursor replaced it. Hot Dog!

    So maybe tomorrow I can try the setup commands, or maybe try the brightness controls. Seems there are no knobs on this unit, all the functions are done via software. The CRT display is very dim. In order to see the screen the lights had to be turned off. I'm a little concerned that the CRT tube is on it's last legs. Mike

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    It's monochrome: you can increase the brightness one way or another. But hopefully you figure out how to do it by software first.

    Either way, the only thing I'd worry about with the tube is if it has burn-in.

  9. #9
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    Even then, they can often be rejuvenated -- Sark/RetroHacker_ has done that for several of my CRT-containing devices when the option was that or replace the tube anyway. You can't really reverse burn-in, but you can get enough brightness out of the CRT that it's not really an issue.

  10. #10
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    Great results Mike! So CIT-101 keyboard protocol has some part in common with VT100 keyboard protocol. On the other hand, as glitch said these two keyboards are not compatible. Iím interested in how they are different.

    I found this thread, dmemphis worked on restoration of his CIT-101. He uploaded several photos of the CIT-101 main board. Yes, itís totally different from VT100. I can see the CPU 8085 and some 16k bit DRAMs. Keyboard port is routed to the edge connector toward the front. There is an 8251 UART near the edge connector, I canít find LM393 comparator IC though. A few transistors on the board could make up the same function, but I am not sure.

    Mike, as discussed on the PM, I will send you my adapter shield anyway. If you are going to try it, I would recommend to make sure that the keyboard signal line of CIT-101 is tolerant to the 12V signal from VT100 keyboard. Ryu

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