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Teletype interface

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    Teletype interface

    Hello all!

    I repair a minicomputer and have some problems with teletype. I want to believe, that you'll help me.
    Unfortunately, English is not mine native language. Excuse me for my errors, please.

    Teletype was the main interface of this computer, and, unfortunately, the TTY was thrown away. I know only, that it was "an ancient American TTY with tape punch" - probably, ASR-33.
    I want to construct a PC-emulator of teletype. Probably, after that I'll try to use some soviet teletype.
    I already found out, that teletype interface is similar with COM - but use current loop instead voltage control. There is some scheme of converter in attachment to this message. Software emulator of teletype is included in standard Windows hypertrm.exe .

    But I have some questions:

    1). Are all ASCII-teletypes have the same interface? I mean, could I use this scheme for my unknown teletype controller?

    2). I could attach either TTY controller or teletype to the system "converter-COM-emulator", couldn't I?

    3). There are 5-line TTY on this scheme - but my TTY controller has only 4 lines (3+gnd). Which line is unnecessary on this scheme?

    4). Author told in description of this scheme: some additional diodes should be there. Where should they be?

    Thanks for replies!
    Attached Files
    Last edited by Arif-Ru; December 24, 2010, 01:27 PM.

    Wow, I'd love to see the computer you are working with that needs a teletype to interface to it. I was under the impression that anythign a teletype could talk to a serial port equpiied computer could, too.
    The ancients knew *more* than us... They're baaaack!

    Come and see for all your gaming needs! We have Zero-ohm resistors!


      It doesn't sound like a current loop. Some tty connections
      had RS232 adapters in the teletype. You might look at the
      circuits in the computer to see if it was a current loop
      or an RS232. Current loops would usually have optical
      isolators or at least just a transistor. RS232 will have
      a RS232 level shifter chip.


        1 - Not all, but most do use a current-loop interface.

        2 - I don't understand the question.

        3 - Actually, I think three lines are unnecessary; ground, DTR and RTS are for controlling non-TTY devices.

        4 - The diodes he mentions are in fact shown on the schematic as far as I can see.

        But the circuit seems unnecessarily complicated for what you want to do; have a look at this one:

        And you might want to trace back from the current loop interface; it may in fact be a converter and perhaps there is already a 'standard' RS232 interface in the computer 'on the other side' so to speak (although it may need voltage conversion).
        Last edited by MikeS; December 24, 2010, 09:26 PM.


          Well, now it is only a wardrobe with many blinking lamps. But I promise - I'll make a little photosession, after the end of repairing, .
          Yes, this computer has many interface cards, but after startup it receive commands exactly from TTY controller. By the documentation and memories of users, at least.

          Dwight Elvey
          Thanks! I'll check it by the next session (I'll go to this computer in Tuesday).
          And yes, you gladden me, .

          Thanks for the long answer.

          2). I mean:
          I want emulate TTY on PC - connect PC to TTY controller.
          I want emulate TTY controller on PC - connect PC to TTY (when I'll find one).
          Should I use the same scheme?

          3). So, TTY is linked to computer only by one line? Thanks! In fact, most of schemes told the same - but I thought, they are for 5-bit TTYs.

          4). Thanks, I see the same, but was not sure.

          (5). - Yes, it'll be the most easy way - I'll try to do it.
          Last edited by Arif-Ru; December 25, 2010, 02:44 AM.


            But the circuit seems unnecessarily complicated for what you want to do; have a look at this one:
            Exactly four lines are used in this scheme, by the way.


              Please note: many (most?) serial terminals (what you call "TTY") and the computers that use them are set up to use full duplex communications. This means; one line that transmits data from terminal to computer, and one other line that transmits data from computer to terminal (not including handshaking etc). I write this because one of the suggestions you got was for a rs232-to-current loop circuit which was half duplex.
              Last edited by tingo; December 28, 2010, 12:45 PM. Reason: added the bit about handshaking


                I'll keep it in my mind.


                  Well, the situation is changing slowly.

                  1). I have copied distal part of the teletype's controller scheme:

                  2). Voltages after powering on the processor:
                  Pin 1 to GND: +2V
                  Pin 2 to GND: -20V
                  Pin 3 to GND: +4V

                  3). No impulses have been seen after powering on or starting processor (tested by oscillograph).

                  4). I get ASR-33 teletype, but it is in very bad condition - all knobs have been cut off, some wires have been cut too.
                  - All knobs have been cut off.
                  - Wires from the reader have been cut.
                  - Two wires from main unit have been cut. But on the other ASR-33 this wires absent too:
                  I mean two unsoldered contacts above the distributor, in front of yellow mark "Caution".
                  - Upper cover does not exist too.
                  - And I can not find on photos and schemes many strange grey wires.

                  I have started the motor, but trip mechanisms do not work. Distributor and all cluthes rotate permanently. Printing mechanism does not reply to keypresses from keyboard.

                  So I have two questions:
                  1). How could I connect teletype and this controller?
                  2). Why could teletype work like it works? Is it result of cut wires or something else?

                  Thank you for answers!

                  The minicomputer:


                    Originally posted by Arif-Ru View Post
                    The minicomputer:
                    Looks like a french Intertechnique Multi 20 minicomputer, from the early 70's. It could carry RAM from 4K up to 64K and do computation in 8 bits up to 20 bits. It seems it was used for medical applications, and to control radiological machinary...

                    This is an earlier version Intertechnique Multi 8, in working order:


                    There are pointers to hardware manuals in that link.


                      Most excellent machine you have running there, love that it is still in service!

                      Also it sounds like your teletype was stripped for parts, at least partially, being that the knobs and wiring have been damaged and/or missing. Is there a different unit available, or a terminal?

                      Can a term emulator on a PC be used here?? Seeing this beautiful machine makes me want to travel over and get to work on it
                      The ancients knew *more* than us...
             They're baaaack!

                      Come and see for all your gaming needs! We have Zero-ohm resistors!


                        Yes! This is Multi-20-based "Plurimat S" from ~1978.
                        I have not succeeded to find anything about this computer (so I was sure, that computer is very unusual, and nobody knows about it) until I got your link.
                        Moreover, I found, that this computer is similar to MicroData 1600.
                        And MicroData 1600 is well documented:
                        So your comment was very helpful. Thank you!

                        Well, I do not know about any other ASR-33 in Russia. But, of course, it does not mean nothing.
                        Items, most similar to TTY:
                        Typewriter with extension port:
                        A console:
                        And PC with RS-232, of course.

                        Now I know pinout of TTY controller, so I'll try to make a PC converter.
                        Last edited by Arif-Ru; January 14, 2011, 04:25 AM.


                          By the way, what is "firmware"? Is it ROMs images?
                          I could not find any information about this page.

                          It looks like the computer could not start without some bootable disc or punched tape. So I am searching bootable file...


                            Like most computer terms "firmware" can have different interpretations and be in different forms but it is the level between the hardware, i.e. the transistors, chips, wiring, etc. that are permanent and can't easily be changed, and software (including the OS if any) which is transitory and changes constantly; not 'hard', not 'soft' either, but in between, i.e. 'firm'. And yes, although many older systems loaded it the same way as software, i.e. via paper tape or special diskettes, in most modern systems it is indeed stored in non-volatile memory like ROMs or EEPROMs.


                              My question was formulated poorly.
                              I mean:
                              What is "firmware" in case of Multi-20? Could this be the images of bootable punched tapes?