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Interfacing a Calculator Printer

Discussion in 'Microcontrollers' started by BobW, Apr 5, 2017.

  1. BobW

    BobW Active Member

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    I have the printer unit from an old desktop printing calculator that I would like to build an interface to. It's made by NMB, and the model number could possibly be 212-1550FG or 279486 (these numbers appear on the chassis), but I can't find any info on the Internet.

    This one is an 18 column printer, numbers and math symbols only. It doesn't appear to have any interface electronics, just a 27 conductor ribbon cable for connection back to the main calculator PCB.
    It uses a stack of 18 print wheels, one motor, lots of gears, a couple of shaft position sensors, a few trip levers and solenoids for tripping paper advance and print head, plus (I assume) 18 solenoids (hidden inside) for tripping the print wheels. I think I understand the logic of how it works, but not the voltages involved.

    I was wondering if anyone has any experience with these things.

    Edit:
    I still have the original calculator, but it's not working. However, after poking at it for a while, I managed to get it to power the printer, and I took some voltage readings. It appears that everything operates on 15 volts. I can probably work out the rest now.

    BTW, the CPU is a NEC D1251G 'calculator on a chip' circa early 1980's. Can't find any info on it. I suspect it's one of those old MOS devices that requires several voltages.
     
    Last edited: Apr 5, 2017
  2. ci139

    ci139 Active Member

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    a local technical library may have some documents with hints on such
    :eek: i likely visited such over a decade ago . . . the good thing about was that i often was the only person there . . . about tracking your source by authors notes on used sources and the used literature lists at the end . . . took some days/visits but was still faster than to read the repetitious "no information" returned by innumerable web searches
    :meh:
     
  3. atferrari

    atferrari Well-Known Member

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    Google Calcuseum

    Is the brand NMB?
     
  4. dave

    Dave New Member

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  5. BobW

    BobW Active Member

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    Yes the printer brand is NMB. The calculator is a Toshiba BC-1232PV.
    I had checked Calcuseum in the past without any luck, but just now I tried doing a Google site search of Calcuseum for "NMB" and got two hits. Both were for similar Olympia models CPD-5212S and CPD-5214. They give circuit board layouts for these calculators, and they appear to be identical to mine. It doesn't provide much help though, because there's no information on the printer part except that it operates at 16 volts. I'd assumed 15 volts based on my voltage measurements.

    I think I've got the printer figured out anyway. The only difficulty was a small circuit board that contains a transistor and a few resistors. I could only see the component side, and wasn't able to figure out the traces. Then I got the idea to poke a bright LED inside the housing which lit up the PCB from the back, and then I could see the foil pattern from the component side. I've now traced out the circuit. So, it's now just a matter of building an interface and doing some timing tests. This is on the back burner for now, because I don't have any practical use for it, and other projects have taken my interest.

    For anyone who's interested, here's how it works. There's a single motor connected to a complicated set of gears, which rotates the print wheel drum a full revolution and then presses it into contact with the paper, then advances the paper by one line. It has two position encoders. The first is a simple magnet and reed switch that pulses once when the mechanism has completed one full print cycle (one line printed). The second encoder is an optical one mounted on the print wheel drum. This sends pulses back to the controller that tell which character is in position. The controller then operates the print wheel solenoids for each print column, triggering the solenoid for each print column when the required character is in position. The devil is in the details of course. Getting it to work is still going to take a lot of tinkering.
     
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  6. atferrari

    atferrari Well-Known Member

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    Your IC shows up under NEC brand but no pinout.

    Interesting project. Ribbon tape?
     
  7. ci139

    ci139 Active Member

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    Last edited: May 28, 2017
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  8. BobW

    BobW Active Member

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    Actually, on this page: http://www.calcuseum.com/ED/desk_46966.html if you scroll down there is a circuit board layout that appears identical to mine, and there is a schematic that shows the pin functions of the NEC D1251 chip, though it's a bit fuzzy and hard to read some of the text.

    The schematic is useful because it gives the pinout of the the printer connector, which mostly confirms what I already traced, but it's a good back check.
     
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  9. atferrari

    atferrari Well-Known Member

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    Just out of curiosity, could you post a detail of the pins with a hint of what function each one is allocated to?

    Ones and zeros, aren't all the same voltages in all pins? Is the micro's Vcc = +5V?

    If you are busy please do not waste your time in replying. I am being just curious.
     
  10. BobW

    BobW Active Member

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    If you mean the pin functions of the NEC D1251, then the only information I have is what I found on the schematic at the above link. Looking at the schematic, you can get a general idea of what many of the pins are for.

    As far as I can tell, all power supply voltages are negative with respect to ground. I think Vcc is around -12 and Vdd may be -5. Note that both Vcc and Vdd are connected to the anodes of the rectifier diodes. Since Vdd is tapped part way down the tranformer winding, I assume it's lower voltage than Vcc. This is one of those early MOS chips that needed strange power supply voltages—horrible things that are best forgotten.

    If you were wondering about the pinout of the printer, I can post that, no problem.
     
  11. atferrari

    atferrari Well-Known Member

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    I consider myself lucky; when I was born to electronics they were history. Hard for me to grasp the idea of the common hanging from (or sitting on?) the +V.

    Yes, please do. Gracias.
     

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