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Trying to understand serial to parallel circuit

opalko

Hi again folks (after some time away!),
When I was a teen, I had an Atari 800 computer/ The Atari did not have a parallel port interface.  All they had were the joystick ports, which were serial.  A clever company came up with a serial->parallel port interface.  Me, not knowing much about electronics, could not figure out how they did this.  (PS -I still don't know much about electronics!!!).

Flash forward 40 years and this past week I decided to take apart the serial->parallel interface and reverse engineer this thing! I took apart and there are 2 chips - a 74LS14, and a 74LS164, a resistor and a capacitor. Next step was to draw the schematic, which I did with pencil and paper. This became messy, so I resorted to learning to use KiCad, and came up with the attached drawing. (see attachments)

I get what makes this work is the 74LS164 8 bit parallel-out serial shift register. I still don't really understand how it works though.  As I understand it, the data on this register may be read out in parallel (from all flip-flops) at once.
So some questions!

0) Can someone explain how this whole interface works!?

1) How is the data being read on the printer side?  Is the printer polling every data pin on the parallel port at once regardless of how many are actually being used?

2) Since there are really only 2 address data pins (3 & 5) being used here, it doesn't seem a vast improvement over serial?  But I guess because the Atari only has pins 1-4 for I/O and 1 of those sends the data to the 74LS164?

Thanks for any help understanding this.
Robert Opalko

PS There doesn't seem to be nearly as many members on this forum as there was in the past... Where did everyone go?

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  • 0

    I don't get it either. A Centronics printer needs 8 data bits and some handshaking to work - this doesn't even have enough pins connected.

    On a Centronics connector all pins from 19-30 and 33 are ground, so connecting some of the output pins of the 74164 to ground doesn't seem like the best way to establish a ground connection.

    The 74164 is a shift register so it can shift data into its parallel register on every rising edge of the clock, but why they only connect 2 of the parallel outputs to data pins on the Centronics connector is not clear to me.

    The way this is connected doesn't seem to be workable on many different fronts.

    I don't know what Atari uses for their port interface, but it is designated to have +5, ground and 6 input pins. This schematic seems to imply that these inputs are being used as outputs.

    Best guess is that the schematic is not drawn correctly. 

  • 0 in reply to dougw

    I am pretty sure I drew it correctly but will double check. I will post a picture of the board itself in a bit. The traces are pretty easy to see through an opaque board. Thanks for the reply Doug!

  • 0 in reply to opalko

    The photo isn't the best but I can count 12 traces to the PCB connector. Your schematic shows only 8 at the Centronics connector.
    How are you checking this ? with a continuity meter test or by eye ? 

  • 0 in reply to beacon_dave

    Good eye!

    I think what happened was I was drawing the circuit with pen and paper and it quickly got too messy. So Iswitched to KiCad and was using my drawing as the reference instead of the actual circuit....dumb dumb dumb mistake.  I haven't had a chance to redraw the schematic but here is a photo of said board, maybe the traces are easier to see here. I will try to redraw the schematic later tonight. The top IC is the 74LS164 and the bottom is the 74LS14.  I have the image mirrored so that the top left pin in the photo of each IC is pin #1, if that makes sense.

    image

  • 0 in reply to opalko

    The 8 data lines are there and mapping to the correct printer pins. 

    SR Qa pin 3 - ribbon 3 - Centronics 2 - data 0
    SR Qb pin 4 - ribbon 5 - Centronics 3 - data 1
    SR Qc pin 5 - ribbon 7 - Centronics 4 - data 2
    SR Qd pin 6 - ribbon 9 - Centronics 5 - data 3
    SR Qe pin 10 - ribbon 11 - Centronics 6 - data 4
    SR Qf pin 11 - ribbon 13 - Centronics 7 - data 5
    SR Qg pin 12 - ribbon 15 - Centronics 8 - data 6
    SR Qh pin 13 - ribbon 17 - Centronics 9 - data 7







  • 0 in reply to beacon_dave

    Awesome! Apologies for screwing up the schematic and causing confusion earlier.

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