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Simple circuit for RS232 > 12 bit DAC?

Discussion in 'Electronic Basics' started by BR, Oct 3, 2003.

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  1. BR

    BR Guest

    Any schematics of this? I want to control only one motor from a PC. What I
    need is high resolution control of velocity. Position isn't necessary. The DAC
    will connect to the control voltage input of an H-bridge that has a built-in
    PWM generator. The motor is a small Kollmorgen Servodisc.

    Ben

    --
     
  2. Bob Masta

    Bob Masta Guest

    How about the parallel port instead? If you are running under
    DOS (so you can write directly to the port) you can use a simple
    R-2R ladder. You might need to buffer the digital outputs on
    some ports if their levels aren't uniform on all bits. If you only
    use the data out port you just have 8 bits, but you can also
    use the control port to get another 4 if you're careful. (The
    control port bits will almost certainly need buffering since they
    have pull-ups.)

    For the 8-bit version you can check out the LPTX driver
    Help files for my Daqarta system at
    www.daqarta.com/lptx.htm
    This discusses using the port for analog in and digital out
    as well as analog out, so read through for the DAC section.

    Hope this helps!




    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  3. John Fields

    John Fields Guest

     
  4. BR

    BR Guest

    Thanks Bob. I later realized how slow the serial port is. I forgot to check
    this thread and started another. Looked around and found a 12 bit DAC such as
    the MAX508. Should make it easier (I think) to get going since its interface
    is a byte wide (8+4). But for starters it'll be an 8 bit DAC or a R-2R ladder
    as the experiment may work with that resolution.

    A control port will be used for the foward/reverse ports on the H-bridge.

    Ben

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  5. BR

    BR Guest

    John, I made the mistake of considering RS232 as it's too slow. It'll be via
    parallel port. What is an all-hardware solution? I'm using Quickbasic on an
    old 133 machine. Just a bunch of loops to change velocity and will need to
    edit those values frequently. If the experiment (fluid study) shows a result,
    the program can be run again to repeat the result. It would be ideal to
    manually control motor speed with a pot and ADC (looking for certain fluid
    activity) while writing control data to disk, and then later playback the
    data. But this would be a little ahead of myself at this stage. Bet Quickbasic
    sucks for that.

    Ben

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  6. John Fields

    John Fields Guest

    ---
    At 9600 bits per second you can accumulate 12 bits in 2.083ms, including
    the overhead of the RS-232 start and stop bits and the wasted four bits
    of data, so that's doesn't seem too shabby for a motor control
    application!-)
    ---
    For RS-232, basically a comparator or something like Jim Thompson's
    MC1489,(just thought I'd mention it to save him a post!^) or one of the
    zillion chips Maxim's got to change the bipolar RS-232 signal into a 0
    to 5V logic signal, two 8-bit shift registers, a clock, some glue logic
    and your DAC or R-2R ladder.

    For the parallel port, it's a couple of HC373's or 375's and your DAC
    or R-2R ladder and probably a little glue logic if you can't control the
    registers with the port's control pins.
    ---
     
  7. BR

    BR Guest

    It depends on what you want to control. Might as well go with SPP being
    50Kbytes/sec or better. The more throughput, the smaller the steps can be
    during fairly rapid changes in velocity. I ordered a surplus S6M4H servodisc
    "pancake" because it's a DC brushed type with zero cogging. It's design is
    amazing to me.

    http://kmtg.kollmorgen.com/products/product_literature/product_brochure/pdfs/KOL1037.pdf
    They do have zillions and ran into a 12 bit DAC with byte wide loading, like
    MAX508. Didn't know they made them. Maybe easier to use?
    http://pdfserv.maxim-ic.com/en/ds/MAX507-MAX508.pdf

    It's been a while since I've looked into this stuff and really need to move
    beyond the CMOS 4000 series :)

    Tanks

    Ben
     
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