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EPROM to RS232

Discussion in 'Electronic Basics' started by Bart, Nov 9, 2005.

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  1. Bart

    Bart Guest

    I'd like to build a handheld unit that will send what is on an EPROM over
    RS232 to a PC. I'll program ASCII characters (in binary), one at each
    address including line feeds and carriage returns, on the EPROM. A nine volt
    battery with a 5 volt voltage regulator will power the unit and drive a 555
    clock pulse. A kind of flowchart below has the 555 fire the binary counter
    which then fires addresses of the EPROM whose data goes into a shift
    register to take the data in and send it out serially to the RS232 driver,
    and out to the PC. I'm sure my unfamiliarity shows, is my approach way off?
    Any tips/suggestions/help is appreciated.
    Thanks in advance,



    CLOCK PULSE (555)




    RS232 DRIVER

    (created by AACircuit v1.28.6 beta 04/19/05
  2. Andrew Holme

    Andrew Holme Guest

    You could build this with counters, gates and shift registers if you like,
    and that's fine for a hobby project, but it would be easier to use a PIC.
    If you do decide to do it with discrete logic, then you're on the right
    lines. Some more things to consider are:
    1. you need to generate start bits and stop bits as well as data bits, so
    you need some logic to handle this;
    2. you might need a negative supply rail for the RS232 driver, and this
    could possibly be taken from the RS232 port itself).
    3. How many times will the message be sent? What will trigger it?
    4. How long is the message? Do you need to store a special end-of-message

    It would be just as easy, take about the same board space, and be more
    accurate to use a 4060 crystal oscillator / divider to generate the clock.
  3. Brian Lyons

    Brian Lyons Guest

    Interested idea.

    However, you will need to have a means to add stop & start bits if you
    wish to transfer the info from the eprom as ascii.

    You may wish to consider using cheap uart rather than shift register.
  4. John Fields

    John Fields Guest

    Your approach is OK, but you'll need to add a stop and start bit to
    the serial data stream. That's easily done with half of an XX74 "D"
    type flip-flop though, by tying one of the "D" input of a "D" type
    flip-flop to the serial output ("Q") of an XX165 or XX166
    parallel-in serial-out shift register, tying 74's RESET\ input to
    the the 165's LOAD\ input, and tying the 165's serial data input
    high. That way, when you load the 165 and take your data from the
    74's Q output, you atomatically generate the start and stop bits and
    the data will be properly framed.

    You'll also need to arrange for your serial data clock to pump data
    out of the shift register at a baud rate your PC can recognize and
    you'll need to divide that clock down so that it increments the ROM
    address counter by one count every time the stop bit ends and then,
    when data from the ROM settles, load the '165 and start a new cycle.

    Want a schematic?
  5. Bob Monsen

    Bob Monsen Guest

    If you can deal with a parallel port, it'll be easier. The port already
    has enough signals to transfer a byte at a time, so you can get away with
    fewer parts. You could also probably power the device from the port, which
    provides signals at 5V. In fact, you could clock the address counter from
    the PC, wait a bit, then retrieve the data from the data port.

    Using a serial port means having a UART, or alternately a device to
    simulate one. UARTs are usually designed to be driven by a processor.
    However, using a 16 bit parallel-in serial-out buffer, you can probably
    simulate a 10 bit transmission. RS232 wants the line to be logic 1 until
    the 'start bit', which goes to 0, followed by the data bits, followed by a
    logic 1 'stop bit', and more logic 1s until the next character. It also
    likes low to high bit ordering. So, load bits 1111xxxxxxxx0111
    into the 16 bit register, and shift the low order bits out. Make
    sure the serial in bit is set to 1, to keep the line idle. The data would
    go to an RS232 driver chip, like a MAX232 or something.

    One problem is that using a 555 for the clock is going to be tricky, since
    it won't be very stable over time. RS232 assumes synchronized clocks on
    both sides, and won't tolerate more than about 5% of deviation. Thus,
    you might have to tune the clock occasionally. Using a crystal oscillator
    is generally a better way to go. Since you can get a crystal that is
    38.4k, you might use that as your bit rate. Clock your counters (the
    number of bits in the counter should be the number of address bits in the
    eprom, plus 4 (or 5, see below)) with an oscillator built from that
    crystal (an unbuffered inverter is a good way to go), and use D4 on from
    the counters to address the EPROM. You will probably want to use
    synchronous counter, or you'll get glitches in the address lines. Use the
    D0 bit to load the next value into the shift register, since when that
    changes, the eprom will have had a bit time to settle since the address

    Use a modem control signal to reset the counters to 0... also, if you use
    an extra bit in the counter at the top, you can use that bit to inhibit
    the counting, and signal completion using a modem signal. Otherwise,
    you'll get wraparound.

    These are just off-the-cuff ideas, you can clearly improve on them...

    Bob Monsen

    There once was a man from Hornepayne,
    Who tried to transform the whole plane,
    It bent a meridian
    So it wasn't Euclidean,
    And frustration drove him insane.
    - Anonymous
  6. <snippety>

    You can de-complicate your design effort greatly by using an SPROM
    (Serial PROM). Such devices are usually EE-type when it comes to
    programming and erasing (though many are OTP -- shop carefully!), and
    they are accessed via serial data stream as opposed to the parallel
    architecture of a normal EPROM.

    At least one example of such devices, data-wise, may be found

    Further search should turn up others.

    Happy tweaking.

    Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
    (Known to some as Bruce Lane, ARS KC7GR,
    kyrrin (a/t) bluefeathertech[d=o=t]calm --
    "If Salvador Dali had owned a computer, would it have been equipped
    with surreal ports?"
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