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Nicad analyser circuit problem

Discussion in 'Electronic Basics' started by Tim Duke, Jul 14, 2004.

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  1. Tim Duke

    Tim Duke Guest

    Hi all,

    Am having a little problem with a circuit that I have built and wonder if
    someone here can help me. I recently purchased a programmed PIC to build a
    nicad analyser. It connects to the PC and with the software provided, plots
    a discharge curve enabling you to see the health and capacity of the pack.

    Here is a link to the schematic:

    and here is a link to the circuit description:

    The problem I am having, is I believe the software or the interface to the
    PIC. Voltage is supplied by the serial port to power the PIC and i have
    tried powering externally just in case the PC didn't supply enough juice.
    The software does instruct the PIC to discharge at the correct rate, but the
    circuit does not reply properly.

    Q1. How does this circuit get away without using a MAX232 chip to convert
    the TTL signals to RS232 level ?

    Q2. Anyone here built one of these and has managed to get it working (or
    not) ?

    Needless to say that I have contacted the guy who designs and sells the unit
    but am not getting anything helpful back from him.

    Here's hoping someone can help. Thanks in advance,

  2. hamilton

    hamilton Guest

    Looking at the data sheet, (page 4) the 16c71 does NOT have a UART
    built-in. So the code in the chip would have to bit bang the asnyc
    serial protocol. The RS232 chips do two things, 1) level translation
    (+-12V to 5V/Gnd) and 2) invert the signal. Current limiting with a
    series resistor is not the best way to limit the current but it does
    work. The inversion is done in the bit bang software.

    Looking over the schematic this should work fine, but there is not much
    room for error. If this device was on the end of a long cable, higher
    voltage spikes could develop along the cable and blow the input of the

    Are you running the circuit on a long cable ??
    Is that why you are using an external supply ?

    The response from the PIC maybe a problem over a long cable.
    Limiting 12v to a 5V input can work, but 5V into a 12V input will be
    hard to do. Be as close as possible to the PC and it should work OK.
  3. Tim Duke

    Tim Duke Guest

    Looking at the data sheet, (page 4) the 16c71 does NOT have a UART

    Thanks for the response Hamilton......

    From the PCB to the D9 connector that connects to the serial port is no
    longer than 2 inches ! And I only tried using an external supply as I was
    running out of ideas. I will scope the Txd line from the circuit and post
    the results.

    All the best,

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