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Will a null modem cable fry these devices?

Discussion in 'Hardware' started by Ian Clowes, Oct 30, 2003.

  1. Ian Clowes

    Ian Clowes Guest


    I have a device with a 9-pin serial port, and the warning 'Never
    connect anything to the serial port'. Rather tempting...

    The device is showing -5.65V on pins 3 and 7 (TX and RTS). My laptop
    (which I would connect with) shows -7.4V on pins 3, 4 and 7 (TX, DTR
    and RTS). These voltages are relative to the local pin 5 (GND).

    Is anything bad likely to happen if I connect the devices using a null
    modem cable?

    Assuming not, is there anything that would let me get the port
    settings right easily on the laptop?

    Ian Clowes, Oct 30, 2003
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  2. Ian Clowes

    Ron Cook Guest

    As long as the two devices follow the specifications for RS-232
    voltages, you should be fine.

    The basic null modem cable / adapter just connects the transmit line
    (3) to the receive (RX) line (2, as I recall) on the other device and
    carries the signal ground.

    If you know the requirements for the serial port on the 'device' (port
    speed or BPS, word length, stop bit, and parity) it should be a simple
    matter to enter them into a basic communication or terminal program on
    the other machine.

    Under recent-ish Windows you can use the supplied HyperTerminal.
    Earlier Windows came supplied with 'Terminal'.

    Downloadable programs such as QModem, Procomm, Telix, and several
    other DOS-based applications will also run well under Windows.

    Linux offers applications such as Minicom which work equally well.

    Ron n1zhi
    Ron Cook, Oct 31, 2003
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