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Serial DTR/RTS confusion

Discussion in 'Electronic Repair' started by Vey, May 31, 2006.

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  1. Vey

    Vey Guest

    When a program asserts these to go high, the voltage is supposed to
    increase to more than +3VDC, right?

    Looking at my multimeter, mine aren't doing anything, yet when I connect
    DTR to DSR or RTS to CTS, they pass the loopback test.

    Yes, my multimeter is set and working correctly. Yes, I am sure I am
    looking it the right pins. I have tried every pin, just to make sure
    (it's a 9 pin connection) and I don't see any voltage, plus or minus out
    of any of the pins. Yet the loopbacks are working just fine.

    I'm using Win2k and the B&B Electronics Comtest program.
  2. Vey

    Vey Guest

    Well, I measured again. I see +0.03VDC and -0.03VDC on RTS Hi/Lo (pin4)
    and -0.001VDC and -0.02VDC RTS Hi/Lo (pin7).

    So maybe that explains the loopback test passing, but that's no where
    near 3 volts and the literature says those readings are considered to be
    "transitory" voltage.
  3. Franc Zabkar

    Franc Zabkar Guest

    IME these voltages toggle between approximately +10V and -10V. If the
    COM port is idle, then the RTS and DTR outputs should be sitting at
    one or the other of these levels. Just to be sure, are you using pin 5
    as the signal ground?

    I've written the following program to test COM ports:

    It runs in DOS only. Each output pin is toggled at 1Hz, 0.5Hz, and
    0.25Hz. A loopback function is also supported.

    This old post of mine describes how to use the DOS Debug command to
    directly control the DTR and RTS pins:

    - Franc Zabkar
  4. Guest

    Are you measuring the output pins at the serial connector without a
    load such as a modem on them. Remember a wrap block places the load of
    a receiver onto the transmitter. Take a bare jumper and wire DTR to
    DSR and look at the voltage level. You can do the same with RTS to
    CTS. +3VDC is too low. I have seen good ones run anywhere from + or -
    6volts to + or - 15 depending on the device.

    Van Gardner
  5. Vey

    Vey Guest

    Pin 5? As a ground? I had two different circuit diagrams I was working
    from. Neither one mentioned connecting Pin 5 to the circuit's ground.
    They both showed just the ground symbol.

    Then when I was measuring, I was using the clam for a ground. Once I
    used pin 5, everything measured right and then I connected Pin 5 to the
    circuit and it started working and then just to check, I ohmed out the
    clam to the ground on the PC and got nada.

    Looking at a third similar circuit, it does mention ground going to pin 5.

    Thanks Franc!
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