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Serial port

Discussion in 'Electronic Design' started by Fred, May 3, 2005.

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  1. Fred

    Fred Guest

    After my question on the parallel port, I must ask you something about
    the serial port. Why I cannot see on my scope the data signal if I put
    my scope directly to the pins. To see it, I must connect the serial
    port on an other device and then, put the scope on the line.

    Thanks for all yours valuable input!!!
  2. Rich Grise

    Rich Grise Guest

    You have to jumper DSR to DTR and RTS to CTS, otherwise the computer
    thinks that what it's connected to isn't ready to receive.
    Do a google search on "null modem".

    Good Luck!
  3. Colin Dawson

    Colin Dawson Guest

    I'm no experts on serial ports, or electronics in general for that matter
    but here goes. As I understand it the serial data is actually transmitted
    via a circuit. When it's left open circuit (not plugged in) a detection
    circuit knows about this and so the line doesn't get raised or lowered.

    When the circuit is fully connected up, the circuit is complete and the line
    can be raised and lowered. I can't remember offhand but I think that the
    line needs to be held in a naturally low state so that it can be driven

    If this sounds like a laymans explaination, that's because my understanding
    of this is very crude.


    Colin Dawson
  4. John Fields

    John Fields Guest

    Connect your scope ground to SIGNAL GROUND in the serial port
    connector and then connect the probe input to either RX DATA or TX
    DATA. Center your trace and set the vertical sensitivity to something
    like 5 volts per box. Set your sweep speed for a convenient display
    of the data rate you're expecting and set the sync to auto.

    That should get you started, and then you can fiddle with the settings
    all you want. Be aware that you'll be looking at data constantly
    changing in time, so unless you're sending (or receiving) a repeating
    pattern and you can get the sweep speed and sync set up just right you
    won't see a stable display.
  5. Key thing missing here, is to go into device manager, select the port,
    select the setup options, and 'advanced', and set 'flow control', to
    'none'. If this is not done, the port will usually default to using
    hardware flow control, and data won't be sent, unless the right flow
    control lines are 'made'. With this done, you should be able to type data
    in a terminal program, and see the serial data on the scope.

    Best Wishes
  6. Si Ballenger

    Si Ballenger Guest

    I had to do the null modem wiring on my win95/98 mcachines lto
    echo bytes to the com port, but on XP the extra wiring wasn't
  7. Joel Kolstad

    Joel Kolstad Guest

    Most terminal programs allow you to specify what (if any) handshaking you want
    to use (even Hyperterminal, the piece of trash MS includes for free with
    Windoze) so one can avoid worrying about whether or not a proper null
    modem-type cable is needed.
  8. Si Ballenger

    Si Ballenger Guest

    In some situations like my page below, using a terminal program
    isn't an option. The bottom link shows the simple loopback wiring
    I used on the DB-9 plug for use with my win95/98 machines.
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