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bare minimum serial cable

Discussion in 'Hobby Electronics' started by C3, Dec 10, 2003.

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  1. C3

    C3 Guest

    I want to quickly build a serial cable to connect a modem to my serial port.
    What are the absolute minimum pin connections I need to make?

    Obviously, I'll need RX<-->RX, and TX<-->TX. Are all the other signals

  2. MC

    MC Guest

    Definitely include the Ground.

    The modem *might* want DTR asserted. (depends on how you've configured it)

    It usually doesn't hurt if you connect RTS to CTS at each end.

    This should work if you have 9-pin connectors at each end.

    2 ---- 2
    3 ---- 3

    5 ---- 5

    4 \ /4
    6 / \6

    7 \ /7
    8 / \8

    If you have a 25-pin at one end and a 9-pin at the
    other you'll need to (at least) swap pins 2 & 3.
  3. Leon.

    Leon. Guest

    And ground.
    In general, yes.

    RTS CTS is required with PC bios programs. Even though it didnt work and
    they had to use XON/XOFF. Just loop CTS back to RTS at PC end, set modem to

    Modem wont like it at first, but you can tell it to ignore DTR.

    Windows dialling programs might not like it if you dont connect DCD. (loop
    DTR back to DCD at PC end )

    Linux can be set for three wire - stty commands would be like -crtscts
    +ixon +ixoff -modem
  4. C3

    C3 Guest

    thanks for your help, guys.

  5. David Milne

    David Milne Guest

    There are a number of ways to connect devices together using rs232.
    Try this website.



    Reply to

    dmmilne at ozemail dot com dot au
  6. Connect all 9 pins on your PC serial connector.

    If you do this, it will work, and you can get on with your life. If you get
    smart and leave out a wire, you will have the luxury or working out a
    spacial initialization string for your modem, and even then it may not work,
    or your fax program may not work etc. Trust me, I work with modems, serial
    comms & PCs and you need the wires.

  7. I tend to agree with Roger.
    Hardware flow control is more efficient than software flow control. Sure,
    it is possible to use less wires, but making up a cable with less than 9
    wires may work okay with one device and cause merry hell with another.

    Is there any real reason for wanting to use a minimum of conductors other
    than having less to connect / solder or whatever? Any problems with
    debugging / working out which wires are essential and which are not can
    rapidly consume more time than connecting all the conductors.

  8. C3

    C3 Guest

    Well, I may as well reveal what I'm doing. I wanted to hook up my old
    Telstra CyberSurfr cable modem up to my computer using the serial port, just
    to see whether you could talk to the modem through a shell of some sort. I
    tried hooking it up using three serial lines (GND,RX,TX) but even at a
    variety of bit rates, it would not talk back.

  9. Ken Taylor

    Ken Taylor Guest

    Try using a null modem cable then. Check the DSE catalog or the web if you
    need the wiring pinout.

  10. Rod Speed

    Rod Speed Guest

    Yes, you can use those RJ11/RS232 adapters and use standard
    phone extension cords for a very quick and convenient connection.
  11. C3

    C3 Guest

    I tried swapping the RX and TX wires, but I still couldn't get the modem to

  12. Franc Zabkar

    Franc Zabkar Guest

    Modems use "straight-through" connections.

    - Franc Zabkar
  13. Leon.

    Leon. Guest

    See page 36 (section 1 -26 )

    for the cross connection between a 25 pin connector on a PC and the 9 pin on
    the cybersurfer.

    This is not a data modem, this serial port is for diagnostics and

    You might get by without RTS CTS- but you could just wire it correctly.

    If you do try 3 wire ..

    On the PC tell your terminal software to use no flow control.
    On the cybersurfr, you might need to short RTS to CTS at the cybersurfer.

    That should be enough.

    But you might need to short DTR, DSR and DCD at the cybersurfer.

    AS for speed, the cybersurfr might autodetect, or it might be fixed to some
    speed and bit pattern.

    If it autodetects and you dont have rts cts flow control, choose a nice slow
    speed, or else it will outpace your terminal program and cause garbled text.
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