Serial cable connection question.

Discussion in 'General Electronics Chat' started by foolios, Jan 17, 2015.

  1. foolios

    Thread Starter Active Member

    Feb 4, 2009
    160
    1
    I am trying to figure out why a serial cable only works one way when I connect it to a pc and device.
    I would think that it wouldn't matter which way the serial cable is connected, but for some reason data transmission will only work between the pc and device if I make sure to connect a particular end of the serial cable to the pc and that opposite end to the device. If I switch which end of the cable is connected to them, data will not transmit. I tried a new cable and the same thing occurs.
    Why would this be so?

    Thanks in advance.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,505
    2,367
    What serial format and connection?
    RS232, RS485? Any handshake or handshake jumpered?
    USB? 9 pin D socket?
    What is the nature of the device.
    Max.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,641
    2,344
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,505
    2,367
  5. foolios

    Thread Starter Active Member

    Feb 4, 2009
    160
    1
    RS232, 9 pin

    sorry about that

    thanks for the links, i will take a look
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    If you are referring to a 9-pin serial cable, there are almost an infinite ways to connect 9 pins from one connector to 9 pins of the other connector.

    Cables and connectors are not necessarily connected straight through, pin-to-pin. You can also find cross-over cables where one pin is connected to a different pin.

    For RS-232 serial communications cables there multiple complications.

    You need to be knowledgeable about the differences between DTE and DCE.

    DTE - Data Terminal Equipment
    DCE - Data Communications Equipment

    You can have a cable that connects DTE to DCE.
    Or you can have DCE to DCE, or DTE to DTE.

    Flow control signals are not necessarily connected as one would expect. Sometimes it is easier functionally to simply jumper some pins on the same connector rather that wire them RTS to CTS and DSR to DTR, etc.
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,641
    2,344
  8. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    There are many idiosyncratic legacies created by the IBM PC debacle.
    One of them is the DCE/DTE confusion.

    IBM in their infinite wisdom at the moment made the PC a terminal. In their minds, the PC was to be used as a time-share terminal connected to a mainframe (IBM 360, no doubt) via telephone wires.

    Hence the PC was designated DTE to be connected to a modem (DCE) using a DTE-DCE cable.

    So when ever you wanted to connect a PC to a printer or plotter (both are DTE) you had to use a DTE-DTE cable.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,505
    2,367
    Null Modem connection for 3 wire RS232.
    To set Null Modem do this by jumpering the 1-4-6 & 7-8 pins at each end.
    Max.
     
Loading...