Re: RS232 Full Duplex to Half Duplex

From: Tauno Voipio (
Date: 03/25/05

  • Next message: Paul Burke: "Re: Understanding DMA"
    Date: Fri, 25 Mar 2005 08:21:31 GMT
    > Well I am not an expert on rs232 communication, but I was told by my
    > professer that in half duplex rs232 there was only 1 wire between the
    > two devices. What I am trying to do is control a device that works at
    > 57600 baud rate but in Half Duplex mode. But when I was studying the
    > diagrams for the device I am trying to control it has both a Tx and Rx
    > wire. If anyone could point me in the direction of a good rs232 serial
    > communication protocol document I would appretiate it.

    That's not correct - the RS-232 standard specifies the 25 pin
    connector between a DTE (Data terminal Equipment, a computer or
    terminal) and a DCE (Data Communication Equipment, a modem). The
    spec contains, among a lot of control lines, separate transmit
    and receive lines (even another pair for reverse channel).

    If your RS-232 means the common misuse meaning in fact asynchronous
    serial byte-by-byte transfer, then the single data line configuration
    is possible. The connection is often called a party line: all receivers
    are connected directly to the line and all transmitters are connected
    via enabling/disabling gates to the line. The enable logic may be
    internal to the tranmit interface chip or a separate chip.

    It's up to you to decide on the connection. You'll need some
    kind of interface chips anyway - the RS-232 standard signal
    levels are not compatible with the processor and UART signals

    If the RS-232 spec is not to be taken too literally, one solution
    is to use RS-485 connection and interface chips (Google for it).
    Please note that you need a couple of resistors to feed the idle
    state to the line when all transmitters are turned off.

    Tauno Voipio
    tauno voipio (at) iki fi

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