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                     RS232    Communication Configuration

              IBM PC                          IBM AT
      Pin  25-Pin Signal (DTE)         Pin  9-Pin Signal (DTE)

          1   Chassis Ground  (GND)      1   Carrier Detect (CD)
          2   Transmit Data (TD)         2   Receive Data (RD)
          3   Receive Data (RD)          3   Transmit Data (TD)
          4   Request to Send (RTS)      4   Data Terminal Ready (DTR)
          5   Clear to Send (CTS)        5   Signal Ground (SG)
          6   Data Set Ready (DSR)       6   Data Set Ready (DSR)
          7   Signal Ground (SG)         7   Request to Send (RTS)
          8   Carrier Detect (CD)        8   Clear to Send (CTS)
        9-19  (not used)                 9   Ring Indicator (RI)
         20   Data Terminal Ready (DTR)
         22   Ring Indicator (RI)

        There are two general cable configurations used with the RS-232C
        Communications Standard:

          Data Terminal Equipment (DTE): IBM PC's, printers, plotters, etc
          Data Communication Equipment (DCE): modems, multiplexors, etc

        DCE to DTE requires all lines run straight through
        DTE to DTE usually requires swapping of the following lines

                RD  and TD              RD  and TD
                RTS and CTS     or      RTS,CTS and DCD
                DTR and DSR             DCD and RTS,CTS

      Signal Functions

        GND    Ground               protective safety ground
        TD     Transmit Data        DTE output data
        RD     Receive Data         DTE input data
        RTS    Request To Send      DTE output, DTE would like to transmit
        CTS    Clear To Send        DTE input, DCE is ready to transmit
        DSR    Data Set Ready       DTE input, DCE is ready to communicate
        SG     Signal Ground        provides a Zero reference voltage
        DCD    Data Carrier Detect  DTE input, data link established, also
                                    known as Receive Line Signal Detect (RLSD)
        DTR    Data Terminal Ready  DTE output, device ready
        RI     Ring Indicator       DTE input, announces incoming call


        - RTS/CTS is used for half duplex line turn around
        - in half duplex DCD is asserted only by the receiving device
        - full duplex modems tie CTS & DCD together (no CTS/RTS handshaking)
        - most modems require DTR to be present to respond to commands
        - maximum voltages are between -15 volts and +15 volts
        - binary outputs are between +5 to +15 volts and -5 to -15 volts
        - binary inputs are between +3 to +15 volts and -3 to -15 volts
        - input voltages between -3 to +3 are undefined while output voltages
          between -5 and +5 are undefined
        - positive voltages indicate ON or SPACE, negative voltages indicate
          OFF or MARK