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Tips, tricks and tweaks for getting the most out of your hardware.
Serial Connection Wiring
RS232 (Serial) Communication Configuration
IBM PC
Pin 25-Pin Signal (DTE)
IBM AT
Pin  9-Pin Signal (DTE)
1 Chassis Ground  (GND)
2 Transmit Data (TD)
3 Receive Data (RD)
4 Request to Send (RTS)
5 Clear to Send (CTS)
6 Data Set Ready (DSR)
7 Signal Ground (SG)
8 Carrier Detect (CD)
9-19 (not used)
20 Data Terminal Ready (DTR)
22 Ring Indicator (RI)
1 Carrier Detect (CD)
2 Receive Data (RD)
3 Transmit Data (TD)
4 Data Terminal Ready (DTR)
5 Signal Ground (SG)
6 Data Set Ready (DSR)
7 Request to Send (RTS)
8 Clear to Send (CTS)
9 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
RTS and CTS
DTR and DSR
or RD and TD
RTS,CTS and DCD
DCD and RTS,CTS

Signal Functions

GND Ground Protective safety ground
TD Transmit Data DTE utput 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 conunands
- maximum voltages are between -15 volts and +15 volts
- binary outputs are between +5 to +l5~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