Online HelpPC 2.10 Quick Reference Utility Copyright 1991 David Jurgens
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
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