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
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