Common RS-232 cable wirings
This document includes information how to make common wires for RS-232 connections. I have shown pinouts for standard 25 pin connection and de-facto 9 pin connector used in PCs.
Normal DTE-DCE connection
Thise wirings can be used with normal Data Terminal Equipment (DTE) to Data Communications Equipment (DCE) connections. The wiring are standard way to do asyncronous DTE to DCE connection and they support hardware handshaking. Good wires for PC to modem connections. 9 pin to 25 pin wiring can also be used for making 9 pin to 25 pin connector adapters.
DTE (25 pin) DCE (25 pin)
TD 2 ------------------------> 2
RD 3 <------------------------ 3
RTS 4 ------------------------> 4
CTS 5 <------------------------ 5
DSR 6 ------------------------> 6
DCD 8 ------------------------> 8
DTR 20 ------------------------> 20
SG 7 ------------------------- 7
RI 22 <------------------------ 22
DTE (9 pin) DCE (25 pin)
TD 3 ------------------------> 2
RD 2 <------------------------ 3
RTS 7 ------------------------> 4
CTS 8 <------------------------ 5
DSR 6 ------------------------> 6
DCD 1 ------------------------> 8
DTR 4 ------------------------> 20
SG 5 ------------------------- 7
RI 9 <------------------------ 22
Two-wire DTE-DCE wiring
This wiring can be used between DTE and DCE where hardware handshaking is not needed.
DTE (25 pin) DCE (25 pin)
TD 2 ------------------------> 2
RD 3 <------------------------ 3
RTS 4 ----, ,----- 4
CTS 5 <---' '----> 5
DSR 6 <---, ,----> 6
DCD 8 <---| |----> 8
DTR 20 ----' '----- 20
SG 7 ------------------------- 7
DTE (9 pin) DCE (25 pin)
TD 3 ------------------------> 2
RD 2 <------------------------ 3
RTS 7 ----, ,----- 4
CTS 8 <---' '----> 5
DSR 6 <---, ,----> 6
DCD 1 <---| |----> 8
DTR 4 ----' '----- 20
SG 5 ------------------------- 7
Null modem cable
This cable can be used when you must connect two DTE equipments like computers to each other directly without using any data communication quipment in between computers. This wiring supports hardware handshaking.
DTE (25 pin) DTE (25 pin)
TD 2 ---------\ /------------- 2
RD 3 <--------/ \------------> 3
RTS 4 ---------\ /------------- 4
CTS 5 <--------/ \------------> 5
DSR 6 <---, ,----> 6
DCD 8 <---+----\ /-------+----> 8
DTR 20 ---------/ \------------- 20
SG 7 ------------------------- 7
DTE (25 pin) DTE (9 pin)
TD 2 ---------\ /------------- 3
RD 3 <--------/ \------------> 2
RTS 4 ---------\ /------------- 7
CTS 5 <--------/ \------------> 8
DSR 6 <---, ,----> 6
DCD 8 <---+----\ /-------+----> 1
DTR 20 ---------/ \------------- 4
SG 7 ------------------------- 5
DTE (9 pin) DTE (9 pin)
RD 2 ---------\ /------------- 2
TD 3 <--------/ \------------> 3
RTS 7 ---------\ /------------- 7
CTS 8 <--------/ \------------> 8
DSR 6 <---, ,----> 6
DCD 1 <---+----\ /-------+----> 1
DTR 4 ---------/ \------------- 4
SG 5 ------------------------- 5
Two-wire DTE-DTE connection
This cable can be used when you must connect two DTE equipments like computers to each other directly without using any data communication quipment between computers. This wiring needs only two signal wires and ground between computers but does not support hardware handshaking.
DTE (25 pin) DTE (25 pin)
TD 2 ---------\ /------------- 2
RD 3 <--------/ \------------> 3
RTS 4 ----, ,----- 4
CTS 5 <---' '----> 5
DSR 6 <---, ,----> 6
DCD 8 <---| |----> 8
DTR 20 ----' '----- 20
SG 7 ------------------------- 7
DTE (25 pin) DTE (9 pin)
TD 2 ---------\ /------------- 3
RD 3 <--------/ \------------> 2
RTS 4 ----, ,----- 7
CTS 5 <---' '----> 8
DSR 6 <---, ,----> 6
DCD 8 <---| |----> 1
DTR 20 ----' '----- 4
SG 7 ------------------------- 5
DTE (9 pin) DTE (9 pin)
TD 3 ---------\ /------------- 3
RD 2 <--------/ \------------> 2
RTS 7 ----, ,----- 7
CTS 8 <---' '----> 8
DSR 6 <---, ,----> 6
DCD 1 <---| |----> 1
DTR 4 ----' '----- 4
SG 5 ------------------------- 5
PC to printer cable
This is a wiring for cable for connecting printer to computer. Both printer and computer are DTE quipment and printer uses DTR busy signalling. This is good cable for PC to printer connection when printer uses DTR bysy or XON/XOFF signalling.
Computer Printer
DTE (25 pin) DTE (25 pin)
TD 2 ------------------------> 3 RD
RD 3 <------------------------ 2 TD
CTS 5 <--+--------------------- 20 DTR
DSR 6 <--'
DCD 8 <--,
DTR 20 ---'
SG 7 ------------------------- 7 SG
Computer Printer
DTE (9 pin) DTE (25 pin)
TD 3 ------------------------> 3 RD
RD 2 <------------------------ 2 TD
CTS 8 <--+--------------------- 20 DTR
DSR 6 <--'
DCD 1 <--,
DTR 4 ---'
SG 5 ------------------------- 7 SG
Loopback tester
This is a simple and useful tool for testing RS-232 ports in DTE equipment are working working or not. This plug is connected so that every sent character is echoed back.
DTE (25 pin)
TD 2 --------,
RD 3 <-------'
RTS 4 --------,
CTS 5 <-------'
DSR 6 <-------,
DCD 8 <-------|
DTR 20 --------'
DTE (9 pin)
TD 3 --------,
RD 2 <-------'
RTS 7 --------,
CTS 8 <-------'
DSR 6 <-------,
DCD 1 <-------|
DTR 4 --------'
Tomi Engdahl <[email protected]>