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]>