RS-232 Modem Cable Pinout

This is the tested pinout for an RS-232 Modem Cable, being capable of synchronous transfers.

Pinout

Both ends utilize D-Sub 25 Pin connectors:

• Female for the "machine end" (DTE),
• male for the "modem end" (DCE).

These pins should be connected one-to-one:

Pin	Signal
2	TxD
3	RxD
4	RTS
5	CTS
6	DSR
7	Gnd
8	DCD
15	ST (Clk)
17	RT (Clk)
20	DTR
22	RI
23	Signal Rate Select
24	TT (Clk)

In addition, on the DTE end's female connector, the otherwise unused pins 1, and 13 must be connected to signal ground, Pin 7. Older AS/400's require this to sense if, and which type of cable is connected to the serial port.

You could easily just solder a short run of ribbon cable between two plugs, and use an existing "PC style" modem cable to gain length. It's nevertheless very important that all 13 wires are connected. Especially the clock signals are very important to support synchronous transmission.

See also

• Serial Cables IDs
• Integrating LAN-less Machines into a local Token Ring

Weblinks

• RS-232, Wikipedia
• AS/400 Service Functions, SY44-5902-03