Meanings of pins in DB25 order:

DB25 PinDB9 PinDescription1FG Frame Ground23TD Transmit data (Out)32RD Receive data (In)47RTS Request to Send (Out)58CTS Clear to Send (In)66DSR Data Set Ready (In)75SG Signal Ground81DCD Data Carrier Detect (In) 9+V (In) 10-V (In) 11QM (also called SSD- Secondary Send Data)(In) 12SDCD Secondary Carrier Detect (In) 13SCTS Secondary Clear to Send (In) 14STD Secondary Transmit Data (Out) 15TC Transmitter Clock (In) 16SRD Secondary Receive Data (In)
17RC Receiver Clock (In) 18Unused19SRTS Secondary Request to Send (Out)204DTR Data Terminal Ready (Out)21SQ Signal Quality Detect (In)229RI Ring Indicator (In)23Data Rate Select (Out)24(TC) External Transmitter Clock (Out)25Unused

Meanings of pins in DB9 order:

DB25 PinDB9 PinDescription81DCD Data Carrier Detect (In) 32RD Receive data (In)23TD Transmit data (Out)204DTR Data Terminal Ready (Out)75SG Signal Ground66DSR Data Set Ready (In)47RTS Request to Send (Out)58CTS Clear to Send (In)229RI Ring Indicator (In)

Connecting to other devices

The unfortunate thing about RS232 is that aside from the simplest TD to RD and SG to SG hookups, and computer to modems (essentially straight through because a modem is DCE- just the reverse of a computer), everybody does it differently. For the diagrams below, leave off FG when you don’t have it (DB9).

Async modems need FG (if available), SG, TD, RD, CTS, RTS, DSR, DCD and DTR at a minimum. Synchronous modems also need SCTS, STD, TC, RC, RI and (TC).

There is tremendous variance in printer wiring. If you don’t know how your printer expects to be wired, you’ll have to wire it just TD,RD and SG, and rely on Xon/Xiff flow control.

Some printers handshake with RTS and use DTR to indicate on-line or off-line.

ComputerPrinter (handshake DTR and CTS)FGFGRDTDTDRDDCDRTSRTSDCDCTS and DSRDTRDTRCTS and DSRSGSG

Others use DTR for everything

ComputerPrinter (handshake DTR)FGFGRDTDTDRDCTS, DCD and DSRDTRDTRCTS, DCD and DSRSGSG

Okidata printers have an unusual setup:

ComputerOkidata Serial PrinterFGFGRDTDTDRDCTSQMDSRDTR and DSRSGSG

Connecting a computer to another computer needs only TD, RD and SG if you are using Xon/Xoff flow control.

ComputerTerminal or ComputerFGFGRDTDTDRDRTSCTSCTSRTSDCD and DSRDTRDTRDCD and DSR
SGSG

See also Serial Wiring and Serial Printers

A.P. Lawrence provides SCO Unix and Linux consulting services http://www.pcunix.com