Signal Connections

Connecting devices (or computers)
When you connect a data set (e.g. a modem), use this connection:

GND1 to GND2
RxD1 to RxD2
TxD1 to TxD2
DTR1 to DTR2
DSR1 to DSR2
RTS1 to RTS2
CTS1 to CTS2
RI1 to RI2
DCD1 to DCD2

In other words, simply connect each pin of the first plug with the corresponding pin of the other. This can easily be done using a 25-wire ribbon cable and two crimp connectors.

When you connect another computer, this is the wiring you need

GND1 to GND2

RxD1 to TxD2

TxD1 to RxD2
DTR1 to DSR2
DSR1 to DTR2
RTS1 to CTS2
CTS1 to RTS2

If software wants it, connect DCD1 to CTS1 and DCD2 to CTS2
If hardware handshaking is not needed, a so-called null-modem connection can be used. Connect:

GND1 to GND2

RxD1 to TxD2

TxD1 to RxD2

Additionally, connect (if software needs it):

DTR1 to DSR1

DTR2 to DSR2
RTS1 to CTS1 and DCD1
RTS2 to CTS2 and DCD2

You won't need long wires for these!
The null-modem connection is used to establish a XON/XOFF-connection between two PCs (see the Handshaking section for details about XON/XOFF).
Remember: the names DTR, DSR, CTS and RTS refer to the lines as seen from the PC. This means that for your data set DTR and RTS are incoming signals and DSR and CTS are outputs! Modems, printers, plotters etc. are connected 1:1, ie. pin x to pin x.

Cable Lengths
The RS-232C standard imposes a cable length limit of 50 feet. You can usually ignore this "standard", since a cable can be as long as 10000 feet at baud rates up to 19200 if you use a high quality well shielded cable. The external environment has a large effect on lengths for unshielded cables. In electrically noisy environments, even very short cables can pick up stray signals. The following chart offers some reasonable guidelines for 24-gauge wire under typical conditions. You can greatly extend the cable length by using additional devices like optical isolators and signal boosters. Optical isolators use LEDs and Photo Diodes to isolate each line in a serial cable including the signal ground. Any electrical noise affects all lines in the optically isolated cable equally - including the signal ground line. This causes the voltages on the signal lines relative to the signal ground line to reflect the true voltage of the signal and thus canceling out the effect of any noise signals

Baud Rate

Max Distance Max Distance

Shielded Cable Unshielded Cable
110 5000ft 3000ft
300 5000ft 3000ft
1200 3000ft 3000ft
2400 1000ft 500ft
4800 1000ft 250ft
9600 250ft 250ft