Installing and Administering PPP

Appendix A 149
Modem Connections
RS-232 Interface
Null-Modem Cables
You can directly connect the serial ports of two systems with a
null-modem cable. Null-modem cables connect pins of one machine to
their symmetric counterparts on another machine. For example, pin 2,
the Transmit Data (TD) pin of machine A, is paired with the pin 3, the
Receive Data (RD) of machine B. If this is not done, both machines
would try to use pin 2 to transmit data and neither could receive the
others transmission. Sometimes only pins 2, 3, and 7 are connected in a
null-modem cable, but hardware handshaking may require optional
connections of other pins. Data Set Ready (DSR) and Carrier Detect
(CD), pins 6 and 8, may be joined together and connect to the other
machine's Data Terminal Ready pin, number 20.
The table below shows typical null-modem pin connections between two
DB-25 ports.
CPU DIRECTION MODEM
Gnd 1 both 1 Gnd
TD 2 from Modem 3 RD
RD 3 from DTE 2 TD
RTS 4 from Modem 8 DCD
DSR 6 from DTE 20 DTR
GND 7 BOTH 7 GND
DCD 8 from DTE 4 RTS
9 from Modem 22 RI
DTR 20 from Modem 6 DSR
RI 22 from Modem 5 CTS