Hardware manual
4 20 DTR Data Terminal Ready--> I'm ready to communicate
1 8 DCD Data Carrier Detect<-- Modem connected to another
9 22 RI Ring Indicator <-- Telephone line ringing
5 7 SG Signal Ground
9-Pin DB9 Connector 25-Pin DB-25 Connector
1 DCD Carrier Detect 1 Chassis Ground
2 RxD Receive Data 2 TxD Transmit Data
3 TxD Transmit Data 3 RxD Receive Data
4 DTR Data Terminal Ready 4 RTS Request To Send
5 SG Signal Ground 5 CTS Clear To Send
6 DSR Data Set Ready 6 DSR Data Set Ready
7 RTS Request To Send 7 SG Signal Ground
8 CTS Clear To Send 8 DCD Carrier Detect
9 RI Ring Indicator 20 DTR Data Terminal Ready
22 RI Ring Indicator
19.2 Signals May Have No Fixed Meaning
Only 3 of the 9 pins have a fixed assignment: transmit, receive and signal ground. This is fixed by the
hardware and you can't change it. But the other signal lines are controlled by software and may do (and mean)
almost anything at all. However they can only be in one of two states: asserted (+12 volts) or negated (-12
volts). Asserted is "on" and negated is "off". For example, Linux software may command that DTR be
negated and the hardware only carries out this command and puts -12 volts on the DTR pin. A modem (or
other device) that receives this DTR signal may do various things. If a modem has been configured a certain
way it will hang up the telephone line when DTR is negated. In other cases it may ignore this signal or do
something else when DTR is negated (turned off).
It's like this for all the 6 signal lines. The hardware only sends and receives the signals, but what action (if
any) they perform is up to the Linux software and the configuration/design of devices that you connect to the
serial port. However, most pins have certain functions which they normally perform but this may vary with
the operating system and the device driver configuration. Under Linux, one may modify the source code to
make these signal lines behave differently (some people have).
19.3 Cabling Between Serial Ports
A cable from a serial port always connects to another serial port. An external modem or other device that
connects to the serial port has a serial port built into it. For modems, the cable is always straight thru: pin 2
goes to pin 2, etc. The modem is said to be DCE (Data Communications Equipment) and the computer is said
to be DTE (Data Terminal Equipment). Thus for connecting DTE-to-DCE you use straight-thru cable. For
connecting DTE-to-DTE you must use a null-modem cable (also called a crossover cable). There are many
ways to wire such cable (see examples in Text-Terminal-HOWTO subsection: "Direct Cable Connection")
There are good reasons why it works this way. One reason is that the signals are unidirectional. If pin 2 sends
a signal out of it (but is unable to receive any signal) then obviously you can't connect it to pin 2 of the same
type of device. If you did, they would both send out signals on the same wire to each other but neither would
be able to receive any signal. There are two ways to deal with this situation. One way is to have a two
different types of equipment where pin 2 of the first type sends the signal to pin 2 of the second type (which
receives the signal). That's the way it's done when you connect a PC (DTE) to a modem (DCE). There's a
second way to do this without having two different types of equipment: Connect pin sending pin 2 to a
receiving pin 3 on same type of equipment. That's the way it's done when you connect 2 PCs together or a PC
to a terminal (DTE-to-DTE). The cable used for this is called a null-modem cable since it connects two PCs
without use of a modem. A null-modem cable may also be called a cross-over cable since the wires between
Serial HOWTO
19.1 Pinout of 9-pin and 25-pin serial connectors 71