Pinout

Note: DCD is sometimes labeled CD

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".
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).

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 "Serial 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 pins 2 and 3 cross over each other (if you draw them on a sheet of paper). The above example is for a 25 pin connector but for a 9-pin connector the pin numbers are just the opposite.

The serial pin designations were originally intended for connecting a dumb terminal to a modem. The terminal was DTE (Data Terminal Equipment) and the modem was DCE (Data Communication Equipment). Today the PC is usually used as DTE instead of a terminal (but real terminals may still be used this way). The names of the pins are the same on both DTE and DCE. The words: "receive" and "transmit" are from the "point of view" of the PC (DTE). The transmit pin from the PC transmits to the "transmit" pin of the modem (but actually the modem is receiving the data from this pin so from the point of view of the modem it would be a receive pin).

The serial port was originally intended to be used for connecting DTE to DCE which makes cabling simple: just use a straight-thru cable. Thus when one connects a modem one seldom needs to worry about which pin is which. But people wanted to connect DTE to DTE (for example a computer to a terminal) and various ways were found to do this by fabricating various types of special null-modem cables. In this case what pin connects to what pin becomes significant.

RTS/CTS and DTR/DSR Flow Control

This is "hardware" flow control. Flow control was previously explained in the Flow Control subsection but the pins and voltage signals were not.

Then when a DTE (such as a PC) wants to stop the flow into it, it negates RTS. Negated "Request To Send" (-12 volts) means "request NOT to send to me" (stop sending). When the PC is ready for more bytes it asserts RTS (+12 volts) and the flow of bytes to it resumes. Flow control signals are always sent in a direction opposite to the flow of bytes that is being controlled. DCE equipment (modems) works the same way but sends the stop signal out the CTS pin. Thus it's RTS/CTS flow control using 2 lines.

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