Data transmission

Only two of the 25 pins are used for data transmission. The standard also calls for secondary transmit and receive lines, but they are rarely implemented. Pin 2 is defined as Transmit Data and pin 3 is defined as Receive Data.

Since DTE uses pin 2 to transmit data and pin 3 to receive it, and DCE does the reverse, connecting a terminal or computer to a modem or printer (DTE to DCE) requires a straight-through connection, as shown here.

To make a connection between two computers (DTE to DTE), a cable is required with lines 2 and 3 crossed; this is called a null modem or modem eliminator cable.

Pin 1 is a safety ground, and should be connected at one end (the host end of a computer-modem connection, or either end of a direct link between two computers) and left unconnected at the opposite end of the cable. In a proper RS-232 implementation, pin 1 of the port is connected internally to the frame ground of the system.

Pin 7 is the signal ground. It provides the reference voltage against which other signals are measured. It should be connected straight through.

A pin is said to be asserted when a voltage greater than ± 3 volts (relative to signal ground) is present on the pin. On the data lines, a voltage more negative than -3 volts is considered a binary 1, and a voltage more positive than +3 volts is considered a binary 0. (Serial drivers usually assert voltages of ± 5 volts to allow 2 volts of noise margin.)

On the control lines, a positive voltage is considered the on state, and a negative voltage is considered off. This is the direct opposite of the case for the data lines.

If it is not known whether a device is DTE or DCE, measure the voltage on pins 2 and 3. The transmitter should always have a negative voltage, even when idle. If pin 2 is negative, the device is DTE. If pin 3 is negative, the device is DCE.