Hardware handshaking
The remainder of the RS-232 lines are control lines. Some types of equipment
(including modems) aren’t happy just to receive a stream of data. They need to feel more in control
through a process called handshaking. In handshaking, some preliminary
communication between the two pieces of equipment must take place before data can be sent.
Let’s consider what type of handshaking might be necessary between a computer and
a modem in order to dial up another computer system. First of all, on an
outgoing call, the computer needs to know that the modem is available to make the
call. Then the modem needs to tell the computer that it has made a connection.
A computer (DTE) asserts pin 20 (Data Terminal Ready or DTR) to show that it
is ready. A modem (DCE) asserts pin 6 (Data Set Ready or DSR). When the modem
makes a connection with another modem on the other end, it asserts pin 8 (Data
Carrier Detect or DCD) to let the computer know that a connection has been
established. These voltages usually remain high during the entire transmission.