Data Carrier Detect - As Used On Modems

As Used On Modems

DCD is very important on modems, as it is the computer's primary way to find out that the modem has lost its connection to the remote host. Aside from intentional disconnects, modems can lose their connection for a variety of reasons unexpectedly - such as the phone line being disconnected. It is possible to use a modem without the DCD signal, however the only way for the computer to know that a connection is disconnected is by the modem transmitting the words "NO CARRIER" over the data lines. Because the words "NO CARRIER" are also a message that could appear in the context of a normal data session (for example, if typed by a person on the remote end), there is no positive way for a computer program to differentiate the words being sent over the connection versus from the modem.

External modems with LED status lights usually have a light labeled "CD" (carrier detect). This status light is directly coupled with what the modem is sending the DCD line.

By default, when a modem is powered up, the DCD signal stays low. It does not go high until the modem either makes an outgoing call, or answers an incoming call, and then connects with a data modem on the other end. The signal goes high at the same time the modem reports its CONNECT message, and stays high until the call is disconnected. The signal goes low again once the local modem has lost contact with the remote modem, regardless of which side initiated the disconnect. So long as the DCD signal is high, the computer can assume that any data coming from the modem was actually sent from the remote side.