Data Carrier Detect - As Used With Null Modems

As Used With Null Modems

A frequent use of a serial port is for a direct computer-to-computer connection. This requires an adapter called a null modem, which isn't actually a modem in the traditional sense, but rather a connector plug that simply crosses the complementary pins on two serial ports so the two sides can communicate. A null modem connects the DTR output of each computer to the DCD input of the other.

When used in this scenario, DCD is used to simply detect the presence and/or readiness of the other side to start a session. For example, on PCs running Windows, the DTR output is kept low until some program is run to access the serial port and raise the DTR signal high. The remote side will sense this as the DCD input going high. Some equipment will recognize the transition alone as the beginning of a session. Other equipment (such as the console port of a router) may expect characters to be transmitted, but the DCD signal high is still a prerequisite for every communication.