The DTR signal is an important call control signal for a data modem. According to the RS232 standard, dropping DTR from active to inactive for at least two seconds tells the modem to disconnect a call or data connection. When a modem is being used for automatic answering, the DTR signal confirms to the modem that the computer is available to accept a call. When a computer wants to place a call, it asserts the DTR signal before sending commands. If the DTR signal is not asserted and the modem receives a dial command, modems either refuse to place the call, or they silently disable DTR support for the duration of that call; the actual behavior depends on the modem software. Other aspects of responses to changes in DTR can be manually overridden or configured on most newer modems. and higher values are used by some vendors. Many external modems have LED indicators on the front, one of which is TR. This light follows the state of the DTR pin. The light is on when DTR is high, and off when it is low. Modems will typically keep the TR light illuminated when the AT&D0 command is used to force the modem to ignore the DTR signal, regardless of the pin's actual state.
When a serial connection is made between two computers using a null modem adapter, the DTR and the Data Carrier Detect lines are typically paired. This allows both ends of the connection to sense when the connection is active. On many operating systems, including Windows, the DTR line is held low while the serial port is unused and not being controlled by any applications.
On some printers with serial interfaces, the DTR line is used for hardware flow control, similar to the use of RTS and CTS for modems. This practice is not consistent; other printers define RTS for this same purpose. When DTR is used for flow control, it manages the flow of data from the printer to the computer. However, because during printing, the bulk of the data is from the computer to the printer, the importance of flow control in the opposite direction is minimal.
Use as a power pin
On some hardware the DTR line may be used to provide power. The most notable example of this is a serial mouse. The DE-9 serial port on the PC does not provide any dedicated power source. The mouse driver holds the DTR and RTS lines high at all times so that the device has a source of power. Another category of devices commonly powered by the DTR line includes converters between RS-232 and other serial standards such as RS-422 and RS-485.
