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{{Short description|Control signal}}
'''Data Terminal Ready''' ('''DTR''') is a [[control signal]] in [[RS-232]] serial communications, transmitted from [[data terminal equipment]] (DTE), such as a computer, to [[data communications equipment]] (DCE), for example a [[modem]], to indicate that the terminal is ready for communications and the modem may initiate a communications channel.
The DTR signal is present on pin 20 of the 22-wire RS-232 interface using a [[DB-25]] connector, and on pin 4 of a newer [[DE-9]] serial port. The signal is asserted (logic "1") by raising the voltage of the pin from negative to positive. Dropping the signal back to its negative state indicates to the modem that the communications session shall be terminated.
==Signaling for modems==
{{further|Command mode and Data mode#Switching between modes}}
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 (end) a call or data connection.
When a modem is being used for automatic answering (such as with the command ATS0=1), the DTR signal confirms to the modem that the computer is available to accept a call.<ref name="V250_6_2_9" >{{Citation |title=V.250 : Serial asynchronous automatic dialling and control (05/99, 07/03) |chapter=6.2.9 Circuit 108 (data terminal ready) behaviour |publisher=[[ITU-T]]/Telecommunication Standardization Bureau |url=http://www.itu.int/rec/T-REC-V.250/en |format=PDF}}</ref>
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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.<ref name="USR_Courier_Ch8">{{Citation |title=Courier V-Everything Command Reference |publisher=[[U.S. Robotics]] |url=http://www.usr.com/support/3453b/3453b-crg/chap%208-controlling%20EIA-232.htm |chapter=8. Controlling EIA-232 Signaling |accessdate=2009-11-23 }}</ref><ref name="MS_PnP_1995" >{{Citation |title=PnP for COM Devices, rev 0.92 |format=RTF |publisher=[[Microsoft]] and [[Hayes Microcomputer Products|Hayes]] |url=http://download.microsoft.com/download/1/6/1/161ba512-40e2-4cc9-843a-923143f3456c/pnpcom.rtf |date=February 28, 1995 |chapter=A.4 AT&D3 Implementation Issues }}</ref> and higher values are used by some vendors.<ref>{{Citation |title=Alpha Micro GPRS Modem Functional Overview 1.0 |url=http://www.alphamicro.net/resources/GPRS_functional_overview.pdf |accessdate=2009-11-23 |year=2003 |author=Nick Robins |publisher=Alpha Micro Components |
Many external modems have LED indicators on the front, one of which is '''TR''' (''"terminal ready"''). 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.
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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]].
==Use for transmit keying==
In many [[radio modem]] implementations, especially in amateur radio, the DTR is used to control transmission. The radio receives when DTR is not asserted, typically passing audio in to a software modem. When DTR is asserted the radio transmits audio coming from the software modem. In cases where the radio is controlled by serial commands (e.g. a [[computer aided transceiver]] interface) the control data flows regardless of DTR state.
==See also==
* [[Ring Indicator]]
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