Data Terminal Ready: Difference between revisions

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{{Short description|Control signal}}

'''Data Terminal Ready''' ('''DTR''') is a [[control signal]] in [[RS-232]] serial communications, transmitted from a [[data terminal equipment]] (DTE), such as a computer, to a [[data circuit-terminatingcommunications deviceequipment]] (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 ''0''"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 signals for a data modem. Dropping DTR from high to low for at least two seconds tells the modem to disconnect (end) a call or data connection.

The DTR signal is an important call control signalssignal for a data modem. DroppingAccording to the RS232 standard, dropping DTR from highactive to lowinactive for at least two seconds tells the modem to disconnect (end) a call or data connection.

===Software signaling===

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" In>{{Citation their|title=V.250 default: configuration,Serial mostasynchronous modemsautomatic dodialling notand answercontrol calls(05/99, if07/03) the|chapter=6.2.9 DTRCircuit signal108 is(data low,terminal evenready) ifbehaviour auto|publisher=[[ITU-answerT]]/Telecommunication modeStandardization isBureau enabled|url=http://www.itu.int/rec/T-REC-V.250/en |format=PDF}}</ref>

A call may also be disconnected by in-band signaling in software with ''escape codes'' to activate the modem's [[Command and Data modes (modem)|command mode]], typically "+++", followed by other modem commands to disconnect, e.g., the hangup command ("ATH" per the [[Hayes command set]]).

When a computer wants to place a call, it raisesasserts the DTR signal before sending commands. If the DTR signal is not asserted and the modem receives a ''dial'' command, modems either refuse to accept commands, place the call, or they silently disable DTR support for the duration of that call; the actual behavior depends on the modem software. Such behavior can be manually overridden or configured on most newer modems.

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. In their default configuration, most modems do not answer calls if the DTR signal is low, even if auto-answer mode is enabled.

VirtuallyOther allaspects modemsof new enoughresponses to supportchanges errorin correctionDTR andcan databe compressionmanually (alloverridden modemsor 9600 bit/s and above, and some 2400 bit/s ones as well) have the capability of modifying their use of RS-232 signals, dependingconfigured on themost application.{{Citationnewer needed|date=November 2009}} The [[AT command]] for manipulating DTR is typically <code>AT&D</code> followed by a single digitmodems.<ref>{{Citation |title=The Extended Hayes Command Set | publisher=[[KDE]] |url=http://docs.kde.org/stable/en/kdenetwork/kppp/hayes-extended-commands.html |accessdate=2009-11-23 }}</ref> AT&D0 and AT&D2 are mandatory under V.250, and AT&D1 is optional.<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> AT&D3 is non-standard but widely implemented,<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 |formaturl-status=PDFdead |archiveurl=https://web.archive.org/web/20070125091854/http://www.alphamicro.net/resources/GPRS_functional_overview.pdf |archivedate=2007-01-25 }}</ref>

When a computer wants to place a call, it raises the DTR signal before sending commands. If the DTR signal is not asserted and the modem receives a ''dial'' command, modems either refuse to accept commands, place the call, or they silently disable DTR support for the duration of that call; the actual behavior depends on the modem software. Such behavior can be manually overridden or configured on most newer modems.

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's 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.

===DTR configurability on modems===

{{Merge to| Modem| discuss=Talk:Data Terminal Ready|date=November 2009}}

Virtually all modems new enough to support error correction and data compression (all modems 9600 bit/s and above, and some 2400 bit/s ones as well) have the capability of modifying their use of RS-232 signals, depending on the application.{{Citation needed|date=November 2009}} The [[AT command]] for manipulating DTR is typically <code>AT&D</code> followed by a single digit.<ref>{{Citation |title=The Extended Hayes Command Set | publisher=[[KDE]] |url=http://docs.kde.org/stable/en/kdenetwork/kppp/hayes-extended-commands.html |accessdate=2009-11-23 }}</ref> AT&D0 and AT&D2 are mandatory under V.250, and AT&D1 is optional.<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> AT&D3 is non-standard but widely implemented,<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]] |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 |format=PDF}}</ref>

* '''AT&D''' or '''AT&D0''' - Ignore DTR signal. A call will continue regardless of the DTR line, and the only way to end the call is with the escape sequence, or if it gets terminated by the other side. This setting is only used if the computer equipment cannot provide or control DTR.

* '''AT&D1''' - Dropping the DTR signal puts the modem into Command Mode, ''without'' disconnecting the call. The computer may disconnect the call with the ATH command, or return to the call with ATO. This mode is useful if the computer wishes to change settings on the modem during the call (such as activating test modes).

* '''AT&D2''' ''(default on most modems)'' - Dropping the DTR signal will cause a disconnect. Following the disconnect, the modem returns to command mode.

* '''AT&D3''' (not supported by all modems) - Dropping the DTR signal will cause a disconnect, followed by a reset (similar to ATZ).

Many modems, especially older ones, have DIP switches that define the default DTR behavior when the modem is powered on or reset.<ref name="USR_Courier_Ch8" /> Newer modems use nonvolatile memory to hold this behavior, which can be manipulated with the AT&W command.

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's 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.

==Null modem operation==

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==Use for flow control==

On some printers with serial interfaces, the DTR line is used for hardware [[flow control (data)|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.

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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==

* [[RS-232]]

* [[Data Carrier Detect]]

* [[Ring Indicator]]