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==Signaling for modems==
The DTR signal is an important call control signal 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.
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> ▼
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 |format=PDF}}</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.
===DTR configurability on modems===
▲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 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 |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 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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