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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 |format=PDF |
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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