Data Terminal Ready: Difference between revisions

Revision as of 09:57, 29 May 2015

Revision as of 09:45, 2 September 2016

Line 4:

==Signaling for modems==

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>

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>

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.