Query DTR control setting
ATS211?
To set DTR control ON
ATS211=0
To set DTR control OFF
ATS211=1
Always assert DSR
ATS211=2
Dropping DTR gracefully powers off modem
ATS211=4
DSR Control asserted after registration
ATS211=8
Queries DTR mode setting
AT&D?
ignore DTR transitions ( S211 = 1 )
AT&D0
de-asserted DTR powers modem off ( S211 = 4 )
AT&D2
To query the current DSR control setting
AT&S?
No control of DSR. It is asserted all of the time
AT&S0
Control of DSR is enabled after modem registration.
AT&S1