Sony Ericsson GS64 - Manual (Page 56)

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5.11.3.5 Data Set Ready (DSR1)
DSR indicates that the DCE is ready to receive data. The signal is active low.
Behavior is controlled using the AT&S command.
5.11.3.6 Data Carrier Detect (DCD1)
DCD indicates that the DCE is receiving a valid carrier (data signal) when low.
Behavior is controlled using the AT&C command.
5.11.3.7 Ring Indicator (RI)
RI indicates that a ringing signal is being received by the DCE when toggled low.
Users can define the exact behavior of RI with an AT command, including the option
of asserting the RI signal to flag an incoming SMS by using the AT*E2SMSRI
command.