Instructions

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2.5.3 Control Signals – RTS, CTS, DTR, DSR
Request to Send (RTS)
RTS is used to condition the DCE for data transmission. The default level is high by internal pull up. The exact
behaviour of RTS is defined by an AT command. Software or Hardware control can be selected. Hardware flow
is the default control. The application must pull RTS low to communicate with the modem. The modem will
respond by asserting CTS low, indicating it is ready for communication.
Clear To Send (CTS)
CTS indicate that the DCE is ready to transmit data. The default level is high. You can define the exact
behaviour of CTS through an AT command, and can select software or hardware flow control.
Data Terminal Ready (DTR)
DTR indicates that the DTE is ready to transmit and receive data. It also acts as hardware ‘hang-up’,
terminating calls when switched high. The signal is active low. You can define the exact behaviour of DTR with
an AT command. The DTR line can also be used to switch on the modem when activated for 0.2 seconds. The
DTR line must be deactivated prior to switching off the modem to ensure it switches off (powers down)
correctly.
Data Set Ready (DSR)
An active DSR signal is sent from the modem to the application (DTE) to confirm that a communications path
has been established. DSR has two modes of operation, settable using the AT commands AT&S.
Data Carrier Detect (DCD)
DCD indicates that the DCE is receiving a valid carrier (data signal) when low. You can define the exact
behaviour of DCD with an AT command.
Ring Indicator (RI)
RI indicates that a ringing signal is being received by the DCE when low. You can define the exact behaviour for
RI with an AT command.