DSR (data set ready) means that the data is ready. It is a handshake signal in RS232. When the modem is started, DSR is used to declare that it is ready after self detection. Therefore, it is not only the output of the modulator/demodulator, but also the input of the DTE (Data Terminal Equipment), and the signal is low level effective.
The signal will remain invalid to indicate to DTE that it cannot receive and send data, regardless of any reason that the modem cannot connect to the telephone.
The DSR indicator light indicates the operation of MODEM on the DSR signal. Under normal circumstances, the DSR indicator light is on, indicating that the MODEM has been connected to the telephone line and is ready to receive signals from the telephone line.
If the DSR signal is set to forced high, the DSR light will be on as soon as the MODEM is powered on and will remain on until the power is cut off.
When MODEM is set to CTS/DSR hardware data flow control mode, the DSR indicator will reflect the data flow and stop during data transmission.