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Questions Related to Serial Ports

Usage of RI
Incoming messages: The voltage of RI changes from high to low (RI will output low level pulse of 120ms when receiving a message).
Incoming calls: The voltage of RI changes from high level to low level (RI will remain low level until the call is answered).
What can DCD do?

Setting AT&C1: DCD is used as the hardware pin to determine whether the module is in the data mode. DCD is the output pin. When the module is in data mode, the DCD output is low.

What can DTR do?
1) Exiting the sleep mode.
2) Exiting the data mode. The low level of DTR can be used to change from data mode to command mode in data communication.
Is it possible to realize serial communication only by TXD, RXD pin, and module?

Yes. But hardware flow control will be disabled and the module RTS pin is required to be connected to GND.

Why is unreadable code obtained from the serial communication software?

Check if the baud rate between DTE and DCE is matched.

What's the serial electric level of the module? Is the electric level switch necessary if the MCU equals 3.3V?
The working level of the module serial port is 2.97. If the MCU equals 3.3V, the electric level switch is still necessary. You can use the resistance of 100 to 200 ohms. But the level switch chip is recommended for separation.
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