Tc35i module and the problem of sending and receiving short messages by single chip microcomputer

Pay attention to a major problem. The power supply of tc35i should be 4.2V, and the peak current should be 2 amps. 2576-adj is recommended.

The RXD and TXD you mentioned are not connected to any pin of the serial port, which is for sure, because there is a 232-level conversion chip in the middle, which is not directly connected. RXD and TXD of tc35i are ttl levels, which are different from 232 levels.

Please refer to the pictures I posted for the connection of other module components.

I didn't share it, so let's just say so much. Think more and experiment more.