As for RS232 pin ( draw forth IO port) to measure level
RS232 is negative logic, TXD and RXD idle state logic is 1, voltage at 3~-15V, conform to definition.
RTS: Request send, module output pin, valid mean can received
CTS: Clear send, module received pin, valid mean can send
Both side is OK, RTS, CTS level is 3-12V, TTL level is 0, logic is 0
In other word, MCU output 0, can receive or send
Output 1, disable send
Use one serial port software which including hardware flow control, as below:
If serial port receive buffer will full or empty, module RTS will change accordingly.