As for RS232 pin ( draw forth IO port) to measure level

GND reference

TXD -4.92V
RXD -10.21V
RTS +5.36V
CTS +10.65V

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.