Can't RS232 serial port communicate when the communication distance is greater than 15m?

That's true.

If it is a standard RS232 serial port, but it is only transmitted through Tx Rx Gnd3 line, you can use this.

Please see Wuhan Hongwei Optoelectronics.

E232Y RS-232 Passive Isolated Remote Transceiver

trait

Up to 95% serial stealing technology, without external power supply.

Current loop transmission, support TX, RX, GND three-wire serial port.

Signal isolation, transparent transmission, baud rate adaptation, no need to change user protocol.

Built-in 600W/ms lightning protection and 15KV electrostatic protection.

Avoid circuit voltage, surge, induced lightning, static electricity and hot plug damage to equipment.