Modbus Communication Down

[seokgi]

MCU : PIC24FJ512GA606
Communication : Modbus Slave on RS-232

This is an RS-232-based Modbus slave mode.
I'm using a Flash EEPROM component. The Flash EEPROM works fine. When I download a HEX file, communication works fine for a while, then stops. The digital output for monitoring works fine.
The same issue persists even after removing all other routines.
The device I compiled and installed previously on the same board (6/24/2025) works fine. After a reset, it works fine again, but then communication stops.
The same issue occurs with FC10 and FC11.

Please help.
Thank you.