I managed to partially solve the problem.
by inserting this c block:
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// UART1 - PIC18F45K22 - 9600 bps - 4 MHz - 8N1

// Imposta modalità asincrona
TXSTAbits.SYNC = 0; // Asynchronous mode
TXSTAbits.BRGH = 1; // High speed baud rate ...

Good morning Mario,
I made a modbus rtu communication program with a pic series 16, but when I reciprocated it on a series 18f45 K..., the communication didn't work.
Could you show me how you managed to implement the communication ports?
Thanks
Luigi

GOOD MORNING, I MADE A MODBUS COMMUNICATION PROGRAM OVER RS232 THAT WHEN COMPILED ON THE 16F887 WORKS PERFECTLY.
WHEN I RECOMPILE IT FOR THE PIC18F45K22, THE COMMUNICATION DOESN'T WORK AT ALL.
COMPARED TO THE PROGRAM THAT WORKED ON THE PIC 16F887, I ADDED THE UART PORT INITIALIZATION.
BUT IT DOESN'T ...