How to modbus communaciton error read

I read the arduino valuse from PC with RS485. I need to for example RS485 communaciton timeout arduino 1 digital output set?How do I know if it is a mistake in communication?
Also I use the simplemodbus library...
Can you help me?

Provide much more information about your project! What hardware, what feature do you want to achieve?

The ModBus standard has two provisions to establish a reliable communication: it uses a parity bit in the serial communication and it has a checksum for every message transmitted.

Provide a link to the library you're using!