" RS485 serial " or " Modbus Rtu " protocol

RS-485 is just the means of the electrical transmission (it is a balanced signal), Modbus is the protocol at which the DATA is transmitted across the line.

You can transmit/receive Modbus across various electrical schemes
RS-232
RS-485/488
Ethernet
 
Modbus RTU has specific timing requirements that define the size of a communications message 'packet' and calculates the CRC error checking value.

Converters like these generally have a specific Modbus model with firmware that handles Modbus packets. The firmware is specfically designed to keep a Modbus message packet intact, without splitting it up, it constructs a proper Modbus packet from other data with the correct header and byte count values, and it calculates the CRC error check value for the Modbus packet.

The generic RS-485 is just that, RS-485. It does not hold to Modbus packet size and timing requirements or calculate a CRC value. It just transmits and receives on 485.
 

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