Modbus RTU is a common type of serial communications protocol on instruments fitted with RS485 or RS232 communications.
Modbus RTU is a master/slave protocol and only the master can initiate communications. Each slave is given a unique address on the network, and the messages from the master contain the address of the intended slave.
Only this slave will act on the command, even though other devices might receive it (an exception is specific broadcast messages sent to address 0 which are acted upon by all slaves but not acknowledged).
The messages can instruct the slave to change a value in one of its memory registers, or ask it to send back one or more values contained in the registers. The Modbus RTU message format includes a cyclic redundancy checksum (CRC) to ensure that the message arrives undamaged.
Some instrument can act as a Slave only, while others can be master or slave. A typical use for a controller with Modbus master communications it to continuously send its setpoint value using broadcast messages to other slave controllers.