Siemens Modbus RTU is an open, serial (RS-232 or RS-485) protocol based on master/slave or client/server architecture. The protocol interconnects field equipment such as sensors, actuators, and controllers and is widely used in both process and manufacturing automation. The fieldbus environment is the base level group of digital networks in the hierarchy of plant networks.

The SITRANS F Modbus RTU Communication modules comply with the MODBUS Serial Line Protocol. Among other things this implies a Master-Slave protocol at level 2 of the OSI model. A node (the master) issues explicit commands to one of the slave-nodes and processes responses. Slave nodes will not transmit data without a request from the master node, and do not communicate with other slaves.

Modbus is a mono Master system, which means that only one Master can be connected at the time.