This Modbus Master protocol implementation supports only serial modes (COM1 and COM2) and measurements and binary data reading (register or coil addresses).
In a nutshell, the Modbus Master protocol runs in a single loop continuously polling all configured slave-units. The protocol instance provides a set of parameters which allow the user to setup the timing and speed constraints for the transmissions on the bus, e.g. number of retries, response delay, inter poll delay, retry interval and baud rate.
The number of slaves which can be configured is limited by the number of Modbus Master objects’ signals/function designs available in the relay. Additionally, the update frequency required for the polled data sets a limitation to the number of slaves/polls. Since the data polls are performed in a single task, the more Modbus requests the relay needs to perform, the more time it takes to finish each poll cycle round, thus slowing down the data update rate.
Modbus master objects/functions designs are based on the relay’s IEC 61850 data model. The Modbus master polling task will copy the polled data into the designated object’s/function design’s input(s). The respective output(s) can be then readout from a SCADA system over e.g. IEC 61850 or IEC-104.
The Modbus Master’s CMT provides users with the interface where the slaves units are mapped, and poll requests are added. In CMT, the polled data is parametrized e.g. scaling and offset for measurement data and optional value inversion for binary data respectively.