Beluk BLR-CM Modbus, Technical Documentation Manual

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Technical Documentation
BLR-CM Modbus Rev. 05
2018-09
8
2.2 The Modbus protocol
2.2.1 Modbus - description
The Modbus protocol uses the RS485 as an underlying physical layer and implements the data transmission control
mechanisms. Therefore, it is located on layer 2 ("link layer") of the OSI layer model for data exchange systems.
2.2.2 Serial data format
The data is transmitted in fixed frames. The frames are separated by the bus being inactive for at least 3,5 characters.
All data is organized in "protocol data units" (PDUs), which are transmitted over the serial bus system by the underlying
physical protocol layer.
PDU
FC data
1 byte n bytes
Illustration 1 : "Protocol Data Unit" - PDU
The PDU consists of two parts:

The "function code" (FC) is a command, which defines how the slave has to respond.

The data block contains information, which correspond to the FC. Its usage depends on the FC, it can either
contain data or register addresses for slave data access.
The PDU defines a single data unit, which has to reach a certain bus device in order to perform an action. The type of
the transfer differs dependent on the physical layer.
To be able to control the transmission, the PDU is extended by two additional blocks. In RS485, the frame is now called
"application data unit" (ADU).
ADU
slave # FC data CRC
1 byte 1 byte n bytes 2 bytes
Illustration 2 : "Application Data Unit" - ADU