How Modbus Works?

Modbus is indeed a very old (late 1970s) protocol that was created for the communication between among programmable logic controllers, PLC. It is based on a master/slave relation. That means a device acting as master, polls slave devices to read or write information. Registers are data storage repository defined with a register reference address and are slave specific only. Old Modbus implementations are based on serial communication, but today with the spread of network based communication, Modbus also evolved to be operated on TCP/IP networks. Arrow supports TCP based modbus operations, so this document will be restricted to only network side of modbus protocol and focused on Arrow operation.

Similar to other client/server applications, modbus master sends out a request to slave address with a specific function code, register number and count. This requests inform the slave about information to be received by master. The request and response is very similar to each other and are in the following format.

There are different types of register. Function code defines what type of register information exchange will be triggered. The most common function belongs to the operation of “reading holding registers”, and it is 3 (three). This function allows the master to request one or more holding register stored values. Similarly code 16 is used to write into holding registers.