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10. Modbus connection
10.1. Modbus background
Modbus is a master-slave protocol, which means there are two types of Modbus device, Modbus Masters and Modbus
Slaves.
Slave devices simply wait until they receive a command from a Master, act upon that command and send a reply to the
Master. Slaves do not have the ability to send commands to other devices on the bus. Master devices are responsible
for sending commands to slave devices and receiving data. Modbus only permits there to be one Master device on the
bus at any one time, but up to 247 slaves can be connected at a time.
Modbus is most commonly used over RS-485, which is a hardware standard allowing multiple devices to be connected
on the same bus.
Each Slave device must have a unique ID on the bus, which is referred to as a Slave ID. Each Modbus command the
Master sends will contain this Slave ID and only the Slave with that Slave ID will reply.
10.2. Modbus registers
Modbus Slave devices store data in registers. There are four register types and each type has its own register bank.
The register types are summarised below:
Register Name Register Type Description
Discrete Input Digital Input
Read only register used for holding status information which holds
a value of 0 or 1.
Coil Digital Output Read and write accessible register which holds a value of 0 or 1.
Input Register Analogue Input
Read only register used for status information which holds a 16-bit
value (0-65535)
Holding Register Analogue Output
Read and write accessible register used for status information
which holds a 16-bit value (0-65535)
10.3. Modbus connections
For communication over RS-485 all 3 connections are needed. These are labeled A, B and GND. Please refer to the
connection diagrams below.
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