Modbus Connection; Modbus Background; Modbus Registers; Modbus Connections - Procon MelcoBEMS MINI Installation Manual

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
Discrete Input
Coil
Input Register
Holding Register Analogue Output
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.
Caution:
The RS-485 cable must be a shielded data cable. Mains flex or other unshielded cable should not be used. The cable
shield should be connected to GND at one end only.
Caution:
RS-485 has polarised data connections. It is crucial that all 'A's are connected together, all 'B's are connected together
and all 'GND's are connected together.
Caution:
The RS-485 cable must be daisy-chained in a bus network. T-junctions (e.g. star network wiring) are not permitted.
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Register Type
Read only register used for holding status information which holds
Digital Input
a value of 0 or 1.
Digital Output Read and write accessible register which holds a value of 0 or 1.
Read only register used for status information which holds a 16-bit
Analogue Input
value (0-65535)
Read and write accessible register used for status information
which holds a 16-bit value (0-65535)
Description