What is Modbus?

Modbus, originally created by Modicon, is a fieldbus that allows a master and one or more slave

devices to share data. These data are organized into 16-bit registers, which can also be used to share

information single-bit I/O points.

It is a popular protocol with PLC vendors due to its simplicity and the inherent ease of sending PLC

The master may initiate read and write operations on single registers or blocks of registers. While

there is no rule to this effect, it is common for the master to read and write on a periodic time base

(polling), rather than sending and requesting data only w hen it is needed. In this manner, PLC register

data is ensured to be valid and consistent as a representation of the slave device’s status.

The Modbus products of MOONS’ are based on serial communication bus with Modbus/RTU.

Since Modbus is a master/slave protocol, that means only one node is a master and the others is slave

node. Each device intended to communicate using Modbus is given a unique address. In serial

networks, only the node assigned as the Master may initiate a command.

A Modbus command contains the Modbus address of the device it is intended for. Only the intended

device will act on the command, even though other devices might receive it (an exception is specific

broadcast able commands sent to node 0 which are acted on but not acknowledged). All Modbus

commands contain checksum information, to allow the recipient to detect transmission errors. The

basic Modbus commands can instruct an RTU to change the value in one of its registers, control or

read an I/O port, and command the device to send back one or more values contained in its registers.

Wiring

Modbus/RTU:

Modbus/RTU uses the standa rd RS-2 32 or RS-485 physical layer.

RS-232 is a point-to-point communications scheme, and as such the largest possible network would

consist of a single slave drive. Please note that even though it will be the only device on the “network”,

it will still require an address. This address may be an integer value fr om 1-32, and is set through the

configuration software (ST Configurator, STB Configurator, Step-Servo Quick Tuner or M Servo Suite)

during initial configuration.

For drives with RS-485 communications, there are a few things to consider.

It is possible to use 2-wire RS-485 for operational communication over Modbus, however 4-wire

RS-485 is required for use with all MOONS’ configuration and programming software. As such, we

recommend that all RS-485 networks be constructed using the 4-wire method.

Be sure to consult your drive’s hardware manual for specific wiring details.

Data Encode

Big-endian: The most significant byte (MSB) value is stored at the memory location with the lowest

address; the next byte value in significance is stored at the following memory location and so on.

For example: To store a 32bit data 0x12345678 into register address 40031 and 40032. 0x1234 will be

defined as MSB, and 0x5678 as LSB. With big-endian system

When transfer 0x12345678, the first word will be 0x1234, and the second word will be 0x5678