PR Elecronics 4801, Installation And Connection Manual

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4801 V100-UK

Modbus basics

Modbus is a “master-slave” system,

where the “master” communicates with one or multiple “slaves”.  

The master typically is a PLC (Programmable Logic Controller), DCS (Distributed 

Control System), HMI (Human Machine Interface), RTU (Remote Terminal Unit) 

or PC.   

The three most common Modbus versions used are: MODBUS ASCII, MODBUS 

RTU and MODBUS/TCP.

In Modbus RTU, data is coded in binary, and requires only one communication 

byte per data byte. This is ideal for use over multi-drop RS485 networks, 

at speeds up to 115,200 bps. The most common speeds are 9,600 bps and 

19,200 bps. Modbus RTU is the most widely used industrial protocol and is 

supported by the 4511.

Modbus RTU:

To communicate with a slave device, the master sends a message containing:

Device Address - Function Code - Data - Error Check

The Device Address

 is a number from 0 to 247.

Messages sent to address 0 (broadcast messages) will be accepted by all sla-

ves, but numbers 1-247 are addresses of specific devices. With the exception 

of broadcast messages, a slave device always responds to a Modbus message 

so the master knows the message was received.

4511 Supported Modbus Function Codes:

Command

Function code

Read Holding Registers

03

Read Input Registers

04

Write Single Register

06

Diagnostics

08

Write Multiple Registers

16

The Function Code

 defines the command that the slave device is to execute, 

such as read data, accept data, report status. Some function codes have sub-

function codes.

The Data

 defines addresses in the device’s memory map for read functions,

contains data values to be written into the device’s memory, or contains other 

information needed to carry out the function requested.

The Error Check

 is a 16-bit numeric value representing the Cyclic Redundancy

Check (CRC).