Introduction; Modbus Rtu; 4511 Supported Modbus Function Codes; 4511 Modbus Parameter Settings - PR 4222 Configuration Manual

This configuration manual
contains the necessary information for configuring a PR 4222 device which is connected to a
PR 4511 Modbus RTU enabler.
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 slaves, 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
Read Holding Registers
Read Input Registers
Write Single Register
Diagnostics
Write Multiple Registers
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).
Maximum number of registers which can be read or written at once:
For a read command, the limit is 8 registers at a baud rate up to 38,400 bps,
16 registers @ 57,800 bps and 32 registers @ 115,200 bps.
For a write command, the limit is 123 registers at baud rates up to 115,200 bps.

4511 Modbus parameter settings

Automatic Baudrate Detection:
Supported baudrates:
Parity Mode:
Stop Bits:
Response delay:
Modbus slave addressing range:
Modbus Parameter Storage:
(Factory Default Values are marked in bold)

Modbus RTU segment line termination

A 120 Ohm resistor should be installed on both ends of a RS485 Modbus RTU segment loop to
prevent signal echoes from corrupting data on the line.

INTRODUCTION

Yes - can be configured ON or OFF
2400, 4800, 9600, 19.2k, 38.4k, 57.6k, 115.2k bps
Even, Odd or None parity
1 or 2 stop bits
0...1000 ms (0 ms = default)
1 - 247 (247 = default address)
Saved in non-volatile memory in the 4511 device
4222MCM101
Function code
03
04
06
08
16
1