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Modbus RTU - Instance & Item setup

Written by Benjamin Pinnerup

Updated at February 21st, 2024

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Table of Contents

Instance Item

Articles in this section:

  • Modbus TCP - Instance & item setup
  • Word (endianness) & Byte-Swapping
  • Modbus RTU - Instance & Item setup

 

 

Instance

The parameters specific to the Instance setup are related to the initial connection with the Modbus RTU server

Parameter Description
Address

The address of the Modbus RTU device

Integer in range 0-240

Timeout

Timeout in milliseconds

Default: 5000

Max Read Registers

Maximum amount of registers read in a single message

Note: Some servers allow for specific numbers of registers read in a single message. If this is exceeded, the connection could be refused

Default: 123

Write Preset

Chooses which function code used for writing

  • Multiple registers: 0x15 / 0x16
  • Single register: 0x05 / 0x06

Default: Multiple registers

Endianness

Endianness is documented in this article

Default: Big endian

Byte Order

Endianness is documented in this article

Default: First byte high

Interframe delay

Interframe delay in milliseconds

Default: 200

Serial mode

Choose whether you use RS485 or RS232 as a communication method

Default: RS485

Device Interface

Choose which port the Modbus Server is connected to

Default: RS485

Baudrate

Choose the baudrate used by the Modbus Server

Default: 9600

Parity

Choose the parity used by the Modbus Server

Default: None

Databits

Choose the databits used by the Modbus Server

Default: 8

Stopbits

Choose the stopbits used by the Modbus Server

Default: 1

 

Item

Parameter Description
Register

Registers are the addressing syntax used by Modbus.

Depending on the register type, chosen below, there are two ways of addressing.

Both 5-digit and 6-digit registers are supported 

When Register Type is Auto

Here we use the full address of the register. 

The first Holding Register will be 40001 or 400001

When Register type is Holding Register (or any other type)

Here SIA Connect will automatically fill out the full register syntax.

The first Holding Register will be 0

Register Type

Register type defines the method of auto-filling the full register syntax

  • Auto
    Use the full address syntax for the register. 4000X for Holding registers, etc.
  • Holding Register
    SIA Connect will automatically fill out the full register syntax.
    Holding Register 0 equals Register 40001
  • Input Coil
    SIA Connect will automatically fill out the full register syntax.
    Input Coil 0 equals Register 10001
  • Output Register
    SIA Connect will automatically fill out the full register syntax.
    Output Coil 0 equals Register 00001
  • Input Register
    SIA Connect will automatically fill out the full register syntax.
    Input Register 0 equals Register 30001
Type

Chooses both the amount of registers being read, and how they are parsed as data.

If a tag is over 1 register, this is where it is chosen. Nothing changes on the addressing syntax, as it is automatically handled by SIA Connect

Floating Point Precision Defines the decimal precision of Floats 

 

rtu protocol modbus communication

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