Modbus Data Address Format

Introduction

In Standard Modbus Devices, data is stored different four different data tables depending on the intended use of the data:

Output Coils
Discrete Inputs
Holding Registers
Input Registers

The Modbus Data Address is a text string that defines the location of data in a Modbus Device. Fernhill SCADA uses the text string to locate items of data in the Modbus Device.

Standard Modbus Data Address

The address format follows the Modicon PLC convention of a data table number, followed by a 1-based number. The 1-based number defines the coil or register number:

Data Table	Format	Notes
Output Coils	0nnnn	Bit values, read-write
Discrete Inputs	1nnnn	Bit values, read-only
Input Registers	3nnnn	16-bit word values, read-only
Holding Registers	4nnnn	16-bit word values, read-write

Coil Number Register Number and Data Offset

The Modbus Protocol supports a number of commands to read and write data. These commands use a 16-bit 0-based number to define the location in the data table. The 16-bit number gives an address range of 0-65535.

The Modicon numbering convention uses 1 to indicate the first coil or register, 2 to indicate the second, and so on.

The Modicon numbering convention is offset from the Modbus protocol by 1. This can sometimes cause confusion. Traditional Modicon coil and register numbering use 5 digits to choose the data table and the coil or register number:

Coil or Register Number	Protocol Data Offset	Table Name
00001 - 09999	0000h-270Eh	Output coils
10001 - 19999	0000h-270Eh	Discrete inputs
30001 - 39999	0000h-270Eh	Input registers
40001 - 49999	0000h-270Eh	Holding registers

The traditional Modicon 5 digit addressing restricts the protocol to the first 9999 coils or registers.

To access the full range of data allowed by the Modbus protocol, 6-digit addressing is sometimes used:

Coil or Register Number	Protocol Data Offset	Table Name
000001 - 065536	0000h-FFFFh	Output coils
100001 - 165536	0000h-FFFFh	Discrete inputs
300001 - 365536	0000h-FFFFh	Input registers
400001 - 465536	0000h-FFFFh	Holding registers

Fernhill SCADA supports variable length Modbus addressing. For example the first discrete input can represented by 100001, 10001, 1001, 101, or 11.

Where Used

The Standard Modbus Data Address is used in these contexts:

In the IOItemName property of an I/O Analog Data Point Tag when the tag is associated with a Modbus Device.
In the IOItemName property of a I/O Digital Data Point Tag when the tag is associated with a Modbus Device.
In the IOItemName property of a I/O Word Data Point Tag when the tag is associated with a Modbus Device.
In the IOItemName property of a I/O Double Word Data Point Tag when the tag is associated with a Modbus Device.
In the IOItemName property of a I/O Long Word Data Point Tag when the tag is associated with a Modbus Device.
In the Register property of a Modbus Register Block.
In the DataAddress parameter of the DirectWriteBit Tag Command when used with a Modbus Device.
In the DataAddress parameter of the DirectWriteWord Tag Command when used with a Modbus Device.

Data Address Editor

The Data Address Editor is a convenient tool to build a Standard Modbus Data Address String.

To open the Data Address Editor, click the Browse button of the IOItemName or Register property:

The Standard Modbus data address editor.

Further Information

Modbus Device Tag

To learn about Modbus Device Tags.

Modbus Register Block Tag

To learn about Modbus Register Block Tags.

Modbus Driver

For an overview of the Modbus Driver.

Glossary

For the meaning of terms used in Fernhill SCADA.