Modbus Data Address Format

Introduction

Data in Modbus PLCs is stored different four different data tables depending on the intended use of the data. The four data tables are:

The Modbus PLC Data Address Format is a string that specifies the location of data in a Modbus PLC. 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 Read-write
Discrete Inputs 1nnnn Read-only
Input Registers 3nnnn Read-only
Holding Registers 4nnnn 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 specify the data table and the coil or register number:

Coil or Register NumberProtocol 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 just 11.

Where Used

The Modbus PLC Data Address Format is used in these contexts:

Register Address Editor

You can use the Register Address Editor build Modbus Data Address Strings. You can access it by clicking the Browse button Browse button found at the right hand side of the Register field:

Register Address Editor

Further Information

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.