Modbus Register Block Tag

Introduction

The Modbus Register Block Tag Register Block Tag Icon specifies a region of Modbus PLC memory and how often to read the data from the PLC.

Configuration Properties

The Modbus Register Block Tag supports the configuration properties of the Word Register Block Tag plus these properties:

Property	Type	Version	Description
ScanInterval	TIME	1.0	The interval between each read. See Interval and Offset for information on how timed activities are scheduled.
ScanOffset	TIME	1.0	The offset added to the ScanInterval to get the actual read time. See Interval and Offset for information on how timed activities are scheduled.
Device	Reference	1.0	The Modbus device where the register block is located.
Register	STRING	1.8	The address of the first coil or register in the Modbus PLC. See Modbus PLC Data Address Format for information on how this field is formatted. Click the Browse button to access the data address editor.
RegisterCount	UINT	1.0	The number of registers to read. If Register specifies single bit registers, RegisterCount is the number of bits to read. If Register specifies word registers, RegisterCount is the number of words to read.

Modbus Function Codes

Data in Modbus devices is stored in one of four data tables:

Single bit read-write registers,
Single bit discrete inputs,
16-bit read-write registers,
16-bit input registers.

The Modbus protocol uses different function codes to access each data table.

The value of the Register configuration property defines which Modbus Function codes Fernhill SCADA uses to read or write to the device. This table shows which Modbus function codes are used to read or write to the different data tables:

Register	Read Function Code	Write Function Code
Single bit read-write registers	Read Coils (01)	Write Single Coil (05)
Single bit discrete inputs	Read Discrete Inputs (02)	Not applicable
16-bit read-write registers	Read Holding Registers (03)	Write Single Register (06), Write Multiple Registers (16)
16-bit input registers	Read Input Registers (04)	Not applicable

Tag Commands

The Modbus register block supports these tag commands:

Tag Command	Version	Description
Refresh	1.0	Requests an immediate read of the data from the PLC.

Note: Event triggered data reads (from the Refresh tag command) and periodic data reads (defined by the ScanInterval property) are independent. You can use the Refresh tag command, or the ScanInterval property, or both to determine when the data is read from the PLC.

Relationships to Other Tag Types

The Modbus Register Block Tag builds on the features of these tag types:

Further Information

Interval and Offset

To learn how activities are scheduled using Interval and Offset.

Modbus PLC Data Address Format

For a description of the Modbus Protocol and how Modbus addressing works in Fernhill SCADA.

I/O Analog Data Point Tag

To learn about the I/O Analog Data Point Tag that can be associated with the Modbus Register Block Tag.

I/O Digital Data Point Tag

To learn about the I/O Digital Data Point Tag that can be associated with the Modbus Register Block Tag.

I/O Word Data Point Tag

To learn about the I/O Word Data Point Tag that can be associated with the Modbus Register Block Tag.

I/O Double Word Data Point Tag

To learn about the I/O Double Word Data Point Tag that can be associated with the Modbus Register Block Tag.

Modbus Driver

For an overview of the Modbus Driver.

Word Register Block Tag

To learn about properties common to all Word Register Block Tags.

Glossary

For the meaning of terms used in Fernhill SCADA.