reads a region of Modbus Device memory.
The Modbus Register Block Tag
reads a region of Modbus Device memory.
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 start address in the Modbus Device.
The format of this property depends on the DataAddressFormat property of the
Modbus Device Tag.
The options are:
Click the Browse button  to open the Data Address Editor.
|
| RegisterCount | UINT | 1.0 |
The number of registers to read. If Register indicates single bit registers, RegisterCount is the number of bits to read. If Register indicates word registers, RegisterCount is the number of words to read. |
Data in Modbus devices is stored in one of four data tables:
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 |
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.
The Modbus Register Block Tag builds on the features of these tag types:
To learn how activities are scheduled using Interval and Offset.
To learn how to construct standard Modbus data address strings.
To learn how to construct Enron Modbus data address strings.
To learn how to construct DirectLogic data address strings.
To learn about the I/O Analog Data Point Tag that can be associated with the Modbus Register Block Tag.
To learn about the I/O Digital Data Point Tag that can be associated with the Modbus Register Block 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 DriverFor an overview of the Modbus Driver.
Word Register Block TagTo learn about properties common to all Word Register Block Tags.
For the meaning of terms used in Fernhill SCADA.