Common Modbus OPC Items
- Michael Standal
- Emma Fullilove (Deactivated)
Standard Items
See Device Standard Items.
Individual Protocol Items
Each Modbus extension has it's own list of specific OPC Items which can be located under their topic in the navigation.
Common Protocol Items
The Modbus protocol supports register numbers as item names. To access a register, client applications specify the register number that is within one of the register ranges in the selected register set for the device. Item names can be specified as in the table below:
| Item | Data Type | Description |
|---|---|---|
| N | Specified in register range properties | Retrieves the register N from the primary or secondary connection. |
| BlockStart-BlockEnd | UI2 | Use this item to retrieve a block of registers and write them to a file. The actual item name to use is the first register number in the block, followed by a dash (‘-‘) and then the end register in the block. For example, the item ‘40001-41000’ will read one thousand registers and store them in the file. Set the item’s value to any positive non-zero value to start the read. The item’s value will be updated with the number of messages required to retrieve all the registers and will be updated with the count of remaining messages as it retrieves the registers. When the item’s value reaches zero (0), the retrieval is complete. If the item’s value becomes a negative number, an error occurred, and not all of the registers were successfully retrieved. The register data will be written to a file, one line per poll message response. The file name format is <device_name><BlockStart>-<BlockEnd>. For example, registers read from block 40001-41000 from a device called RTU will be stored in the file ‘RTU40001-41000.txt’. Note: Appending an "S" will convert the value to I2. |
Extended Items
The following extended items are common to all Modbus protocol devices and extensions in ACM:
| Item | Data Type | Description |
|---|---|---|
| LastTimeSync | Date | Date and time of the last RTU time synchronization. |
| LastTimeSyncResult | I4 | Result of the last RTU time synchronization. 0 = In progress 1 = Success 2 = Alarm condition (timestamps are out of range) 3 = Error (failed to synchronize) |
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