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A tag is a logical name that represents a variable in a network device or in a computer’s memory (RAM), simply means variable in computer language. SCADA is a rapid development application for industrial control & monitoring. The nature of the system requires SCADA to communicate with huge range of external devices like PLC & industry specific equipments/meters. In order to link up and access with these devices, various open or proprietary communication protocols can be used. And these devices need to have a mechanism to open up its parameters or data for SCADA to access, either for read, write or both.
Normally the data will be given in the form of addresses or registers, but also possible being given directly in the form of variable string depending on the protocol used. As such the SCADA must assign a variable or memory location to be associated with each and individual interested parameter or data that provides by any device, regardless of hybrid or integrated setup. And this variable is called “Tag”. More specifically, it’s called I/O tag because that’s the only input & output channel of a SCADA. Another name for it is external/physical tag simply because that’s external/physical device out there to access.
HMI tags provide scale and offset capabilities, which are not supported by some data servers. Use scale and offset to modify the raw data that comes from the network device before it is saved in the computer’s memory, or to modify a value specified in FactoryTalk View before it is written to a device or data server. Scale is a multiplication factor — the value from the device is multiplied by the scale. Offset is a fixed value — after the value from the device is multiplied by the scale, the offset amount is added.
Each time a tag is updated a time stamp is added to the value. This allows Eru to have a historian that knows when a specific tag was updated to create charts and trends.
Sometimes you do not want to show the raw value in the tag value result but a custom string instead. Let's say you want to display ERROR when the read value of a height of a windows is nonsense, well you can easily do that using a value map.