?WHAT IS TAG OR POINT IN SCADA/HMI/DCS
We often receive basic questions like “What is Tag”, so this is an article dedicated to our beloved newbies. Because of your support, we strive to make SCADA as easy as possible without compromising its rich functionalities.
A specific term used in SCADA, “Tag”, 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.
In contrast to I/O tag, the internal used variable is called virtual tag, alternatively runtime tag, memory tag, or simply internal tag. Some SCADAs further identity non predefined tag or on the fly creation tag as runtime tag. And some SCADAs also further identify database used tag as archive tag.
SCADA is a niche engineering product that do not have a mass market to spread its heavy development cost. And it will appears to be unaffordable for small scale projects but merely a small fraction of cost for mega projects. As such the quantity of tag used in a SCADA often relates as project sizing. And the price of a SCADA also corresponds to I/O tag count to spread its cost according to projects’ budget. Furthermore a SCADA does need more software engineering works for large scale of memory/tags handling. So it’s fairer to charge more for tag usage. Anyhow some SCADA vendors also charge on virtual or archive tag count. Some other SCADA vendors even offer unlimited tag count as sales like zenon by COPA-DATA.