WebAccess uses two concepts of tags used in communication with plant floor devices: Tags and Blocks. Both IO Tags and Blocks describe real-time measurements and outputs (reads and writes) between the SCADA node and automation devices (PLCs, controllers, DDC systems, etc).
Tags are easiest to understand and are frequently used in small projects.
Blocks are productivity tools, that require significant planning, but can dramatically reduce the time to implement a large automation project or repetitions of the same automation project.
Tags can represent analog, discrete or text type data. Discrete type data is also called digital. Analog data is a usually a floating point number. Discrete data is an Integer between 0 and 7, but is often only 0 or 1 (hence the term digital). Text type data is an ASCII string up to 72 characters.
Parameters are the individual elements of a Block. Users reference the data in a block by appending the parameter to the block name: Blockname:Parameter.
For example, to read the setpoint of flow controller, type FIC101:SP in the Point Info Dialog box.
IO Tags and Blocks are built from template type objects: Parameter Types and Block Types.
Tag Fields describe the multiple dimensions of a Tag including description, Alarm State, Alarm Limits, Output Limits, Engineering Units, Span Hi, and Span Lo. Tag fields are read by appending the Field name to the Tagname. The format is:
Tagname.FIELD or Blockname:paramter.FIELD
For example, to read the high output limit of the setpoint in the above example, type FIC101:SP.OUTPH
Many Tag Fields are Read/Write and can be used to adjust Alarm Limits during Run time, change setpoint limits etc.
WebAccess provides Internal Tags that do not read IO directly: Calculation Tags, Accumulation Tags, and Constant Tags . The internal tags are used for calculations and for constants used by scripts or calculations. Internal Tags have all the alarm and security features of IO Tags. These Internal Tags are available to every other tag, script, display, and client in the system.
Local Screen Tags are temporary internal tags used by scripts and graphic displays. Typically, Local Screen Tags hold intermediate values or enhance display animation. Local Screen Tags disappear when the display they are assigned is closed.
%DAQ System Tags and System Point Tags are used to access system data like Time, date, Alarms, communication status.