Rockwell Automation T6200 Compressor Anti-Surge and Capacity Controller - Manual (Page 181)

Data Structures and Expressions
A-3
Tags
Tags are user or factory defined symbols or code names. Tags are assigned to system variables
and are used to access and/or alter the value or status of a variable or it may be used just as a
reference to an item, such as a table.
Tags should have from 1 to 16 alphanumeric, and/or underscore(_) characters. Tags cannot
contain spaces (blanks) or other symbols. The first character of a tag cannot be a dash or number.
Both upper and lower case letters are accepted but A/S VIEW will convert lower case letters to
upper case for processing.
Tags assigned to variables by the user are global and can be accessed from any Controller or
Operator Interface within the system network. Tags in the firmware are assigned at the factory in
each Controller. These tags are local to the loop and cannot be accessed from outside of the loop
that they are assigned to, without the user assigning a prefix to the tag. A tag can be assigned by
the user to only one variable within a system network and cannot be assigned to another variable
in a different loop or Controller.
A tag assigned in the firmware can be accessed from outside of the loop by adding an extension to
the user assigned loop tag and separating them with a period(.).
For example, in a loop where the user has assigned FIC254 as the tag for the loop, the firmware
assigned tag PV (process variable) can be accessed from outside the loop via using the tag:
FIC254.PV
Tags are also used for one Controller to access the value or status of a variable from a different
Controller via the peer-to-peer communication. When a Controller makes a request for access to a
tag from a different Controller, it must put the tag name of the Controller (that the information is
being requested from) followed by two colons (::) in front of the tag for the information that is
being requested.
For example, a Controller with a tag name of UC254 and an analog input with the tag FT254, the
input could be accessed by a different Controller by requesting the following tag:
UC254::FT254