B&B Electronics VFG3000 - Manual - Manual (Page 66)

A LL A BOUT T AGS V LINX F IELDBUS G ATEWAY M ANAGER U SER M ANUAL
P AGE 50
equal to the contents of a PLC register, it is not truly mapped to that register, in
that it can never be written to and thus cannot be considered to be equivalent to
that register. The need for this restriction is obvious if you consider a formula
such a
Tank1+Tank2 . What would it mean to write to this expression?
• Arrays represent a collection of data items within the terminal. These are
typically used to store recipe data, or to build up collections of data for statistical
analysis. They are not used in the majority of simple applications, but provide a
powerful tool for more complex projects.
T
AG
T
YPES
Each family contains five tag types, each of which holds a different kind of data…
Flag tags represent a single true or false condition. When they are mapped to a
register in a remote device, they will typically correspond to an internal coil or to
a single digital I/O point. Flag formulae typically represent combinations of such
items, or comparisons of numeric values.
Integer tags represent 32-bit signed numbers. These tags can store values between
–2,147,483,648 and +2,147,483,647. Even if a tag is mapped to a PLC register
which contains only 16 bits of data, Fieldbus Gateway Manager performs its
internal operations at the higher level of precision to ensure large intermediate
values can be handled with ease.
Multi tags represent numeric values that correspond to a number of distinct states.
Thus, while an integer might represent a tank level, a multi tag will represent,
say, one of three states of a machine, such as Stopped, Running or Paused. The
distinction is obvious when you consider that a multi tag is displayed as one of a
set of strings, while an integer is displayed as a number.
Real tags represent 32-bit single precision floating-point numbers. These tags can
store values between ±10
-38 and ±10 +38 with a precision of about 7 significant
figures. While it is seldom necessary to use real tags to represent physical
quantities—which typically have more tightly defined ranges—they are useful for
performing statistical operations or other math functions.
String tags represent an item of text made up of a number of characters. They are
used to store such things as recipe names, or to process data received using Raw
Port device drivers. Strings cannot be mapped to PLC registers, but can be used
to store such data within the Master itself.