B&B Electronics VFG3000 - Manual - Manual (Page 190)

D ATA V ALUES V LINX F IELDBUS G ATEWAY M ANAGER U SER M ANUAL
P AGE 174
W
RITING
E
XPRESSIONS
You will recall from the earlier sections of this manual that many fields within Fieldbus
Gateway Manager are configured as what are called expression properties. You will further
recall that these fields are configured by means of a user interface element similar to that
shown below…
In many situations, you will be configuring these properties to be equal to the value of a tag,
or to the contents of a register in a remote communications device, in which case your
selection will be made simply by clicking the appropriate option on the drop-down menu, and
then selecting the required item from the resulting dialog box.
There will be situations, though, when you want to make a property dependent on a more
complex combination of data items, perhaps using some math to combine or compare their
values. Such eventualities are handled via what are known as expressions, which can be
entered in the property’s edit box whenever General mode is selected via the drop-down.
D
ATA
V
ALUES
All expressions contain at least one data value. The simplest expressions are thus references
to single constants, single tags or single PLC registers. If you enter either of the last two
options, Fieldbus Gateway Manager will simplify the editing process by automatically
changing the property mode as appropriate. For example, if you enter a tag name in General
mode, Fieldbus Gateway Manager will switch to Tag mode, and show the tag name in the
selection field.
C
ONSTANTS
Constants represent—not surprisingly—constant numbers or strings.
I
NTEGER
C
ONSTANTS
Integer constants represent a single 32-bit signed number. They may be entered in decimal,
binary, octal or hexadecimal as required. The examples below show the same number entered
in the four different number bases…
B ASE E XAMPLE
Decimal 123
Binary 0b1111011
Octal 0173
Hexadecimal 0x7B