B&B Electronics VFG3000 - Manual - Manual (Page 72)

E DITING F LAG T AGS V LINX F IELDBUS G ATEWAY M ANAGER U SER M ANUAL
P AGE 56
• The Bit Number property is used when a flag variable is mapped to a PLC
register which contains more than a single bit of information. The property is
then used to indicate which bit within the register is to be accessed by the tag.
• The Access property is used to specify what sort of data transfers should be
performed for a mapped variable. You may indicate that data is to be both read
and written, or just read or written as appropriate. Write-only tags can be used to
avoid unnecessary read operations on data that can only be changed by the
terminal. They will typically be set to retentive as their value cannot be obtained
from the PLC, and must therefore be stored by the terminal.
• The Storage property is used to indicate whether Fieldbus Gateway Manager
should allocate FLASH memory within the Gateway in order to retain the value
of the tag during power-down. Mapped tags that are not write-only cannot be set
to retentive, as their values will in any case be read from the PLC, and it does not
therefore make sense to waste local storage to retain data that will be
overwritten.
• The Simulation property is used to select the value that Fieldbus Gateway
Manager will assign to this tag when displaying it within the display page editor.
This facility can be useful for documenting databases, in that it allows a display
page to be configured to represent a particular machine state, such that a screen
capture can then be pasted into an operator manual or other documentation.
• The Setpoint properties are used to indicate whether a setpoint will be specified
for this tag, and what that setpoint will be. Setpoints are used by certain alarm
modes, and allow the actual state of a tag to be compared to its intended state.
For example, a tag that represents the state of an input from a speed switch for a
motor might have the motor’s control output specified as a setpoint. This allows
an alarm to be programmed to activate if the motor fails to start.
• The On Write property is used to define an action that will be executed when a
change is made to the tag. This action may be used to update dependent values,
or to perform other actions specific to the database. Care should be taken not to
perform actions that are too complex, or system performance may be reduced.