Appendix A HMI Function Reference
© National Instruments Corporation A-69 BridgeVIEW User Manual
Write Tag on Change (discrete)
Use the Write Tag on Change (discrete)VI to update the Real-Time Database with a new
value for memory, output, and Input/Output tags. The value also is sent to the server if it is
an output or Input/Output tag. The value is updated and sent to the server only if the tag value
is different from the previous time the VI was executed. Use this VI if you do not need to
pass output values to the RTDB and server unless there really is a value change. This saves
you from adding code to your diagram to check value changes.
If the tag is an input only tag, Write Tag on Change (discrete)VI causes a system error because
input tags only can be updated by servers. If the tag is configured as an Input/Output tag, the
tag value is passed to the server when Write Tag on Change (discrete)VI is called but not
written to the RTDB. The RTDB is updated with the new value when the server polls it and
passes it back to the BridgeVIEW Engine. This maintains correct time synchronization in
the RTDB.
tag name is the name of the output tag.
value is the value to be written to the output tag.
generate event (F) determines whether a user change event is generated
for the write operation on the tag. If the tag is configured with event logging
on, this tag event can be displayed in the Event History Display in your
HMI and logged to a .evt file. By default, generate event is FALSE.
status returns the current status of the value written in the Real-Time
Database. If status is greater than or equal to 0, the Write Tag on Change
(discrete) operation was successful. If status is less than 0, either the device
server has reported an error indicating there is a problem with the tag, or
BridgeVIEW has reported an error indicating there is a problem using
the tag.
error indicates that an error occurred when executing Write Tag on
Change (discrete) or that the status of the tag is bad. See the status output
for the specific error condition.
shutdown indicates that the BridgeVIEW Engine is shutting down. In this
case, Write Tag on Change (discrete) no longer waits for the tag to be
updated and returns immediately with both timeout and shutdown TRUE.
You can use shutdown to exit any loop that uses Write Tag on Change
(discrete).