Appendix A
HMI Function Reference
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National Instruments Corporation
A-67
Write Tag on Change
Use the Write Tag on Change 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 VI causes a system error because input
tags can only 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 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
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 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. You
can use shutdown to exit any loop that uses Write Tag on Change.
