National Instruments BridgeVIEW User Manual download pdf (Page 374)

Appendix A HMI Function Reference

BridgeVIEW User Manual A-54 © National Instruments Corporation

Read Tag

Use the Read Tag VI to read the latest value of a tag from the Real-Time Database. For

immediate polling of the tag value, leave timeout (secs) unwired. To wait until the value is

updated before reading it, wire a timeout value in seconds to timeout. The Read Tag VI

returns with the most recent Real-Time Database value when it is updated, the timeout is

exceeded, or the Real-Time Database is shutting down, whichever occurs first. Use the

changed? output to determine whether the value changed since the last read.

Note

Use a separate Read Tag VI for each tag you want to monitor. Do not put the Read

Tag VI in a loop to read a different tag each iteration of the loop. This results in

slower program performance. The Read Tag VI is designed to save information

about the tag internally for efficient operation. This information is updated every

time the tag name changes.

tag name is the name of the tag.

timeout (secs) (0) specifies how many seconds to wait for the tag value to

be updated in the Real-Time Database before reading the Real-Time

Database for the latest value. If timeout is 0, the Read Tag VI reads the

Real-Time Database immediately and returns the tag value without

waiting. If timeout is –1, Read Tag waits indefinitely until the tag value is

updated, or the Engine shuts down, whichever occurs first. If a timeout

occurs before the value is updated, Read Tag returns the most recent value

from the Real-Time Database, and is set to TRUE. The default value is 0.

in alarm is TRUE if the tag is in alarm.

value status returns the status of the value. If value status is greater than

or equal to 0, the value returned by Read Tag is valid and there is a warning

about the tag value. If value 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.

value is the latest value of the tag read from the Real-Time Database.

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National Instruments BridgeVIEW User Manual Page 374