National Instruments BridgeVIEW 用户手册
Appendix A
HMI Function Reference
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.
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 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.
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.
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.