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9.6 Diagnostic Logic Blocks (DLBs)
A Diagnostic Logic Block (DLB) is a block of Ladder Diagram logic that can be downloaded to the
controller for independent execution. These blocks are useful tools for interacting with an
application that is running in the PACSystems controller. DLBs may be used to:
Collect information from a running application to analyze and diagnose problems
▪
▪
Test modifications and corrections to a running application before actually incorporating them
into the application.
▪
Test the devices that will be controlled by the application.
DLBs are intended to accomplish a specific task that is temporary in nature, such as diagnosing the
source of a problem or testing tuning parameters. When you have finished using a DLB, it should be
removed from the host controller. At this point the application logic and its variable allocation return
to what it was before the DLB was downloaded.
You can also remove the DLBs from the Logic Developer target, at which point the target's logic and
variable allocation will be identical to what they were before the DLBs were introduced.
Note that, although the DLB is removed from the controller, any changes the DLB made to the
system are not removed. For example, if the DLB logic changes a hardware parameter, the
parameter does not return to its previous value when the DLB is removed.
DLB logic can be executed with the controller in STOP IO Enabled Mode, which allows debugging the
application without the main application program running.
Note: Redundancy CPUs do not support DLBs.
GFK-2950C
Do not use a DLB as a permanent part of a production
application, because a DLB is stopped and deleted from
memory when Logic Developer loses its Programmer-
mode connection with the host controller. This could
happen if the programmer's communications cable is
disconnected or if a second programmer connects
serially to the same RX3i and establishes a
Programmer-mode session.
February 2018
Chapter 9. Diagnostics
Caution
443
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