Chapter 9. DiagnosticsGFK-2950C February 2018 4439.6 Diagnostic Logic Blocks (DLBs)A Diagnostic Logic Block (DLB) is a block of Ladder Diagram logic that can be downloaded to thecontroller for independent execution. These blocks are useful tools for interacting with anapplication 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 theminto 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 thesource of a problem or testing tuning parameters. When you have finished using a DLB, it should beremoved from the host controller. At this point the application logic and its variable allocation returnto 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 andvariable 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 thesystem are not removed. For example, if the DLB logic changes a hardware parameter, theparameter 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 theapplication without the main application program running.CautionDo not use a DLB as a permanent part of a productionapplication, because a DLB is stopped and deleted frommemory when Logic Developer loses its Programmer-mode connection with the host controller. This couldhappen if the programmer’s communications cable isdisconnected or if a second programmer connectsserially to the same RX3i and establishes aProgrammer-mode session.Note: Redundancy CPUs do not support DLBs.
