User manual

Publication 1756-RM093F-EN-P - January 2010 69
Monitor Status and Handle Faults Chapter 7
inputs to their de-energized (safety) state, and the associated input
status to faulted. If an output connection failure is detected, the
operating system sets the associated output status to faulted. The
output module de-energizes the outputs.
De-energize to Trip System
GuardLogix controllers are part of a de-energize to trip system, which
means that zero is the safety state. All inputs and outputs are set to
zero when a fault is detected. As a result, any inputs being monitored
by one of the diverse input instructions (Diverse Inputs or Two-hand
Run Station) should have normally-closed inputs conditioned by logic
similar to the logic in Rung 4 of Ladder Logic Example 2
and Ladder
Logic Example 3 on pages 72 and 73. The exact logic required is both
application and input-module dependent. However, the logic must
create a safety state of 1 for the normally-closed input of the diverse
input instructions.
Use Connection Status Data to Initiate a Fault Via Program Logic
The following diagrams provide examples of the application logic
required to latch and reset I/O failures. The examples show the logic
necessary for input only modules, as well as input and output
combination modules. The examples use a feature of the I/O modules
called Combined Status, which presents the status of all of the input
channels in a single boolean variable. Another boolean variable
represents the status of all the output channels. This approach reduces
the amount of I/O conditioning logic required and forces the logic to
shut down all input or output channels on the affected module.
Use the Input Fault Latch and Reset Flow Chart
on page 70 to
determine which rungs of logic are required for different application
situations. Ladder Logic Example 1
shows logic that overwrites the
actual input tag variables while a fault condition exists. If the actual
input state is required for troubleshooting while the input failure is
latched, use the logic shown in Ladder Logic Example 2
. This logic
uses internal tags that represent the inputs to be used in the
application logic. While the input failure is latched, the internal tags
are set to their safety state. While the input failure is not latched, the
actual input values are copied to the internal tags.
IMPORTANT
You are responsible for providing application logic to latch
these I/O failures and to make sure the system restarts
properly.