GFK-0467K Chapter 3 Fault Explanation and Correction 3-5
3
Fault Reference Definitions
The alarm processor maintains the states of the 128 system discrete bits in %S memory. These fault
references can be used to indicate where a fault has occurred and what type of fault it is. Fault
references are assigned to %S, %SA, %SB, and %SC memory, and they each have a nickname.
These references are available for use in the application program as required. Refer to Chapter 2,
“System Operation,” for a list of the system status references.
Additional Fault Effects
Two faults described previously have additional effects associated with them. These are described
in the following table.
Side Effect Description
PLC CPU Software Failure Whenever a PLC CPU software failure is logged, the Series 90-30 or
90-20 CPU immediately transitions into a special ERROR SWEEP
mode. No activity is permitted in this mode. The only method of
clearing this condition is to reset the PLC by cycling power.
PLC Sequence Store Failure During a sequence store (a store of program blocks and other data
preceded with the special Start-of-Sequence command and ending with
the End-of-Sequence command), if communications with the
programming device performing the store is interrupted or any other
failure occurs which terminates the download, the PLC Sequence Store
Failure fault is logged. As long as this fault is present in the system,
the PLC will not transition to RUN mode.
PLC Fault Table Display
The PLC Fault Table screen displays PLC faults such as password violations, PLC/configuration
mismatches, parity errors, and communications errors.
The programming software may be in any operating mode. If the programming software is in
OFFLINE mode, no faults are displayed. In ONLINE or MONITOR mode, PLC fault data is
displayed. In ONLINE mode, faults can be cleared (this may be password protected).
Once cleared, faults which are still present are not logged again in the table (except for the “Low
Battery” fault).
I/O Fault Table Display
The I/O Fault Table screen displays I/O faults such as circuit faults, address conflicts, forced
circuits, and I/O bus faults.