Allen-Bradley GuardPLC 1753, Safety Reference Manual

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26 Publication 1753-RM002C-EN-P - September 2008
Chapter 2 GuardPLC Central Functions
Test of the I/O Bus Within the System
The connection between the CPU and the related I/O points or I/O
modules is checked.
Reactions to Detected Errors in the CPU
A hardware comparator within the central area constantly compares
whether the data of microprocessor system 1 are identical to the data
of microprocessor system 2. If this is not the case, or if the test
routines in the central area are negative, the system automatically goes
into FAILURE_STOP mode and the watchdog signal is switched off.
Input signals are no longer processed, and outputs go to the
de-energized, switched-off condition.
GuardPLC Controllers and
I/O Modules Error
Diagnostics
Because the GuardPLC 1200, 1600, and 1800 controllers are compact
systems, error diagnostics are summarized in a collective error LED
indicator.
Each GuardPLC distributed I/O module has its own LED indicator to
display errors in case of module failures or faults in the external
wiring, providing a quick error diagnosis in case of module failure.
The evaluation of system variables that contain the status value of the
I/O or the CPU can also be monitored in the application program.
An error signal is transmitted only if the error does not impede
communication with the CPU, that is, an evaluation via the CPU is still
possible.
An extensive diagnostic record of system performance and faults is
stored in the diagnostic memory of the CPU and the COM. This record
can be viewed in the programming software, even after a system fault.