Allen-Bradley GuardPLC 1753, Safety Reference Manual

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Publication 1753-RM002C-EN-P - September 2008 21
Safety Concept for GuardPLC Controllers and GuardPLC I/O Chapter 1
GuardPLC Reaction Time
The maximum reaction time of working GuardPLC systems is twice
the cycle time of the system. The cycle time of a system consists of the
following items.
•
Reading local inputs
•
Processing the application program
•
Writing local outputs
•
Testing routines
•
Communication (for example, GuardPLC Ethernet, DeviceNet
Safety, Profibus, Modbus networks)
In addition, when considering the worst case for the entire system, the
switching times of the inputs and outputs must be taken into account.
When a network is used for communication data, reaction times are
affected.
Refer to Chapter 9 for reaction time calculations.
Watchdog Time of the CPU (in the PES)
The watchdog time of the CPU depends on system configuration.
The watchdog time of the CPU is the maximum permissible time
allowed for a RUN cycle (cycle time). If the cycle time exceeds the
default watchdog time of the CPU, the CPU goes into FAILURE STOP
mode. The watchdog time of the CPU must be a value between 2 ms
and half the safety time of the PES. The maximum permitted value is
5000 ms. The default setting for controllers is 50 ms. The default for
distributed GuardPLC I/O modules is 10 ms.
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