Allen-Bradley GuardPLC 1753, Safety Reference Manual

Page: 89 / 120

Publication 1753-RM002C-EN-P - September 2008 89
Technical Safety for the Application Program Chapter 8
The essential characteristic is the encapsulation of the functions into
self-created function modules. Thus a program can be clearly
structured in modules (function modules). Every module can be seen
individually and the final, complex function results from linking these
modules into a larger module or ultimately into a program.
Functions of the
Application Program
Programming is not subjected to any restrictions imposed by
hardware. The functions of the application program are freely
programmable.
When programming, follow the closed-circuit current principle for the
physical inputs and outputs. This is best used in combination with the
I/O module channel [nn] state.
Only components which comply with IEC 61131-3, and their
corresponding functional requirements, may be used with the logic.
•
Appropriate logical and/or arithmetic functions are used by the
application program, regardless of the closed-circuit principle of
the physical inputs and outputs.
•
The I/O module uses the closed-circuit principle which requires
the safety state of the inputs and outputs to be 0. The logic in
the controller does not rely on the closed-circuit principle, so
you can determine the safety state for connections between
function blocks to be 0 or 1. However, we recommend that you
use a safety state of 0 between function blocks.
•
Design and document the logic to simplify troubleshooting. Use
flow charts and write good documentation of the program logic.
This does not replace any of the documentation requirements
for your applications. Flow charts and logic documentation
should be included if they are not already required by your
documentation procedures.
•
Any number of negations are permissible.
•
The programmer must evaluate input, output, and logic module
error signals.
Allen-Bradley Parts