S7-200 SMART
System Manual, 09/2015, A5E03822230-AC
85
Guidelines for designing a PLC system
There are many methods for designing a PLC system. The following general guidelines can
apply to many design projects. Of course, you must follow the directives of your own
company's procedures and the accepted practices of your own training and location.
Partition your process or machine
Divide your process or machine into sections that have a level of independence from each
other. These partitions determine the boundaries between controllers and influence the
functional description specifications and the assignment of resources.
Create the functional specifications
Write the descriptions of operation for each section of the process or machine. Include the
following topics: I/O points, functional description of the operation, states that must be
achieved before allowing action for each actuator (such as solenoids, motors, and drives),
description of the operator interface, and any interfaces with other sections of the process or
machine.
Design the safety circuits
Identify equipment requiring hard-wired logic for safety. Control devices can fail in an unsafe
manner, producing unexpected startup or change in the operation of machinery. Where
unexpected or incorrect operation of the machinery could result in physical injury to people
or significant property damage, consideration should be given to the use of electro-
mechanical overrides which operate independently of the CPU to prevent unsafe operations.
The following tasks should be included in the design of safety circuits:
● Identify improper or unexpected operation of actuators that could be hazardous.
● Identify the conditions that would assure the operation is not hazardous, and determine
how to detect these conditions independently of the CPU.
● Identify how the CPU and I/O affect the process when power is applied and removed, and
when errors are detected. This information should only be used for designing for the
normal and expected abnormal operation, and should not be relied on for safety
purposes.
● Design manual or electro-mechanical safety overrides that block the hazardous operation
independent of the CPU.
● Provide appropriate status information from the independent circuits to the CPU so that
the program and any operator interfaces have necessary information.
● Identify any other safety-related requirements for safe operation of the process.
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