Creating The Functional Specifications; Designing The Safety Circuits - Siemens SIMATIC S7-200 System Manual

Creating the Functional Specifications

Write the descriptions of operation for each section of the process or machine.
Include the following topics:
Input/output (I/O) points
Functional description of the operation
Permissive states (states that must be achieved before allowing action) for
each actuator (solenoids, motors, drives, etc.)
Description of the operator interface
Interfaces with other sections of the process or machine

Designing 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.
S7-200 Programmable Controller System Manual
A5E00066097-02
Basic Concepts for Programming an S7-200 CPU
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