Guidelines For Designing A Micro Plc System - Siemens SIMATIC S7-200 System Manual

S7-200 Programmable Controller System Manual

Guidelines for Designing a Micro PLC System

There are many methods for designing a Micro 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 S7-200 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.
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Identify the conditions that would assure the operation is not hazardous, and determine how
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to detect these conditions independently of the S7-200.
Identify how the S7-200 CPU and I/O affect the process when power is applied and
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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
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independent of the S7-200.
Provide appropriate status information from the independent circuits to the S7-200 so that
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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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Specify the Operator Stations
Based on the requirements of the functional specifications, create drawings of the operator
stations. Include the following items:
Overview showing the location of each operator station in relation to the process or
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machine
Mechanical layout of the devices, such as display, switches, and lights, for the operator
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station
Electrical drawings with the associated I/O of the S7-200 CPU or expansion module
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