7PLC OPERATION

Topics:

• The computer structure of a PLC

• The sanity check, input, output and logic scans

• Status and memory types

Objectives:

• Understand the operation of a PLC.

For simple programming the relay model of the PLC is sufficient. As more complex functions are used the more complex vonNeumann model of the PLC must be used. A vonNeumann computer processes one instruction at a time. Most computers operate this way, although they appear to be doing many things at once. Consider the computer components shown in Figure 7.1 Simplified Personal Computer Architecture.

 

Figure 7.1 Simplified Personal Computer Architecture

Input is obtained from the keyboard and mouse, output is sent to the screen, and the disk and memory are used for both input and output for storage. (Note: the directions of these arrows are very important to engineers, always pay attention to indicate where information is flowing.) This figure can be redrawn as in Figure 7.2 An Input-Output Oriented Architecture to clarify the role of inputs and outputs.

 

Figure 7.2 An Input-Output Oriented Architecture

In this figure the data enters the left side through the inputs. (Note: most engineering diagrams have inputs on the left and outputs on the right.) It travels through buffering circuits before it enters the CPU. The CPU outputs data through other circuits. Memory and disks are used for storage of data that is not destined for output. If we look at a personal computer as a controller, it is controlling the user by outputting stimuli on the screen, and inputting responses from the mouse and the keyboard.

A PLC is also a computer controlling a process. When fully integrated into an application the analogies become;

inputs - the keyboard is analogous to a proximity switch

input circuits - the serial input uart is like a 24Vdc input card

computer - the x86 CPU is like a PLC CPU unit

output circuits - a graphics card is like a triac output card

outputs - a monitor is like a light

storage - memory in PLCs is similar to memories in personal computers

 

It is also possible to implement a PLC using a normal Personal Computer, although this is not advisable. In the case of a PLC the inputs and outputs are designed to be more reliable and rugged for harsh production environments.

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