HPR4768: Programmable Logic Controls - Episode 7

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01 Introduction

This is the seventh episode in an 8 part series.

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In the previous episode we took a brief look at PLC networking, together with a detailed look at one network protocol, Modbus.

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In this episode we will look at where personal computers fit into the factory floor.

This includes

* Personal computers in industry, including specialized hardware,

* SCADA systems,

* HMI systems,

* Test and measurement systems,

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04 Personal Computers in Industry

In a previous episode we how the first microprocessor based computer sold on the commercial market was the MICRAL from France, based on the Intel 8008.

It was sold as a cost effective replacement for minicomputers used in industry.

It preceded what are generally considered to be the first Personal Computers.

So you can see that industry was not slow in adopting general purpose computers in roles for which they were suitable.

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However, personal computers suffered from a number of problems which made their use on the factory floor difficult.

They did not withstand heat, vibration, dirt, or oil very well.

Power supplies, hard drives, keyboards, and monitors were particularly problematic points of failure

If enclosed in a sealed cabinet, they put out a lot of heat of their own which must somehow be gotten rid of and the cabinet itself was difficult to fit into the available manufacturing floor space.

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None the less they offered certain advantages over PLCs in a few very specific applications.

Their hard drives could save data recorded from production.

Their monitors allowed the creation of large, complex user interfaces.

The previous two advantages combined with their powerful CPUs, large memory, and highly flexible programming allowed the creation of highly specialized production test systems.

07 VME and STD Bus

There were however several multi vendor standard computer designs intended for embedded applications.

A well known one is VME Bus.

This uses cards which plug into a backplane in a rack.

The backplane was not part of a "motherboard".

Rather, the CPU was just another board that plugged into the backplane.

If you are familiar with S-100 bus systems, the concept was similar, but designed to be more rugged.

However, these were very expensive and so saw little use in industrial applications.

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More common was STD Bus.

Again, these were boards which plugged into a common backplane bus in an open rack.

The individual boards were much smaller than S100 or VME boards.

The system was designed for lower cost and more compact installation than VME bus.

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While VME and STD Bus systems offered standard hardware, they came without the user friendly software that defined a PLC.

The user would write software that either ran without any supporting operating system at all, or at most a very limited operating system that would allow the program to be loaded and run.

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They were intended for manufacturers who were producing many units of manufacturing equipment that they would offer to customers as complete turn key off the shelf products.

They were not of much use to machine builders who were building one off entirely bespoke machines that would be supported by the customer's own maintenance personnel after delivery.

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11 Passive Backplane PCs and "Industrialized" PCs

In an attempt to provide a sort of compromise between commodity desktop PCs and things like VME or STD Bus systems, some manufacturers came out with "industrialized" PCs that