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 provided computers that were more suited to the factory environment while still being compatible with PC operating systems and software.
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These took two forms. One was to simply put a standard desktop PC motherboard into a more solid case with better cooling.
These cases often had shock mounting for the hard drives, positive pressure air cooling with changeable filters on the inlets, and even redundant power supplies in some instances.
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The cases were built to match rack mount server dimensions, and typically rack mounting hardware kits were available for them as well if you didn't intend to put them on a shelf.
Keyboard and mouse connections were often at the front, although that became less relevant when USB replaced dedicated mouse and keyboard connectors.
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The other form is what are termed "passive backplane" systems.
Instead of a conventional PC motherboard, there is a board which contains a number of slots conforming to standard PC expansion bus standards, such as ISA, PCI, and PCIe.
These could have anywhere from 6 to as many as 20 slots.
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The functionality of a full motherboard was integrated into a board which plugged into the backplane.
This held the CPU, RAM, disk interfaces, networking, and other chips required to provide the equivalent to a standard PC motherboard.
Some boards had both PCI and ISA connections, allowing the use of both types of bus, provided the passive backplane supported this.
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Putting what amounted to the motherboard on a plug in board allowed for the equivalent of a motherboard change by simply opening the top of the case, uplugging the old CPU board, and plugging in a new one.
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The main advantage that passive backplane systems provided however was that they allowed for a large number of slots to accomodate special boards for measurement and control of test or other equipment.
These specialized boards were often the real heart of the system and the PC is there to support them.
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The main advantage of industrialized PCs in general were
their large, roomy cases making them easy to work in and replace components such as power supplies,
the good filtering systems,
the shock mounting for hard drives,
and the hard drives often being mounted in removable carriers that allowed them to be simply pulled out and replaced through the front of the case.
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With these systems often being used for things like production test systems where a production line could be down if the computer is not working, high reliability and speed of repair is very important.
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There are a number of vendors of these systems, with Advantech being one of the best known.
21 Other Standards
There are a variety of other designs for adapting PC hardware standards to the industrial environment, but I won't go into them here as that would be outside the focus of this series.
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22 Single Board Computers
There were also numerous small vendors selling what were called Single Board Computers, or SBC for short.
These were small programmable boards, often using a Z80 or 8032 series CPU.
Many of them came with a Basic interpreter built into the ROM.
Again, these were mainly used by specialist vendors who needed some sort of very limited hardware to do one specific task in a standard turnkey product.
The Raspberry Pi seems to have taken over much of that market these days.
Without the software that defines a PLC however, they occupied a different niche in the automation market and were not a real competitor to PLCs.
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23 Software Systems and Applications
Having outlined the hardware environment, I will now turn to what sort of software was used on these computers.
24 Spreadsheet Horrors
First I will start with tales of software that will chill your bones.
One particularly horrifying use of PCs in connection with PLCs was to use spreadsheets as a database or user interface.
In the early days this was done with Lotus 123 and later on Microsoft Excel.
With the right communications boards, drivers, and other components, you could link cells in a spreadsheet to the PLC data table.
People used, and still use, spreadsheets as a development environment and database to create PC software.
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Knowing how to program a PLC did not necessarily equip you to write a complete stand alone program in languages such as C, Basic, or other common PC programming languages.
For these people, being able to create applications with spreadsheet macros or VBA was much simpler.
One of the major problems with these monstrosities is that later on, nobody, often not even the author, could figure out how they worked or what they did or what to do when they stopped working.
I will leave off on this topic before I give anyone nightmares.
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26 SCADA
You may have heard of this term before.
It stands for "System Control And Data Acquisition".
A SCADA system is a customized program that runs on a PC that has a complex GUI, some sort of database, and some sort of scripting or other configuration to make it do what is wanted in that application.
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These are most commonly found in process industries such as chemicals, food processing, water utilities, and other places like that.
They are much less commonly found in assembly manufacturing.
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A SCADA system is generally built using a special IDE that has a drag and drop GUI builder, database, and various other related things.
The IDE is very closely tailored to developing SCADA applications.
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Communications with the PLC is generally done via drivers following a standard known as "OPC".
OPC originally stood for "OLE for Process Control".
"OLE" stands for "Object Linking and Embedding", and was a Microsoft inter-process communications method.
OPC was later re-branded to "Open Platform Communications".
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What OPC does is allow PLC vendors with proprietary communications protocols write Windows communications drivers which allow other vendors with proprietary SCADA development systems to use them.
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I won't go into any more detail on this, but historically, it was usual for the only way for a third party to write software that could talk to most PLCs was through an OPC server.
Open protocols such as Modbus TCP were in fact very rare.
This in turn explains to a large degree the very strong position of Microsoft Windows in manufacturing.
32 SCADA Security
If you have read articles on security problems in industrial control systems, these are almost invariably problems with SCADA systems.
As these are normally just Windows applications running on a Windows operating system and typically using a Microsoft SQL Server database, SCADA systems are open to all the usual Windows security problems.
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There is really nothing special about this, except that for most companies that have such a system, it is often critically important to their business.
On the other hand, as a factory floor system maintenance of it is subject to all the usual penny pinching and postponement of upgrade or replacement until a crisis occurs that characterizes most other factory floor operations.
To put it simply, SCADA security is just normal IT security and is best understood in that context.
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34 HMI Systems
HMI stands for "Human Machine Interface".
In the context of PLCs, it basically refers to a way to give a display, and either a keyboard or touchscreen to a PLC so that an operator can interact with it.
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These can be anything from a small LCD screen and keyboard to a large PC based system running a custom GUI program.
These differ from SCADA systems in that they focus on display and entering of data and generally lack control or data logging functions.
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The HMI will display the status of the machine, display fault and diagnostic messages, show QC related information, and various other related things.
The operator can also use it to change modes, control the machine manually, enter set points and recipes, etc.
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Typically special HMI GUI builder software is used to create these programs, which may be traditional GUIs or be web based.
At one time ActiveX was very widely used for this until browser support for it was killed off.
At the back end of the HMI will be a communications driver to connect to the PLC where a block of PLC memory will be used to hold numeric data which is read from or written to the PLC by the HMI.
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Smaller HMIs were run on custom designed hardware, but PCs were used for larger ones.
These PCs however would often be built into the back of the display to give a compact and easy to mount system.
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39 Test and Measurement Systems
Many manufacturing processes require some sort of quality assurance or other tests to be integrated into the production system.
Very simple tests can be conducted by a PLC and the parts accepted or rejected based on measurements or other checks.
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However, some tests require sophisticated algorithms to be applied, or require large samples of data, or otherwise are not suitable for being done by PLCs.
A further consideration may be the necessity to record the data permanently for future reference, such as making decisions on product recalls.
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Test and measurement systems often include computers, either to conduct the actual test, or to just record the readings.
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Readings on sophisticated tests may require thousands of samples to be taken at high speed at precise intervals and then have advanced algorithms applied to them.
Data acquisition hardware may involve boards that plug into a PC's bus inside the case, or it may be housed in an external rack and the data transferred to the PC by USB, Ethernet, or other communications link.
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I won't go into any real detail here except to mention a few important product names that you may hear in this field.
One of the biggest companies in this field is National Instruments, who make both hardware and software.
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They have two software products that are very well known in this field.
These are LabView and LabWindows.
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LabView came out in 1986, roughly 40 years ago.
It provided a GUI based graphical development system which allows connections of "virtual instruments" in a drag and drop manner.
It first came out on the Macintosh, later being ported to Windows and Linux.
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In 1987, National Instruments came out with LabWindows, which provided a GUI builder, analysis libraries, and interface libraries to connect to their data acquisition instrumentation hardware.
Despite the name, LabWindows ran on MSDOS, not Microsoft Windows.
Applications were written in C or Basic.
Several years later, in what amounted to a major re-write, LabWindows was ported to Microsoft Windows, where it was renamed LabWindows/CVI.
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LabView was better suited to hacking a lab test system together quickly, particularly for people who may not be familiar with programming in a conventional programming language.
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LabWindows on the other hand was better suited to production test systems where it was important to be able to tailor the behaviour of the program more precisely in order to optimize test and production rates.
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In the present day, Python is very well suited to writing production test software and should probably be the first item on any list for evaluation for such a project.
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50 Episode Conclusions
In this episode we looked at the following.
* Personal computers in industry,
* SCADA systems,
* HMI systems,
* Test and measurement systems
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In the next episode we will look at the following.
* Soft logic systems,
* The IEC-61131 standard,
* A free and open source PLC
* What a hypothetical free and open source soft logic system suited to industrial use may look like.
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This has been the seventh episode in an 8 part series.
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