Table Of ContentPCs IN THE FACTORY
2nd Edition
SEPTEMBER 1990
a
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© Copyright 1990 Architecture Technology Corporation. All rights reserved. No part of this publication may be reproduced, photocopied, stored on a
re\neva\ system, or transmitted without the express prior written consent of the publisher.
PCs IN THE FACTORY
2nd Edition
SEPTEMBER 1990
a
ARCHITECTURE DISTRIBUTED OUTSIDE THE USA/CANADA BY:
ELSEVIER ADVANCED TECHNOLOGY
TECHNOLOGY
MAYFIELD HOUSE
CORPORATION 256 BANBURY ROAD
ELSEVIER
OXFORD 0X2 7DH
SPECIALISTS IN COMPUTER ARCHITECTURE
ADVANCED UNITED KINGDOM
P.O. BOX 24344 · MINNEAPOLIS, MINNESOTA 55424· (612) 935-2035 TECHNOLOGY
© Copyright 1990 Architecture Technology Corporation. All rights reserved. No part of this publication may be reproduced, photocopied, stored on a
re\neva\ system, or transmitted without the express prior written consent of the publisher.
DISCLAIMER
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purpose.
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could have occurred. Architecture Technology assumes no responsibility for any incidental or consequen-
tial damages caused thereby.
Further, Architecture Technology Corporation reserves the right to revise this guide and to make changes
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revision or changes.
This disclaimer applies to all parts of this document.
FOREWORD
There is much to say for decentralized control, whether of an organism or of an organization. In fact,
distributed intelligence has become the watchword for many veterans of the microcomputer revolution, and
they are implementing the philosophy: put computing power where it is most needed and best used.
Computing need not be centralized to be efficient; in fact, many users have found, the opposite is true.
Personal computers (PCs), when located nearer to where the action is, are better for decision making, data
analysis, or information manipulation than distant, unapproachable corporate mainframes or even dedicated
minis.
As computing costs have plummeted, so have the price o fPCs. Analysts continue to predict a drop in the cost
of computing power, perhaps as much as 35% per year for the next severa lyears. However, market forces are
likely to keep the price around $2,000 for a non-gray market, business machine. Falling costs will mean more
features such as better graphics and more computing power.
In software, as well as in hardware, PC proliferation has opened up intense competition. No longer does a
software development firm feel constrained to re-invent the wordprocessor in order to address a potentially
large market. It can concentrate on writing an excellent statistical quality control package or an easy-to-use
tool management program.
Manufacturing and control systems are quickly being changed by an increasing use of and dependence upon
PCs, which have been introduced belatedly as new but integral parts of these systems.
This very basic change in system design philosophy is often glibly explained away by quoting from the monthly
barrage of ever-brighter reports about the increasing capabilities of PCs. It should be noted that almost all
of these reports come from publications narrowly designed to serve the PC industry and which, therefore, serve
themselves in making the future look as bright as possible.
Th numbers of engineers who have become personally familiar and comfortable with PCs is obviously growing
in both user vendor companies. The PC has become their indispensable professional tool. Thus, the PC
magazines inevitably provide some help to manufacturing/control engineers, as to all other users of PCs.
Readers should be aware that PC publications, with a few exceptions, fan the fires kindled by industry press
agents.
Although there are plenty of "environmentally hardened" PCs to choose from, their software still leaves plenty
to be desired. PC/MS-DOS is still single-tasking, and multi-tasking is essential in manufacturing control applications.
IBM wouldn't give up a single line of commercial MS-DOS capability just for control applicability of its
"hardened" PCs. Instead, it redesigned the whole shooting match to run a new operating system, OS/2, which
is multi-tasking but which also consumes memory to a new order of magnitude.
In a word, new speed and memory capability developments are used heavily just to get us the minimum
requirements for control, and the attractive low cost o fPCs is no longer as low. This new cost must be added
to the cost of hardened hardware.
PCs in the Factory
List of Figures
Figure 1: Single Communications Network 4
Figure 2: Serial Multidrop Lines 10
Figure 3: PC Connections to the Real-world, Directly or Through PLCs 26
Figure 4: Block Diagram of IC-DOS 28
Figure 5: PLC to PC Communications Co-Processor 29
Figure 6: PLC Programming on a PC 30
Figure 7: Flow Diagram Programming on a PC 31
Figure 8: Sub-multiplexing technique for I/O Expansion of PCs 47
Figure 9: Private Bus Solution to Expanding PC I/O Capability 48
Vll
PCs in the Factory
1. Personal Computers in Industry
There is no doubt that computers play a big part in industrial control, but what form they take and where
they are located also play an important part. A mainframe business computer performing the paperwork
and assisting management in a factory or plant is one thing, but the micro or mini industrial computer on
a factory or plant floor or in a control room, no matter what task it assigned to it, is another thing.
1.1 PCs in the Factory
Major changes have been taking place in the industrial and process control field. Influenced by the flurry
of activity and innovation in the personal and office computer markets, control system users have been
evaluating and using smaller, less expensive alternatives to large-scale, computer-based process control.
1.1.1 PCs Industrialized
A key factor behind the move of the computer into control rooms and onto the factory floor was its
flexibility; engineers could easily reconfigure their systems as control needs changed. The computer's
capabilities could often be expanded, in many cases without having to purchase additional equipment or
increase the area of the control room. Although the computer could not perfectly emulate an analog
controller or strip chart recorder, it opened new possibilities for the optimization of control and
generation of more detailed and accurate management information.
The hardening of computer systems against harsh industrial environments further encouraged their use in
control applications. They were repackaged with larger power supplies, became more tolerant to heat and
humidity, and used air filtration systems, to name some industrial adaptations. The advent of multitasking
and real-time operating systems enabled computers to handle many tasks simultaneously, making them
more cost effective and flexible than dedicated, single-use systems.
1.1.2 Early Disadvantages
Early systems had three serious disadvantages:
• Most were programmed in assembly language or a proprietary language, thus requiring
ongoing vendor or inhouse software maintenance.
• The computer was relatively expensive. A significant number of loops had to be placed on the
computer (or a number of jobs eliminated) to justify system expenses.
• The systems had limited man-machine interface capabilities-black-and-white text or
semigraphic CRTs were standard, color was a luxury. In some cases, electomechanical panels
were included in the system to give the operator a familiar method for performing their work.
Major system vendors spent a great deal of time and money engineering their systems to overcome this
weakness. As a result, today's computer tools feature:
• Menu-driven configuration that requires no programming on the part of the end user;
• High system security (including data integrity, operator safeguards, memory-resident software
and in some cases, redundancy); and
1
PCs in the Factory
• Man-machine interfaces that cater to the operator by providing unambiguous displays and
commands.
Note that many of the above were available before the introduction of the IBM PC. While there are still
many good systems based on fine equipment from such companies as DEC, Hewlett-Packard, and others,
the PC has flooded the general market and, in the process, changed many points o fview. A careful
examination suggests that:
• The control industry is going through the same qualification and justification steps that it
went through 30 years ago, when process computers were introduced;
• A changing economy has made justification of multi-million dollar systems more difficult,
forcing users to look for cost-effective alternatives;
• Users expect large-system support at PC prices;
• New, qualified vendors have entered the market with small systems, while some of the
traditional vendors have not produced small microcomputer systems;
• The burden is on the user to determine whether his claims for system features exist and
whether a given system will solve the problem.
1.1.3 Commercial Expectations
With the tremendous number of influences and changes in this marketplace, everyone has great
expectations of one sort or another. Some of these will lead to false promises and disenchantment.
Others, however, will define the path technology and applications of the future will take.
A closer look at these expectations shows that they can be divided into three major categories: commercial,
emotional and technical.
Even though traditional rules and customs regarding pricing, support, and buying procedures are changing
as new PC-based systems enter the market, some people still cling to expectations that were born of
yesterday. For example, they expect PC-based control systems to:
• Maintain the same hardware/software pricing ratio that has been common for the past several
years.
• Emulate the pricing structure of popular PC and business computer systems. That is, they
expect to pay $7,000 for the computer and S500 for the software package (e.g. a word
processor or spreadsheet analysis program).
• Be distributed to qualified users through traditional channels (e.g., manufacturer's reps and
direct sales).
The above pricing expectations are very unrealistic. The cost of computer hardware keeps going down as
manufacturing practice and technology improve. However, the cost of good engineers and programmers
keeps going up. As a result, this hardware/software pricing ratio has changed to the point where it's not
uncommon to see the cost of the system software equal the cost of the hardware.
Control software could be priced at $500 only if a minimum of engineering was done in developing the
software or if the true cost of the software were leveraged in the price of the hardware.
2
PCs in the Factory
Ten or 12 years ago, a hypothetical control system might have had a hardware cost of 5100,000 and a
software cost of $30,000. But the cost of some "add-on application services" usually added another $70,000
to the final cost. Thus, the true lifetime cost of the system increased significantly over the anticipated cost.
The real cost problem stems from the market size for microcomputer-based process monitoring and
control systems. While it is possible to sell ten million micro computers into office applications in a year,
only about three percent of that number will go on to the factory floor. So, the industrial system
developer must recover their investment over a smaller number of installations.
1.1.4 Emotional Expectations
Today's emotional expectations are very similar to those expressed 30 or so years ago by engineers as
computers first entered the control room. They are negative expectations, derived from our natural
tendency to be cautious when it comes to new technology. The emotional expectations are that:
• The PC is too slow for process control.
• The PC isn't reliable enough to have round-the-clock operation in an industrial environment.
• The operator won't be able to use the system.
Several companies offer industrialized versions of microcomputers. These feature heavy-duty power
supplies, multiple fans, modular assemblies, and low chip counts. They are as reliable as the well-accepted
minicomputers and are similar to the dedicated microcomputers known as programmable controllers.
Therefore, the new microcomputer systems can be used in direct digital control applications with a feeling
of security.
Whether the operators trust and accept such a system is a function of how it was designed for the
application. Operators expect the machine to tolerate their errors and misunderstandings. They also
expect the machine to act like the traditional control systems they already know. For example, the system
should have familiar color graphic displays and simple keyboard commands.
1.1.5 Technical Expectations
Technical expectations span a full range of topics. With the tremendous turnover in the PC market, we've
come to expect new products, new features, and new software just about every month. These expectations
have driven vendors to pre-release products and quote unrealistic features and delivery dates. However,
other market expectations have caused innovation.
Some products provide form multitasking in a streamlined and easy-to-use system. The multitasking
shouldn't require any changes to the standard system software or hardware. Non-standard configurations
or third-party operating systems can spell trouble if the system is to be changed or expanded.
The system should have an open architecture that makes it possible to add additional features. These
might include support for special hardware or data access for user-written application programs.
Speed that is consistent with the hardware being used and the process being controlled is a must if the
process uses serial I/O hardware, a speed measured in seconds, rather than milliseconds, may be sufficient.
Claims of high I/O speed may not take into account processor or interrupt loading on the part of the PC
or the process control device.
3
PCs in the Factory
People have come to expect pixel-oriented graphic displays at the man-machine interface. While a great
deal of effort is spent in providing drawing tools for the manager and engineer, the operator is sometimes
neglected. The system should provide clear and fast graphic displays.
Documentation should be clearly written. Online help screens are ideal documentation, because they are
available while working on the system.
Networking (Figure 1) is another expectation for these small systems. With a PC-based distributed control
system, one should expect the same features as in a large system.
Process Control
Packaging
Real-time
data exchange
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Figure 1: Single Communications Network
1.2 PCs in the Process
The PC can bring several benefits to even very small plant processes in a cost effective manner. These
benefits capitalize on the PC's abilities to gather, store, present, and manipulate data in much the same
wav PCs are used in the office environment.
In the office environment where PCs are a way of life, they are regarded as powerful information
processing machines. Facts, figures, and the printed word are manipulated by professionals striving to
maximize the useful work of this indispensable tool.
4
PCs in the Factory
PCs may someday become as widespread in the process industries as they are in the office. Process control
specialists are beginning to look at the PC in much the same way as their business counterparts. Efficient
information processing becomes the objective with a great deal of thought given to the amount of data
needed and manner in which it may be presented, archived, or manipulated. The purpose of this paper is
to examine the areas where the PC has the greatest potential for increasing plant effectiveness.
1.2.1 Process Connections
Off the shelf hardware and software available from a variety of sources has brought PCs within the reach
of the typical plant for a variety of uses. Their greatest value lies in their ability to improve the
productivity of the people using them. Information can be gathered, stored, and presented in a more
useful manner that has direct benefits in terms of spotting problems quicker, facilitating control strategy
improvement, and making better data analysis possible.
PCs can interact with the process in the following way:
• Monitor only device-all operation through other devices.
• Monitor and auxiliary operations-operate through other devices or PCs.
• Monitor and primary operations center-operate mainly through PC, with other devices used
backup.
In each case the PCs function is to gather data, manipulate the data in some form, and present it in a
light that enables plant personnel to take action upon the information in a beneficial manner. Differences
only exist in the devices used to actually effect changes to the process.
Implementation of the philosophy desired occurs when the process devices the PC is to interface to are
selected along with matching software. The process devices include I/O or microprocessor-based
controllers. The software provides communications drivers, menu driven configuration, and application
programs such as standard displays.
1.2.2 Operations
As an operations tool, the PC provides many of the same benefits found in large computerized systems. It
becomes a centralized point facilitating quick recognition, interpretation, and manipulation of the process.
This is done through data grouping, data trending, and chronological alarm presentation.
Data grouping is the presentation of related information in a logical format. By using the PC's
programmable nature to easily place related data on the same display, operations personnel can see how a
change in one variable affects another.
Graphic displays in the form of semigraphic panels have been used for many years to serve as a starting
point for discussion on the corporate tour, train new operations personnel, and quickly overview plant/unit
status for experienced personnel. Graphic displays on PCs serve the same functions at a greatly reduced
cost. The economic advantage is apparent also when a change in equipment usage occurs. Updating the
semigraphic panel is much more expensive than rearranging a programmable graphic display.
Status displays allow a much higher concentration of data within the view of an operator. This permits
operators to gain a greater feel for the process. This type of display helps overcome problems where an
important variable is far away in another control room or on another physical section of the panel not
within a reasonable viewing distance. It is also useful when changes in equipment usage necessitate
5
