ONE'S A TOOL, THE OTHER'S A SOLUTION
A spate of articles has recently appeared in computer journals attacking the functionality of the Programmable Controller (PLC). In rebuttal I would submit that PLCs offer a number of advantages lacking in VMEbus and the other open systems these articles advocate for industrial automation. PLC advantages include higher net real-time performance, lower life-cycle cost, broader customer acceptance, and a larger selection of systems integrators .
The VMEbus (VERSAmodule Europe) bus standard, used with off-the-shelf boards and modular chassis, can be used to rapidly construct high-performance computer systems. The standard offers advantages such as full 32-bit data path and 32-bit addressing.
To begin, I have to point out that what we're really talking about is two entirely different things. The computer is a tool. The programmable controller is a solution. Programmable controllers, word processors, auto engine electronics, and kitchen appliances all have customized "computers" embedded within them that allow them to do one thing well. On the other hand, VMEbus technology can be used to make a PLC or a word processor or anything else you want. But that which is made won't be a standard product. The open system that is created to address the application will be one of a kind.
Customer want standard solutions with good reason. Envision a plant with 300 control boxes. Suppose each is customized for its particular role. Each is almost the same, i.e., totally different. It ends up being like a Chinese restaurant where the menus so expensive you don't know what to order. If you want steak, it's better just go to a steak house.
I contend that although VMEbus is useful for repetitious applications such as telecommunications, it difficult to use in solving control problems. The very versatility that makes VMEbus attractive to me and other computer nerds is a problem for industrial applications. We don't design personalized workstations or word processors. Why should we design a new PLC for each application?
Non-application-specific operating languages are typically used to design tools and only seldom to implement solutions. Tool languages, e.g., LISP, Forth, and C++, are not solution languages. Examples of solution languages are Ladder, Spreadsheet, WordPerfect, HyperCard and industrial workstation facilities from vendors such as Wonderware. Recent attempts to use operating systems as "the" solution have had limited success. UNIX is not a real time system , although Posix and Realix are promising.
Some of the advantages of VMEbus systems most often mentioned are actually illusory or temporary. FDDI communications available on VMEbus systems will soon be available for PLCs. Any new communication protocol will soon find its way into PLC solutions, once it's proven itself in the computer industry. Sonet, for example, is one of the protocols that will emerge in the next decade. Even E-net electronic mail can be used in a real-time system if the occupancy level is reduced.
Recently , I visited a very large science project where the various options available for industrial inputs and outputs (I/O) were the subject of considerable conversation. Interested parties asked each other, should customers be advised to chose I/O on the bases of life-cycle costs or simply on entry costs? To me, the answer is clear. Looked at over the entire life cycle, PLC vender I/O offerings are very cost-effective. What's more important, they are very reliable. Frankly, the prospect of having an elevator, airplane or furnace controlled by a random mix of products purchased from the back pages of Byte magazine makes me nervous.
This is not a tirade against VMEbus. It is a tirade against faith in an earth centered universe. No single tool is the center of an expanding universe of possible solutions. I like VMEbus foe the applications for which it is suited. I like PLC's for solving automation problems. That's not to say that PLCs don't suffer from lacks. PLCs can lack flexibility in utilization, lack data analysis capabilities, lack the ability to interchange vendor products, and suffer from lack of progress in language enhancements.
But the PLC solves problems. The real issue is software, not hardware. PLC software is "easy-entry" and it works. It is not dependent on a single engineer; it operates in real time, and it is designed specifically from the problem that it addresses. Let the market decide. Why have PLCs become a global business, in spite of perceived lacks? Because they work.
As appeared in Manufacturing Systems Magazine June 1992 Page 16
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