INTRODUCTION
In previous decades (and now) PLC manufacturers favored “proprietary” or
“closed” designs. This gave them control over the technology and customers. Essentially,
a proprietary architecture kept some of the details of a system secret. This tended to limit
customer choices and options. It was quite common to spend great sums of money to
install a control system, and then be unable to perform some simple task because the manufacturer
did not sell that type of solution. In these situations customers often had two
choices; wait for the next release of the hardware/software and hope for a solution, or pay
exorbitant fees to have custom work done by the manufacturer.
“Open” systems have been around for decades, but only recently has their value
been recognized. The most significant step occurred in 1981 when IBM broke from it’s
corporate tradition and released a personal computer that could use hardware and software
from other companies. Since that time IBM lost control of it’s child, but it has now
adopted the open system philosophy as a core business strategy. All of the details of an
open system are available for users and developers to use and modify. This has produced
very stable, flexible and inexpensive solutions. Controls manufacturers are also moving
toward open systems. One such effort involves Devicenet, which is discussed in a later
chapter.
A troubling trend that you should be aware of is that many manufacturers are mislabeling
closed and semi-closed systems as open. An easy acid test for this type of system
is the question “does the system allow me to choose alternate suppliers for all of the components?”
If even one component can only be purchased from a single source, the system
is not open. When you have a choice you should avoid “not-so-open” solutions.
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