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.