Personal computers have been driving the open architecture revolution. A personal computer is capable of replacing a PLC, given the right input and output components. As a result there have been many companies developing products to do control using the personal computer architecture. Most of these devices use two basic variations;

• a standard personal computer with a normal operating system, such as Windows NT, runs a virtual PLC.

- the computer is connected to a normal PLC rack

- I/O cards are used in the computer to control input/output functions

- the computer is networked to various sensors

• a miniaturized personal computer is put into a PLC rack running a virtual PLC.

In all cases the system is running a standard operating system, with some connection to rugged input and output cards. The PLC functions are performed by a virtual PLC that interprets the ladder logic and simulates a PLC. These can be fast, and more capable than a stand alone PLC, but also prone to the reliability problems of normal computers. For example, if an employee installs and runs a game on the control computer, the controller may act erratically, or stop working completely. Solutions to these problems are being developed, and the stability problem should be solved in the near future.

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