In the past, equipment control for manufacturing processes on the shop floor tended to be carried out by discrete systems running arcane real time operating systems controlled by proprietary management systems that had no connection through to other systems.

In essence, this meant that the systems were secure, being "air locked" from everything else around them. The sensors fed information to a low-intelligence control unit that would send simple control sequences to actuators on the equipment. Sensor A shows that the temperature is too high here, actuator B opens up a little to let more coolant through, and so on.

At The Register.

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