Process control is primarily enabled by the PLC (programmable logic controller) or the PAC (programmable automation controller). Today the controllers are quite similar, but both began with slightly different roots. The modern PLC is the result of technology advancement from the control realm, whereas the modern PAC is the result of technology advancement from the data communications realm. Now PLCs have communications capability and PACs have control capability, so the differences are blurred. We will use the term “PLC” to refer to both controllers. The PLC's block diagram can be simplified to the one shown below. In larger, modular PLCs, I/O modules exist as plug-in cards with their own microcontroller per card and a backplane connection to a high-speed bus that runs between each I/O module and a plug-in CPU module. Additionally, there is usually a power-supply module and a communications module. For simplicity, we have not broken out this detail. All the solutions presented in this guide are intended for use in industrial process control, HVAC and building automation, motor control, and other industrial applications.
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