A programmable logic controller (PLC) is a microprocessor-based control unit designed for an industrial
installation (housing, terminals, ambient resistance, fault tolerance) in a power switchboard to control
machinery or an industrial process. It consists of a CPU with memories and an I/O interface housed
either in a compact box or in modules plugged in a frame and connected with proprietary buses. The
compact box starts with about 16 I/O interfaces, while the module design can have thousands of I/O
interfaces. Isolated inputs usually recognize industrial logic, 24 V DC or main AC voltage, while outputs
are provided either with isolated solid state switches (24 V for solenoid valves and contactors) or with
relays. Screw terminal boards represent connection facilities, which are preferred in PLCs to wire them
to the controlled systems. I/O logical levels can be indicated with LEDs near to terminals.
Since PLCs are typically utilized to replace relays, they execute Boolean (bit, logical) operations and
timer/counter functions (a finite state automaton). Analog I/O, integer or even floating point arithmetic,
PWM outputs, and RTC are implemented in up-to-date PLCs. A PLC works by continually scanning a
program, such as machine code, that is interpreted by an embedded microprocessor (CPU). The scan
time is the time it takes to check the input status, to execute all branches (all individual rungs of a ladder
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