Memory provides permanent storage to the operating system for data used by the CPU. PLCs work with inputs, outputs, a power supply, and external programming devices. The CPU runs memory routines, constantly checking the PLC (PLC controller is redundant) to avoid programming errors and ensure the memory is undamaged. The CPU directs the PLC to execute control instructions, communicate with other devices, carry out logic and arithmetic operations, and perform internal diagnostics. It is a -16 or -32 bit microprocessor consisting of a memory chip and integrated circuits for control logic, monitoring, and communicating. PLCs also cannot display data well, so external monitors are often required.Ī central processing unit (CPU) serves as the brain of the PLC. When dealing with data that requires C++ or Visual Basic, computers are the controllers of choice. They do not perform well when handling complex data. Replacing relays with PLCs helps prevent overheating of contacts. Arcing generates high temperatures that weld contacts shut and cause degradation of the contacts in the relays, resulting in device failure. Relays switching under load can cause undesired arcing between contacts. PLCs are modular so they can be plugged into various setups. Their programming language is easily understood, so they can be programmed without much difficulty. PLCs are robust and can survive harsh conditions including severe heat, cold, dust, and extreme moisture. Technicians then had to troubleshoot an entire wall of relays to fix the problem. Relay systems at the time tended to fail and create delays. PLCs were introduced in the late 1960s by inventor Richard Morley to provide the same functions as relay logic systems. The image above is an Allen-Bradley PLC rack, a common example of a PLC setup that includes a CPU, analog inputs, analog outputs, and dc outputs.
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