Durrais

Since PLCs were designed initially as an electronic replacement for relays, the programming language used was similar to the relay logic used, so that the technician could easily understand and program it without much additional training. However, this has become less relevant today, since most PLC programmers have a software background and are familiar with conventional programming languages.

In ladder logic, a particular sequence of events is described using the open and close contacts of the relays and the flow of current / power. The “OR” condition is defined by connecting relays in parallel, the “AND” condition by relays connected in series. For example a process (output) may be started only if the operator presses the start button (input 1) and there no failure in any equipment(input 2). There may be many sensors or switches for indicating failure of an equipment, and even one failure indication could cause operational problems of the entire equipment.

Special software is available to write ladder logic software on standard PCs. This is then uploaded to the PLC CPU by connecting it to the serial port of the PC.

All manufacturing processes can be defined as a sequence of operations and this sequence was first implemented using relay logic. The relays were hard wired together and since they were mechanical systems, they had a limited life span. If a single relay failed, it had to be rewired by a skilled technician, resulting in manufacturing downtime and loss of production.

The Programmable Logic Controller (PLC) was first designed to be an electronic replacement for the hardwired relays for use in automotive manufacturing processes. Though the microcontroller CPU used in PLCs may be similar in specifications to that used for consumer electronics, the PLC systems are designed for operation in industrial environment, where electrical noise levels and temperature variations are high, with very high reliability rates.

As they are designed to control machinery, assembly lines , material handling system and other process automation, PLCs incorporate a sophisticated control systems, which process the input from a number of input modules connected to the PLC. Based on the programming sequence loaded in the PLC, suitable commands will be given to activate valves and other equipment connected to the system.

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