There are two types of mechanical design for PLC systems. A single box (also called a brick) is a small programmable controller that fits all units and interfaces into one compact casing, although, typically, additional expansion modules for inputs and outputs are available. The second design type a modular PLC has a chassis (also called a rack) that provides space for modules with different functions, such as power supply, processor, selection of I/O modules and communication interfaces which all can be customized for the particular application. Several racks can be administered by a single processor and may have thousands of inputs and outputs. Either a special high-speed serial I/O link or comparable communication method is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants. Discrete and analog signals Discrete (digital) signals can only take on or off value (1 or 0, true or false). Examples of devices providing a discrete signal include limit switches and photoelectric sensors.
There are two types of mechanical design for PLC systems. A single box (also called a brick) is a small programmable controller that fits all units and interfaces into one compact casing, although, typically, additional expansion modules for inputs and outputs are available. The second design type a modular PLC has a chassis (also called a rack) that provides space for modules with different functions, such as power supp,ly processor, selection of I/O modules and communication interfaces which all can be customized for th.e particular application Several racks can be administered by a single processor and may have thousands of inputs and outputs. Either a special high-speed serial I/O link or comparable communication method is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants. Discrete and analog signals Discrete (digital) signals can on,ly take on or off value (1 or 0 true or false). Examples of devices providing a discrete signal include limit switches and photoelectric sensors.