Programmable logic controller: Difference between revisions

Revision as of 22:16, 10 November 2024

Revision as of 22:22, 10 November 2024

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[[File:PLC AB InstaladoV1.JPG|alt=Modular PLC with EtherNet/IP module, digital and analog I/O, with some slots being empty.|thumb|Modular PLC with [[EtherNet/IP]] module, discrete and analog I/O, with some slots being empty]]

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{{snd}} a ''modular'' PLC{{snd}} 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{{snd}} which all can be customized for the particular application.<ref>{{Harvnb|Bolton|2015|pp=12–13}}</ref> 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.{{Citation needed|date=April 2020}}<!--[[User:Kvng/RTH]]-->

===Discrete and analog signals===

[[Digital signal|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 switch]]es, and [[photoelectric sensor]]s and [[Encoder (position)|encoders]].<ref name=":8">{{Harvnb|Bolton|2015|pp=23–43}}</ref>

[[Analog signal]]s can use voltage or current that is proportionalanalogous to the size of the monitored variable and can take any value within their scale. Pressure, temperature, flow, and weight are often represented by analog signals. These are typically interpreted as integer values with various ranges of accuracy depending on the device and the number of bits available to store the data.<ref name=":8" /> For example, an analog 0 to 10&nbsp;V or 4-20 mA [[current loop]] input would be [[analog-to-digital converter|converted]] into an integer value of 0 to 32,767. The PLC will take this value and transposetranslate it into the desired units of the process so the operator or program can read it. Proper integration will also include filter times to reduce noise as well as high and low limits to report faults. Current inputs are less sensitive to electrical noise (e.g. from welders or electric motor starts) than voltage inputs. Distance from the device and the controller is also a concern as the maximum traveling distance of a good quality 0<!-10 V signal is very short compared to the 4-20 mA signal.{{Citation needed|date=January 2021}} The 4[[User:Kvng/RTH]]--20 mA signal can also report if the wire is disconnected along the path as a <4 mA signal would indicate an error.{{Citation needed|date=September 2023}}>

===Redundancy===