![]() ![]() ![]() The “path” just described represented electrical current flow through the logic.ĭavid M. If the path to the right side completes (there is a complete “true” path across the ladder), the ladder is complete and the output coil will be set to “true” or “energized.” If no path can be traced, then the output remains “false,” and the relay remains “de-energized.” 1 This was implemented before PLCs, with a (+) bus on the left-hand side and a (−) bus on the right-hand side. If an input relay is “true” the path continues, and if it is “false” it does not. A path is traced on the left side, across “rungs” consisting of various inputs. Ladder logic follows a relay function diagram, as shown in Figure 4.2. Ladder logic gets its name from the legacy method of implementing discrete logic via electromechanical relays and was initially referenced as “relay ladder logic.” Ladder logic can be thought of as a set of connections between inputs (relay contacts) and outputs (relay coils). Programmable logic controllers can use “ladder logic” or “ladder diagrams (LD),” which is a simplistic programming language included within the IEC-61131-3 standard that is well suited for industrial applications. However, implementation of safety interlocks in programmable electronic systems is not universally accepted and many organizations require that all safety interlocks be hard-wired. Programmable logic controllers are a potential candidate for implementing interlocks due to excellent capability of implementing discrete logic. Therefore, for applications that require extensive use of discrete logic, most DCS suppliers will incorporate PLCs into their system. The process control modules of a DCS often implement discrete logic, but their discrete I/O capabilities and slow scan rates rarely match that of a PLC. Scan rates much faster than 100 msec are common. The manufacturers of PLCs provide robust, cost-effective discrete I/O modules that can scan discrete data points (sensors) very rapidly. ![]() They can be programmed using the same ladder logic diagrams or other textual and graphical programming languages to represent logic and are an excellent tool to implement discrete logic. Programmable logic controllers (PLCs) were developed in the 1970s in order to replace hard-wired relay logic. Edgar, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 VII.C Programmable Logic Controllers ![]()
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