- PLCs replaced relays because they were more flexible and cost-effective. A relay control system had a fixed number of relays wired a certain way. Changes required replacing or adding relays, and changing the wiring at substantial cost. PLCs were inexpensive, and their programs were easy to change.
Even today it is much easier to change the program of a PLC than a computerized system. This advantage comes from the fact that PLCs are process-oriented, showing the technician the inputs to a particular module, and asking him to choose what the corresponding output should be. Once the program is finalized, it stays in the PLC memory, even during power failures. - Compared to relays, PLCs are more reliable because they don't have contacts. All outputs are solid-state electronic modules that are much more reliable than mechanically switched contacts. Today PLCs are more reliable than other control systems due to their simplicity. They use electronics to scan all the inputs, run through the programmed logic and determine the outputs as a result. Programmers work directly with the inputs and outputs. There is no separate operating system that takes time to load and has the potential for bugs or viruses.
- PLCs are built up of modules that control individual inputs and outputs. When a PLC fails, it is easy to check which outputs are not operating correctly. Many PLCs have built-in fault finding circuits that give an alarm when an output is not controlled properly. For most faults, technicians can quickly isolate the problem and replace the corresponding modules. PLC modules are comparatively inexpensive, allowing companies to stock spare modules for critical processes.
- Programming a PLC means telling it which inputs are connected to particular terminals, what operations it should perform with the input values and what it should output as a result. Ladder diagrams, showing relay contacts the way they were used in relay control systems, are often used to program PLCs.
With more powerful PLCs able to perform more complex operations, logic diagrams, block diagrams and even low-level computer languages are used to program PLCs. However, in most cases, companies don't need computer programmers to run the PLCs, and process technicians can make the changes often required to optimize a process. - When they replaced relays, PLCs had to be able to switch the same voltages and currents that relay contacts were able to handle. Later, this ability became an advantage since computer systems were not able to switch higher power levels on and off. Where PLCs can switch motors, valves, heaters and similar equipment directly, computer systems require amplification of their output signals to be able to operate equipment. This self-contained nature of the PLC has yet to be duplicated with other control systems.
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