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PLC70

Main Features of a CPU

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I want to know that what are the main features which we must consider when make comparison between two CPU . For instance number of inputs and outputs a CPU can handle is a feature of this CPU means if a CPU can handle 100 inputs/outputs and other can handle 200 inputs/outputs then we will say that the CPU which can handle 200 I/Os is much better then the CPU who can handle 100 I/Os. Similarly i want to know other features, we must consider when make comparison between two CPUs.

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This is an exersize to waste time and talent.The only valid comparison to compare a CPU's features with what is needed for its intended use. For instance if a CPu can handle 1024 I/O but your project only has 187 I/O what good is the higher count ability? So determin your most demanding system and choose a processor that is capable for it. You may want to look at: RAM, Speed, Software Ease of use, Available ports, Available protocols on these ports, Expandability, Physical size, CostParts availability And more... Edited by RussB

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Tell your teacher he's asking the wrong questions. All of the specifications published by the makers of PLCs are nothing more than tools for you to use to determine whether or not a particular model is capable of doing the task you need it to do. If there were a single set of specifications that defined the "best" PLC, then all manufacturers would offer a model that met those specifications. If the CPU that can handle 100 I/O points costs $100 and the CPU that can handle 200 I/O points costs $1000 and the job you need a PLC for requires 75 I/O points, which is better for the task at hand? On the other hand, if your facility already has 50 of the $1000 model and your entire maintenance staff is familiar with it, does it make sense to use the less expensive model? But wait, suppose the $1000 unit of which you already have 50 in use has been declared obsolete by the manufacturer?

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Actually i want to say that how you will determine that one CPU is better then the other. Let suppose if you have SIEMENS 315-2 and 318-2 then how you will determine, which one is better?

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Better for what? I'm not familiar with the 318-2, but in general one model may have better motion control capability but might scrimp on other I/O while the other model has better I/O but no motion. "Better" is like buying a car. Your specific needs determine what's the best solution for your problem.

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A few things to look at: Processing speed. Size of memory. I/O capacity. Feature set of the programming languages. Support for communication protocols. Communication performance. Support for distributed I/O. Distributed I/O capacity. Supports integrated motion functions. Supports integrated safety functions. Supports RIUP. Support for CPU redundancy. Support for I/O redundancy. Support for network redundancy. Reliability. Documentation. Tech support. Availability of spares. Price.

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everyone is correct....and not answering your specific question....well sort of.....they're not answering better....they are answering necessary....its important to understand the requirements of the application....a processor that handles 1024 i/o is not 'better' for a specific application than a unit that handles 256 i/o if the application requires 10 i/o. Buts its inherently a 'better' processor (this is a generality as I'm not considering speed, etc.) because it has more capability. Your professor (or teacher) is probably asking the latter as a reference but using the word 'better'. Whats a more capable processor? It could be equated again to the application...but if an application is not specified (this is actually silly from an engineering standpoint) a faster CPU, with more memory, more functions, and more support for i/o, communications etc., would be termed the 'better' processor. I'm guessing that you're taking a plc course....and its very likely your teacher is not an Engineer, nor an actual accredited teacher. He, or she, is most likely someone who has several years of experience or training. They'll judge you on what they expect, not necessarily on what would be an answer.....silly thing, I've actually worked for Engineers that would ask and expect the same thing....go figure....I certainly would not argue with my professor that a faster cpu is not 'better' than a slower cpu. bob

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