What is the difference between AC800 and AC500? I have PLC AC800m but I ALSO found AC800 libraries in HMI. ????
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Totally different PLC's.
The only similarity is that both have IEC61131 like languages - ie you can easily learn to program them both if you understand the basics of FBD, ST and SFC etc.
The AC500 is aimed at a small scale automation and so called "low-voltage" applications. It can easily handle up to several hundred IO. Its cheap, easy to program and great for small projects. Its basically an expandable brick PLC, programmed using CodeSys http://www.codesys.com/ ( Actually, for a small "brick" type PLC its pretty powerfull and I like it a lot )
The AC800 is aimed at larger projects. Its more expensive but can easily handle several hundred or more IO per controller and when linked into an 800xA system can accomodate many thousands. It is aimed more at the DCS and large scale market - but can also be used for smaller projects.
My own experience is that the AC500 needs more "programming time" while the AC800M is much faster to engineer. At a certain project size you'll find the the low price of the AC500 will be offset by the increased engineering required and the AC800 becomes more economical.