May I ask a question?What's the maximum number of PID loops of AC 800M(ABB 800xA)
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I guess there is no real limit. The limits on any given AC800M controller model is based on the availability of RAM, & CPU cycles. These are in turn limited by what interface cards (CI8xx) and whether you use Modulebus IO or Profibus IO. Profibus S800 IO has a lower load than Modulebus but costs more. Other effects are cyclic loads and the number of Applications. For example, if you have a large load running at 0.5sec cycle, this may use up all your given RAM/CPU cycles. The same application running at say 5 sec cycle, will use a lot less CPU cycles (same RAM overall), but therefore having less load and possibly you could a few more PID's.
Other issues are heavy loads based on convoluted code (not optimised code like "homegrown" libraries), high IO counts, lots of MMS interfacing, and so on
As mentioned in other responses, the manuals do give you some idea on loading effects. As the AC800M is a controller that can handle any situation, PID codes vary (type dependant), answering this type of question is always difficult.
I know that this does not specifically answer your question, but I would ask: how PID's do you need? What library are you using? What are the cost restraints? Can you split the controller code into two?....