Alternative to Modulebus when configuration exceeds maximum number of I/O-clusters?
I´m reluctant to "downgrade" to a single CPU configuration x 2 just to double the capacity....
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Direct IO is really only an option when all of your IO is centrally located AND you need to scan that IO faster than your Bus update rate will allow.
You can also now run S800 I/O modules on "Ethernet" - the same as Select I/O - but the real secret here is that this is actually Profinet. Profinet does work very well and has much faster bus update rates than Profibus - as well as being much easier to install. Everything runs on Ethernet hardware.
Or you can run Profinet to third party IO modules or remote PLCs using a CI871.
What is the 800xA system Version planned to use? PM858 Only compatible with System 800xA 6.0.2, Compact Control Builder 6.0.0-1 and onwards. Please see product Update for more information.
If your requirement is 9 clusters, AFAIK would suggest to go for 2 pairs of controller with optical module bus configuration for your 3 sections of IO clusters. And also by considering the memory or performance - application type/complexity of Library/Logics and expected Scan Cycle or CPU Load, you can choose PM858 or higher controller version.
I would recommend to use profibus anyway, if distance is great, you can use Profibus to fiber converter. With correct setup, You can achieve similar or even better I/O scan ratio.
If You are looking anyway for other solution, maybe think about changing modules to 32 channels? In this case You can compress existing clusters and maybe You can get enough for future part.
Different solution could be use of Select IO [ethernet connected modules/clusters] , but this will require newest 800xA (but this option is definitely not the cheapest).