How to arrange IO modules in a PLC rack?

Hi,

Based on what we/I used to do/design, the arrangement of IO modules in a PLC rack comes with Digital IO the follow by Analog IO.

But I read it somewhere that, analog consume a lot of data exchange between CPU & Module, thus it should be located nearest to the CPU, while the Digital can be located after all the analog & comm modules.

So, the best arrangement (forget abt the wiring first), should be :

Communication module / Analog IO / Digital IO

Pls correct me if I'm wrong.

Thanks.

It depends.
Some PLCs require special positions for some types of modules. For example, this can be analog modules/high speed counter/interrupt modules are required to be in the same rack as the CPU (Modicon 984s). Sometimes not.
PLC manufactureres also can have their own reccomendations.

Just for information, the new line we are putting in has PLC racks with, 1 power supply, 1 CPU, 1 or 2 Profibus Masters, 1 Ethernet card.
All IO is on the field bus.

If the IO is in the same chassis as the processor, it really doesn't matter if the com and analog modules are first or last in the rack. However, if additional chassis are required then it is preferable to put the comm and analog cards with the processor if faster performance of the analog IO is needed.

This can sometimes vary with PLC too. For example if you are using a PLC5 with 2-slot addressing, you have to alternate input and output modules.

OK, thanks for all the feedback