Hi,
I am quite new to circuit designs. I need to create multiple addressable modules of about 30 I/O pins each which are to be connected to a master board and controlled accordingly. I was thinking of using multiple arduino nano's and multiplexers and somehow connected by a one wire bus system to an arduino mega being the master?
Thanks for your help.
What are the 30 I/O pins connected to? When you say, "multiple", how many do you mean?
Also, what does your post have to do with, "Installation and Troubleshooting"?
I certainly hope you are including serviceability and fault detection in your design.
The 30 I/O pins would be connected to 30 sensors and 30 mosfets accordingly. For my usecase I was thinking of using 70 slave units (arduino nano) so in total the master (arduino mega) needs to handle 2100 inputs and 2100 outputs.
30 sensors and 30 MOSFETs would be 60 I/O. Nano doesn't even have 30 I/O.
How far apart are the modules? What kind of wiring is permissible between them? How will they be powered?
That is why I will be using multiplexers to have more I/O pins.
They will be powered by an external 12v power supply each, and they won't be apart more than 5m.
All of them will be within 5m, or each will be 5m from the next?
Each module will not be more than 5m apart from the next.
What is your budget for this?
That would be accounted for, after I manage to find a way to have multiple slaves connected to a master board 
. However I am accounting some input pins for module fault detection.
Currently the budget is not set, since I need to see if the idea is possible to be implemented or not.
Of course it's possible. We went to the moon.
Are you thinking RF connections or wired connections for the communications? Do you have a data rate in mind and a response time for each slave unit? In other words, how often will a slave have to be polled?
I would prefer if it have a wired connection, since the installation would be used in a crowded environment with a lot of wireless connections. The response time needs to be fast to allow for fast switching of multiple mosfets at once.
Is this a real project, or an academic exercise or pipe dream?
This is going to be a real project using arduino or not, to be able to move multiple RGB lighting in a 3D environment with motors.
Then, what is your budget? The maximum amount that you can possibly spend?
For your 70 modules, you could connect them with RS-485 with some bridging...
DMX motors each cost around €75 to rent, we need around 400, so the maximum budget for the project not to be feasable is of €30,000.
What is your engineering background? This would be a challenging project for even a senior engineer.