A lower level of distribution creates a simple and effective system, with few dependencies. Besides, the implementation takes a shorter time. But they contain disadvantages that you can solve by building a higher level of distribution.
Central Programmable-Unit (PU) with I/O’s.
Disadvantages: The whole system is out of control if the central PU breaks down. In effect, you can’t shut down the plant in a safe way. Unless the plant is simple and small, the wires to sensors and activators are long and hard to install.
A system with first level of distribution has distributed I/O’s and one central PU polling data. You copy the I/O status to or from I/O’s via fast bus systems.
Disadvantages: The whole system is out of control if the central PU breaks down or the bus connection is lost. To clarify, you can’t shut down the different parts of the plant in a safe way.