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.
0th 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.
1st level of distribution
A system with 1st level of distribution has distributed I/O’s and one central PU polling data. The I/O status is copied to/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.
2nd level of distribution
Each cluster has its PU connected to a central PU. The PU’s in the clusters are polling data and hold a copy of the status of each I/O.
The local PU can hold a program to make a safe shut down in the event of a breakdown on the central PU or bus connection.
Disadvantages: Each cluster still depends on its PU being alive and the bus connection to the different I/O modules being intact. In other words, you can't shut down local parts of the plant in a safe way.
3rd level of distribution
Examples of uses are temperature control using analog input or output control using a steam valve.
Each control functions can shut down. The need for a redundant system is much lower, as the damage of an error is limited.
A plant built on the third level limits the effect of a hardware error to a small area, so the rest of the plant can keep working. "A broken finger does not prevent the other hand from calling the doctor”.
During service and when making changes, a level 3 implementation makes it possible to perform a selective shutdown. It also reduces the need for redundant control equipment.
However, the implementation of the third level is often time-consuming unless you are using an integrated development environment specialized to level 3, such as VIGO6.