0th level of distribution:
Central Programmable-Unit with I/O’s.
Disadvantages: The whole system is out of control if the central Programmable-Unit (PU) breaks down, the plant/ machine cannot be shut down in a safe way. Unless the plant/machine is simple and small, the wires to sensor and activators become long and difficult to install.
1st level of distribution:
System with distributed I/O’s and one central PU polling data. The I/O status is copied to/from I/O’s via a high speed bus systems.
Disadvantages: The whole system is out of control if the central Programmable-Unit breaks down or the bus connection is lost. The different parts of the plant/ machine cannot be shut down in a safe way.
2nd level of distribution:
Each cluster has its own PU connected to a central PU. The PU’s in the clusters are polling data and hold a copy of the I/O status in the different I/O’s.
The local PU can hold a program to make a safe shut down in the event of a breakdown on the central PU and/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. Local elements of the plant/ machine cannot be shut down in a safe way.
3rd level of distribution:
Each I/O module in the cluster contains a PU’s. Each PU are used to perform a control function including I/O access. E.g. Valve control with feedback,
A central PU may send commands to the local PU’s, which sends status reports to the central Programmable-Unit. Application: E.g. temperature control using analogue input, Analogue output control a steam valve et.
The individual control functions can shut down individually. The need for redundant system is much lower, as the damage of an error, is limited.
A plant built on the level 3 principle limits the influence of a hardware error to a small area, so the remaining plant can continue working. "A broken finger does not prevent the other hand from calling the doctor”.
During maintenance and reconstruction, a level 3 implementation makes it possible to perform selective Shut down.
A level 3 implementation also reduces the need for redundant control equipment.
The level 3 implementation is normally very time consuming unless using an integrated development environment specialized to level 3, such as VIGO6.