Industrial control systems (ICS) are the controllers and software that run a plant: small computers beside each machine, and a control room above them, where operators watch the plant on screens. Every reading from the plant is also recorded.
There are four main products in this zone. A programmable logic controller (PLC) is a rugged computer beside a machine, which reads its sensors and switches it many times a second. A distributed control system (DCS) does a similar job for a process that never stops, such as a refinery. Both pass their readings up to SCADA, the software on the operators' screens, through which they watch the whole plant and steer it. Finally, the process historian stores every reading with its time, so that engineers can look at it later.
Here is a typical case. At a sugar mill in Kolhapur, a sensor on the juice tank measures its level and sends the value to the PLC beside the tank. The PLC passes the value up to SCADA, where the operator sees the tank as a green bar on a diagram of the mill. If the level climbs too high, SCADA shows an alarm, the operator closes the inlet valve from the screen, and the historian stores the level for the whole season. The four products each do one part of this one journey.
These systems are sold in an unusual way. PLC makers rarely sell straight to the plant. They sell through system integrators, which are engineering companies that design the control system for a plant and install it. This is channel selling. DCS makers, by contrast, often design and install the system for a large plant themselves. And once a plant starts with one maker, it usually stays with that maker, because the equipment is rarely replaced. A controller may run for twenty years or more.
This zone differs from the layer above it. Control systems act within a fraction of a second, because a machine or a valve cannot wait. The layer above, manufacturing operations, works in minutes and hours. It counts orders and batches and does not control machines.
Because the equipment lasts so long, much of it runs firmware, the permanent software inside a device, that was written long ago. The historian is a database for time-series data, which is a list of readings with the time of each one.
The readings from this zone are used by the layer above it, where a manufacturing execution system counts what the plant has made. And since these systems control physical machines, they need OT and IoT security.