The lowest level of the controls hierarchy is the control module. Therefore, controls units perform two primary functions. Additionally, they provide an interface with the physical devices, and they contain basic control algorithms.

A control module is a collection of basic control logic, sensors, actuators, control modules, and associated processing equipment that is operated as a single entity. CMs where the real work is done, in the form of basic control. They only execute basic control, minimal coordination control and perform no procedural control functions.

CMs usually make up the majority of control system code in a batch application. However, they are also the mechanism for defining significant amounts of reusable code.

Equipment modules and controlling modules should be treated as objects in a phase. A phase should not directly act on an output. A phase should send a message to the control module requesting it to take an action.