1.2 Control Structures

Class B controls can be designed according to one of three defined structures (H.11.12.2):

• Single channel with functional test – A single channel structure in which test data is introduced to the functional unit prior to its operation (H.2.16.5).
• Single channel with periodic self-test – A single channel structure in which components of the control are periodically tested during operation (H.2.16.6).
• Dual channel without comparison – A structure which contains two mutually independent functional means to execute specified operations (H.2.16.1).

The term channel refers to the number of MCUs in the appliance. The single channel structure tends to be the preferred structure since it has the lowest cost.

A single channel structure with functional test means that the system is tested at the point of manufacture only. This structure can only be used if the appliance does not use any components that require periodic testing. Periodic testing is a safer option because this allows for faults to be detected during operation of the product. The time interval between the tests must typically be shorter than the time it takes for a Fault in the relevant component to cause a hazard.

Dual channel without comparison is essentially a structure in which two MCUs operate independently with different tasks and either MCU can check that the other is operating correctly. One approach is to use one MCU strictly for supervision, rather than sharing control tasks between the two. In this case, a lower cost device can often be used as the supervisor.

This application note focuses on the single channel structure.