Single Channel Structure with Diversified Software - SELF tests

Description of Technique

This technique is distinguished from the normal type of single channel structure by a single, but nevertheless important feature : the software which has relevance to safety is incorporated twice, albeit in a diversivied form. The two versions of the program run consecutively, and determine, in different ways, the outputs relevant to safety, which are then compared.

The outputs from the first program run have to be put into a temporary store. There are three alternatives in regard to temporary storage and comparison:

a) external temporary storage, fail safe comparator

b) external temporary storage, externally tested comparator

c) internaI temporary storage, internaI comparison.

Characteristics and Special Features of the Technigue

This technique is designed to detect systematic software errors within those portions of the program which are set up using the principles of diversification. On account of the time diversity which occurs at the time the programs are being run, the effects of disturbances are also detected, provided that they make themselves evident up to the the comparison.

Special Factors in the Application/lncorporation of the technique

This technique will replace the verification of the two-channel diversified software sections; the remainder of the software and the hardware should be treated exactly as they would be in the case of a normal single-channel structure.

Checking

The specified properties of this technique are checked by checking for the diversity in the sections of the program which are involved, by means of a check as to the contents of the comparison programs and through an analysis of the switching circuitry which brings about the comparison being made, as well as by fault simulation, at the comparator and in the software by means of a microcomputer development system.

Literature for Further Reading

None known as yet