[SystemSafety] Separating critical software modules from non-critical software modules

[José Faria]

Myriam,

Yes, it is a valid approach. Valid meaning both technically feasible and
acceptable by certification authorities. As Gerry said, the fundamental
issue is to demonstrate that the lower SIL level part cannot compromise the
higher level part.

In the systems I've worked the basic architecture solution was to have 2
separate board groups for the SIL4 and SIL0 software. In such a solution,
you can find the guidance for the safety analysis of the communication
protocol between the two boards in EN 50159 Annex A.

Best,
José

On Tue, Jul 23, 2013 at 9:21 AM, M Mencke <menckem at gmail.com> wrote:

> Dear All,
>
> For any software development project, many software modules are involved,
> where some are defined as safety critical, others are not. For example, in
> railway signaling, communications modules are likely to be defined as
> critical, whereas other modules such as those involving data storage or
> other basic functions are not. An analysis may be performed with the
> objective of demonstrating that the safety critical modules are entirely
> independent from the non critical modules, leading to the conclusion that
> the application of a programming standard for safety critical software is
> only required for those modules defined as safety critical (note the phrase
> “with the objective of demonstrating…”; I would hesitate before drawing the
> conclusion that the analysis really demonstrates what it is supposed to
> demonstrate).
>
> In my field the EN 50128 would be applied, however, it could be any
> standard for safety critical software. Thus, the software is developed
> applying the standard only to the modules which have been defined as
> “safety critical”. In order to supposedly save time/money, etc., the rest
> of the modules are developed as non-critical software, either as SIL 0
> functions or according to a standard programming standard. My question is
> whether such an approach is really valid, given that the application of a
> safety critical standard does not only involve the application of specific
> language features, it involves an entire development life cycle, and I find
> it difficult to see how the modules defined as “non-critical” then do not
> form part of that life cycle. I’m not saying it is not valid, but I would
> like to know how others see this.
>
> Additionally, if the same programmers are involved in the programming of
> both critical and non-critical modules, does it really make sense that they
> only pay attention to the features required for safety critical software
> when programming the critical modules, and modify their programming style
> for the rest of the modules (or revert back to their “usual” style)? These
> questions also depend on what you consider as critical, for example, for a
> control system with a HMI, you could only consider communication modules
> critical, however, you need a GUI to display the status of the elements an
> operator has to control correctly. Some operations performed by the
> operator may not have the potential to generate a hazard with a high
> severity level, because there are mitigations in place. However, that
> doesn’t necessarily mean that the software responsible for displaying the
> information should not be programmed according to a safety critical
> standard. I am aware that these questions don’t have an “easy” answer; any
> opinions would be appreciated.
>
> Kind Regards,
>
> Myriam.
>
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*José Miguel Faria*
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