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

From: Peter Bishop < >
Date: Wed, 24 Jul 2013 13:41:54 +0100

Yes it is similar to a PLC as it uses functions blocks (not sure if it precisely 61131 though)

The other features are
- simulation so application software can be tested before installation on the Teleperm XS
- application timing analysis before installation to ensure applications do not exceed their allocated time slots - Capability to implement N out of M vote architectures with data exchange over comms links

It is used on 74 nuclear power units including the new Areva EPR.

Bit of bumpf here (but not much detail).


NB usual disclaimers - no financial interests in plugging this system.


> Hi, Peter
> Function block languages like the one standardized in IEC 61131?
> This Areva TXS is like a PLC processor?
> Best regards,
> Thierry Coq
> -----Original Message-----
> Sent: mercredi 24 juillet 2013 12:54
> To: systemsafety_at_xxxxxx
> Subject: Re: [SystemSafety] Separating critical software modules from non-critical software modules
> Relatively simple schedulers have been used like the Siemens (now Areva TXS). But there is a strong emphasis on determinism (maximum time slots per process) separate i-o modules, etc. Also the applications are often defined at a higher level using a function block language which reduces the scope for application level errors.
> Peter
> Ignacio González (Eliop) wrote:

>> Peter, I come from the railway world. In nuclear, you cannot even use
>> an OS with safe partitioning, Arinc-style, hypervisors, etc.?
>> With no physical separation you need to be very sure that
>> "non-safety" cannot affect safety (overwriting "safe" memory,
>> crashing the system, hogging cpu, comms...)
>> In nuclear standards, anything in the same box has to be implemented
>> to the level of the most critical function.
>> Peter Bishop
>> <mailto:grcreech_at_xxxxxx >>
>> Malcolm,
>> I agree that for small devices it may be difficult to provide or
>> prove separation between safety and non-safety and therefore all
>> needs to be considered safety in these cases.
>> However, I am also a firm believer that removing complexity
>> increases safety. If I can prove, say 50% of the code, cannot
>> affect safety then I can focus on the 50% that does and not get
>> distracted on the areas that have less effect.
>> Just because there is a safety & non-safety section, doesn’t
>> mean that the programming style needs to be different, after all
>> even in the non-safety section quality is important for any
>> product and the non-safety sections obviously need to be in the
>> document structure clearly documented as non-safety.
>> Once the segregation is in place it has several benefits, for
>> example, although all code needs to be reviewed & tested (from a
>> quality point of view) why focus on software that could be
>> classed as black channel software components where the safety
>> aspect is assured elsewhere, the focus can be where it is
>> needed, complexity is reduced and the amount of important safety
>> code is reduced.
>> This method has the added benefit that proven in use / COTS
>> firmware can be used in the non-safety area knowing that it is
>> unlikely to affect the safety firmware.
>> Best regards,
>> Gerry Creech
>> From: "Watts Malcolm (AE/ENG11-AU)"
>> <systemsafety_at_xxxxxx >> <mailto:systemsafety_at_xxxxxx >> Date: 24/07/2013 02:01
>> Subject: Re: [SystemSafety] Separating critical software
>> modules from non-critical software modules
>> Sent by:
>> ----------------------------------------------------------------------
>> --
>> Our exerience in automotive is that it is effectively impossible
>> for most automotive products to have the kind of separation José
>> speaks of; for example “two separate board groups”. Much of our
>> software (although not SIL4 – often SIL2 equivalent) runs on a
>> single micro in a single device. Very high integrity product
>> might have 2 independent micros in a single enclosure, with some
>> redundancy of function in other devices in the vehicle (for
>> example, data redundancy). Many of the micros used do not have
>> memory-protection units, and micros may be running only
>> scheduling executives, not full operating systems (in the
>> interests of simplicity, proven field use, and testability). In
>> this circumstance, it makes the most sense (to me) to develop
>> all of the software in the micro to the highest integrity level
>> required by any component.
>> I share the concerns raised in to Myriam’s post; as a matter of
>> practicality, few developers are feasibly able to swap back and
>> forth between “safety” and “no-safety” development methodologies
>> (to say nothing of the cost and complexity of maintaining two
>> sets of procedures, two sets of training, duplicated QA, the
>> complexity of planning and tracking, and so on. To answer
>> Myriam’s rhetorical question; no, for me it does not make sense
>> that developers can swap back and forward between two different
>> mindsets without mistakes, and no, it does not make much sense
>> that tightly-coupled modules can be part of significantly
>> different lifecycles without adverse effects on interfaces,
>> assumptions, change management and quality requirements. [This
>> is the same problem faced when incorporating 3^rd -party
>> components. There’s a reason that such a high proportion of
>> defects are in the interfaces].
>> The more conservative approach (taking into account possible
>> changes, and mistakes in understanding whether a component or
>> its interface is safety-relevant or not, under given
>> circumstances, is to develop all software components (in
>> tightly-coupled products typical of automotive) to the
>> highest-applicable integrity level.
>> The benefit you get (in my opinion) is reduced risk due to
>> unexpected interference between modules, reduce risk due to
>> systematic defects, reduced risk due to human-factors effects
>> from the developers, reduced cost due to consistency, and
>> better/faster impact analysis on change.
>> The flip side is increased cost and effort for all components
>> (and their integration ?) that could otherwise have been
>> considered “non-safety-relevant”. This really is a serious
>> disadvantage of the approach. Ignacio mentioned that this may
>> be practical only for small teams and “small software”. Does
>> anyone know of any research in this area ?
>> Best Regards,
>> Mal.
>> Mal Watts ^
>> ----------------------------------------------------------------------
>> ----------------------------------------
>> Functional Safety Manager (AE/ENG11-AU)
>> Robert Bosch (Australia) Pty. Ltd.
>> Automotive Energy and Body Systems,
>> Locked Bag 66 - Clayton South, VIC 3169 - AUSTRALIA
>> Tel: +61 3 9541-7877 <tel:%2B61%203%209541-7877> Fax:
>> +61 3 9541-3935
>> _ <>
>> [mailto:systemsafety-bounces_at_xxxxxx >> Behalf Of *José Faria*
>> Sent:* Tuesday, 23 July 2013 7:58 PM*
>> To:* M Mencke*
>> Cc:* systemsafety_at_xxxxxx >> <mailto:systemsafety_at_xxxxxx >> Subject:* Re: [SystemSafety] Separating critical software
>> modules from non-critical software modules
>> 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
>> 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.
>> _______________________________________________
>> The System Safety Mailing List_
>> __systemsafety_at_xxxxxx >> <mailto:systemsafety_at_xxxxxx >>
>> --
>> --
>> *José Miguel Faria*
>> *Educed *- Engineering made better
>> t: +351 913000266 <tel:913000266>
>> w: _www.educed-emb.com_ <>
>> _______________________________________________
>> The System Safety Mailing List
>> systemsafety_at_xxxxxx >> <mailto:systemsafety_at_xxxxxx >>
>> _______________________________________________
>> The System Safety Mailing List
>> _______________________________________________
>> The System Safety Mailing List
>> systemsafety_at_xxxxxx >> <mailto:systemsafety_at_xxxxxx >>


Peter Bishop
Chief Scientist
Adelard LLP
Exmouth House, 3-11 Pine Street, London,EC1R 0JH
Recep:  +44-(0)20-7832 5850
Direct: +44-(0)20-7832 5855
The System Safety Mailing List
Received on Wed Jul 24 2013 - 14:42:18 CEST

This archive was generated by hypermail 2.3.0 : Tue Jun 04 2019 - 21:17:05 CEST