FIELDBUSES ALWAYS COME IN THREES

Dear Editor

I think it was Dr John Porteous, of the British Automated Homes and Buildings Association (BAHBA), who remarked that, when waiting for a bus, `it was funny how three always came along together’. John was talking of the first of the three-headed monsters; the attempt by CENELEC TC205 (TC105 as it was then) to force fit European Installation Bus, European Home Systems and BatiBUS into a single European pre-standard for home and building electronic systems. Since then we have seen more monsters although the number of heads seems to be rising!

The infamous fieldbus standard, EN 50 170, has frequently been debated and deserves no further comment other than it, too, seems to be budding more heads.

Two weeks ago, CEN TC247 WG4 put the finishing touches to its European Pre-standard for System neutral data transmission for mechanical building services. This magnum opus manages to include eight protocols, including two from EN 50 170, carefully allocated to a three level hierarchy. In alphabetical order, the tally is BACnet (twice), BatiBUS, EHS, EIB, FND, LonWorks, Profibus and WorldFIP.

One good thing, perhaps the only good thing, that can be said for this standard is that the working group will be meeting (in two years time) to attempt to whittle the list down to a single protocol for each level!

Meanwhile, in another corner of the woods in Brussels, CEN ELEC TC1 7B is quietly fabricating a new Frankenstein.

Starting from a failed (or at least significantly delayed) attempt to standardise the Actuator Sensor Interface system (AS-interface) in IEC, this was subsequently injected into CENELEC. In March, it was agreed that further protocols were present in the market and the draft should be extended to include these. Hence, we expect to see at least DeviceNet, SDS and LonWorks join this effort.

Perhaps it is time we thought this one through. At least one committee, CENELEC TC205, is now doing this, and, rather than arguing the technical merits, or commercial strengths of the various contenders, they are considering the fundamental requirements for the systems. They have already published EN 50 090-22 `System overview – general technical requirements’, which includes common requirements for safety and EMC testing for all the systems.

A working group of TC205 has now circulated a draft on Functional Safety, which reveals that significant safety hazards may be present in some of the systems unless additional safeguards are implemented. Many, but not all, of these hazards relate to the possibility of remotely controlling, managing or reprogramming home electronic systems, without being able to verify the precise devices on those systems. Is this remote control far-fetched and futuristic? On the contrary, it is an essential and easily implemented facility and, with the explosive growth of the Internet, a highly likely scenario.

Many of the bus systems (I use the term loosely) are products which started with simple systems and have grown, topsy-fashion, thereafter. Rather than simply accepting or rejecting these for standardisation on what are, to all intents and purposes, commercial grounds, isn’t it time we looked deeply into the issues, such as safety, both electrical and functional?

Is it possible that, as the systems grew, their designers overlooked these key issues? The evidence from one committee suggests that the answer is `yes’ and we should insist on thorough verification of all new standard proposals, even if they are existing National Standards which the proposers wish to see adopted without question.

Failure to do this may expose the population to new risks which, given the reliance on British Standards which we have inculcated in them, they will not expect, and against which they will have no defences.