HIGHLAND PROTOCOLS

Dick Pountain/03 February 1997/Idealog 30

I suppose it's not so surprising that many folk who are into computers are also interested in other machines - for example at least five (I'm one of them) people who work for PC Pro ride motorcycles. A computer is in one sense the ultimate machine, the one that has an infinity of different uses, but only so long as you want it to shift bits and not atoms. Moving bits can be fun but I like to hear, see, smell and feel atoms in motion - I'm the proud new owner of a 950cc Moto Guzzi which moves my atoms in a very satisfactory way.

What's much less obvious is that nearly all the laws that we learn through computing have been known for centuries by mechanical engineers, except that often they didn't theorise them the way we do. Take communication protocols for example. My brother-in-law Pip has restored a pre-World War 1 diesel launch which he keeps moored at Oban, living as he does in Scotland. It's called the Kami No Michi (Spirit of the Sea), the name given by its original owner, an English army officer who had spent many years in Japan as some sort of attache. A few years ago we took the Kami sailing around the Inner Hebrides, but after a wonderful week we hit some mechanical trouble with the gearbox near Mallaig. The Kami is propelled by a huge 1950's Gardner diesel engine which is quite simple to work on, and neither of us is afraid of spanners, and so I donned a blue overall and began dismantling the gear selector mechanism while Pip stripped down the clutch.

Up in the wheel-house the gearbox's user interface manifests itself as a semi-circular brass plate on the wall - with the words FORWARD, NEUTRAL, and REVERSE cast into it - a brass gear change lever, and a separate brass arrowhead that points to which gear you're in. This mechanism communicates to the  gearbox below decks via a Bowden cable just like a bike brake cable. As I was unbolting bits from the gear box I was rather surprised to discover that there were in fact two of these cables, not one, leading up through the deck into the wheel-house, so I immediately assumed that they pulled the selector in opposite directions (which seemed rather over-engineered but reasonable.) I was missing the point completely. On tracing them backwards I found that one was connected to the gear lever and the other to the brass arrowhead. It suddenly dawned on me that these two cables made up a mechanical full duplex communications channel, running a simple Request and Acknowledge protocol; when you moved the gear lever one cable caused the gearbox to shift gear, and only then did the other cable pull the brass arrow over to the appropriate word.

Had I designed that gear change I'm very afraid that I would have just attached a brass arrow to the gear lever itself and used only one cable - the slatternly solution. That wasn't good enough for Gardner's engineer. He, for it was surely a he (I see him wearing a green cardigan, Ernest Bevin spectacles, and smoking St Bruno in a battered pipe) wanted that arrow to give you real information. Not just the information that you've pushed the gear lever to FORWARD, which you already knew, but the information that the gearbox had indeed gone into FORWARD, and so was operating correctly. This was not over-engineering at all but just great engineering, in a modest sort of way.

Another example. Pip rents a croft, way up North near Dornoch, which is what an estate agent might describe as 'ripe for modernisation'. It has no electricity or telephone; we use paraffin-fuelled, gas-mantled Tilly lamps for light and Calor gas for cooking. The only water supply is from a stone cistern, fed by two small streams, up on the mountain side around a mile from the house, and in dry weather it sometimes gets low (and yes, on the East coast there is such a thing as dry weather.) Then you must avoid drawing a full bath, as otherwise you may break the siphon in the iron water pipe, causing an airlock which stops the water altogether. Resetting from this error condition involves using a stirrup pump, a bucket and many megajoules of muscle power, leaving you partially paralysed, dripping and a good stone lighter.

We agreed that it would be nice, to say the least, if there were a water level gauge on the cistern that could be read from the house. Fuelled by several shots of Glenfarclas I plotted various electronic schemes, ranging from a mile of Twisted Pair cabling to a short-wave radio transceiver - but even half drunk I could guess that any scheme involving batteries, wire or electricity would be gobbled up and spat out in pieces by the first highland winter. A robust mechanical solution was required, a wooden float with a depth scale sticking up on top. But the cistern was barely visible (if you knew exactly where to look) from the mound behind the kitchen door. At that point my eye fell on the pair of binoculars sitting on the table. There it was, a very fast optical communication channel, with zero latency and jitter and a resolution of about six inches at one mile. Of course it can be a noisy and lossy channel when the rain or fog come down, but then it wouldn't be dry weather, would it?