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Ascension Island

The story of when a BBC micro shut down power to an entire Island.

By Dave Parsons
 

Setting the scene

When I arrived on Ascension Island (ARS Atlantic Relay Station) in the mid 80s, in my mid 20s, the control system for the BBC Broadcasting Station, whilst advanced when new, was getting a little long in the tooth and had, for example, only core store memory and an early 2 chip wire wrapped 8080 cpu.

It controlled 4x Marconi BD272 250kw HF Senders, many antenna arrays, 2 tape machines, frequency counters and satellite program feeds. It had been reliable for many years and was essential to controlling the complex 24hr switching operations on station allowing for a shift of only 2 Engineers and an SME.

The day it broke

However, the day had arrived when the system was to fail, permanently. An early logic analyzer was flown to ARS to try and diagnose the issue .. but after many hours of head scratching .. the system was declared retired !

This meant that every scheduled task on the station had to be performed by hand, to within a second accuracy, and the 24hr shifts were doubled up to enable this. At busy times, it was the nearest thing to seeing BBC engineers wafting around the control room in unison like ballet dancers, counting and synchronizing their movement as they flicked switches as you’re ever likely to see!

The proposed solution

As an early computer enthusiast, I teamed up with Bob Weston (hardware) and we built a multitasking Operating System for the BBC micro with a colour graphical real time scrolling control schedule display. The shift could edit the schedule whilst it was running. Very futuristic for its day, (OK I was proud of it !) This would enable the staff to control the station automatically again. Bob saved the day with an interface board to the existing station controller wiring harness that he seemed to rustle up in no time at all and the whole thing both looked and worked beautifully.

Tangle of wires

We then went on to reverse engineer the existing control system Address and Data bus by generating sequential numbers, and having staff posted around to see what happened and call in on walkie talkies when ‘things happened’ !

We filled out a large sheet of graph paper and could extrapolate many control points once we got an idea of how devices originally mapped out .. If you can call a 250kw transmitter a device !

A bit risky on a 4 x 250kw transmitting station but we isolated the 11,000v supply and listened for contactors clicking ! what else could we do ?

Dodgy wiring discovered 20 yrs later

Interestingly, mapping went well except the frequency counters. Used to confirm the correct drive frequency had been manually set, one had a problem with the 3rd digit when read. It was an illegal BCD code and eventually I realized it was wired backwards ! Looking on the equipment bay on the back of the counter, it was discovered the 3rd digit was wired backwards in the plug !! .. Not fancying the idea of getting out my soldering iron, and Bob not up for it either, we decided to do what the original designer had obviously done 20 years or so earlier, and fixed it in code by reversing the read data 4 bit code for that one digit.

Switching it on

Once the mapping was completed, the current schedule was programmed into the new system, and the BBC micro was allowed to take control .. dozens of switches were flicked over to AUTO again, and after several hours of flawless execution, Bob, the station staff and myself proudly retired to the Klinka Club bar for some well deserved refreshment and celebration.

After many long days and nights over the past week or two, life was back to normal ... or so we all thought ..!

Pride before a fall

As the party was really getting underway, around midnight, the lights in the Klinka Club, and as we learned later, the entire island, started to vary in brightness. They grew bright, then dim, very bright, very dim, blindingly bright and then off ..

We were all standing in the dark, suddenly sober, and silent wandering what had happened, when the PHONE RANG (battery powered )!

A bit more info

Now, you have to understand, that Ascension Island is a small island and the BBC transmitting station consumes more power than all of the rest of the island combined. Also, the power is provided by diesel engines. The diesel engines have mechanical governors to ensure the massive engines stay at constant speed as the load varies. If the Island load reduces a little, the engines speed up a little as the generators become easier to turn, and the governors compensate by closing the throttles on all engines, if the load increases .. the opposite happens. We're talking small variations here .. a few percent ..

Suddenly sober

Back to the phone ringing ..

The Senior Engineer was on the end of the phone asking if I was in the bar .. and if so, I needed to go to the transmitting station immediately. We had been celebrating now for several hours and there were not many sober folks around to drive and I was in no state to do much, but we followed instructions, and the situation did help sober everyone up!

I was told by the Power Station Shift Lead how the Island load had reduced by around 80% in a few seconds, then, as the diesel engines rapidly over-speeded, the governors had snapped the throttles hard closed, when a few seconds later, the Island load was immediately back up to its original load .. the diesels were now coasting on a closed throttle but having their entire normal load quickly re-established !! After another round of this, with the voltage on the Island power grid cycling up and down, and the lights with it, the power station had shut down to protect the engines.

The emergency compressed air system to start the engines was depleted I was later told, and the small engine to make compressed air was being serviced.

The Milky Way

So there we all were, a thousand or so people, on the tip of a volcano, in the middle of the Atlantic Ocean, 1000mi to the nearest land, with no power, no lights, no compressed air and the people who could fix it having been down the Klinka club for the past several hours and in no fit state to do much .. !

Outside, looking up into the now very dark sky, I noticed the Milky Way looks fabulous under those conditions .. we walked to the light of the Milky Way.

The problem reveals itself

Thankfully, the powerhouse restored power within an hour or so, I sobered up very quickly and once the software was up and running again, and knowing the problem happened at midnight, that enabled me to find and fix the software bug quickly !

I had forgotten to add 24hours to any operation that was due to happen the following day, so after the last change before midnight, the code suddenly believed all operations were not completed for today, and it needed to catch up quickly !! .. It had switched all 4 transmitters on / off / on / off and nearly broke a crankshaft on one of the engines .. Whoops !

Once the line of code was fixed, the system ran reliably for several years, until replaced with a BBC designed system.

Acknowledgements

To all the station staff on Ascension at that time .. thank you for welcoming me, nurturing me, tolerating me and for allowing me to be a little part of the Ascension story and not being too hard on a 20yr old kid fresh from Wood Norton!

In fond memory of Bob Weston, I hope this made you laugh !

Dave Parsons