Are you sitting comfortably? Then we'll begin. Hang on, the PDP 11/70 has dropped offline

Beware the wandering buttock

Who, Me? As the weekend recedes and the workday begins, tarry a while with a beverage of your choice and today's entry in The Register's file of reader Who, Me? confessions.

Today's comes from a reader we have elected to call "Pete" and returns us to the late 1970s. The Space Shuttle had yet to fly, Douglas Adams had penned The Hitchhiker's Guide to the Galaxy and the very first Wetherspoons pub had opened its doors.

What a time to be alive.

Most computers of the time were notable by their size and noise, and those Pete was responsible for were no exception: "I was working for the IT department of a large manufacturing company," he told us, "as a systems programmer for the PDP-11s, Primes and IBM system/360."

steampunk-looking robot smokes pipe. pic shutterstock

Das blinkenlights are back thanks to RPi revival of the PDP-11


Introduced back in 1975, the DEC PDP 11/70 was a mighty beast for the day. It could support up to 4MB of main memory, enjoyed a speedy CPU and featured the UNIBUS bus architecture. The machines lingered on well into the 1980s (and beyond) and enjoy quite the following.

The vintage computer enthusiasts at Beige-o-Vision in association with the National Museum of Computing demonstrated last year how one could recreate the thing with a Raspberry Pi and a neat working scale reproduction of the PDP 11/70's front panel.

Because who doesn't love flashing LEDs and a toggle switch or two?

Alas, things were not going so well for Pete back in the 1970s: "We had an intermittent fault on the system drive on the PDP11/70. It was going offline at spurious times of the day without any warning."

The system drive was a 40MB behemoth, about the size and weight of a washing machine.

Pete called DEC, who rigged up some test hardware to track down the problem. Sure enough, two or three times a day the drive would go down, at seemingly random times. "It looked like the 'offline' button was being pressed," recalled Pete, "but nobody in the computer room would do that..."

Suspecting a short circuit, the button was replaced, but the intermittent issue continued. Then the logic boards – still the system kept dropping over. Finally the whole drive was swapped out, but the mystery offline events continued.

The solution came when Pete was sat at the console of the PDP. "The development manager popped in for a chat with the operators," he told us.

"He made himself comfortable by ... leaning on the system drive and promptly hitting the offline button with his backside."

Beware the wandering buttock indeed.

Pete solved the issue that had that left DEC's finest baffled: "I made a little plastic box to fit over the disk drive buttons and all was well again."

Body parts and computers never go well together. Ever caused an outage by a badly positioned bit of anatomy, or triggered the gas extinguishers with a careless elbow? Did you get away with it? Confess all with an email to Who, Me? ®

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