This PDP-11/70 was due to predict an election outcome – but no one could predict it falling over

Remember when the only interference was electromagnetic?

Who, Me? Monday has arrived, and with it a tale of election predictions past courtesy of The Register's Who, Me?

Our story takes us back to the 1970s and concerns the exploits of "Neil", a systems manager who is sadly no longer with us (but still merits the tender affections of the Regomiser).

A national broadcaster of the time was starting to explore the potential of these newfangled devices called "computers". A PDP-11/70 had been installed and occupied (with its infrastructure) much of the floor of a downtown office block. The programmers and system managers worked on the floors above.

The PDP-11/70 was quite the beast, capable of addressing a mind-blowing four megabytes of memory. The series lasted into the 1990s, and retro fans can still relive those halcyon days thanks to projects such as the PiDP-11.

Our story takes place during testing for coverage of an upcoming election where the computer was going to predict the outcome based on early results.

Alas, with impeccable timing, it fell over just as the office was going home. The system manager mashed the keys on his VT100 terminal to no avail, and a glimpse into the PDP-11/70's lair confirmed its unhappiness as it vented fury via page after page from the console DECwriter following an unexpected reboot.

Mindful of how career-limiting such an incident might be midway through election night, a call was placed to DEC to deal with the recalcitrant beast (thank heavens for 24-hour support).

The field engineer (FE) duly showed up and Neil returned to his desk, a few floors above. A few board swaps later and the terminal sprang back to life. The FE came up, ready for customer sign off and… the terminal died once more.

The FE swapped more boards. The terminal was restored, but just as he arrived to collect a signature, death once more stalked the circuits.

And so it went on, until the engineer either ran out of boards or patience, and called for moral support from the computing giant in the form of more pairs of hands.

More and more DEC people turned up, probing, scoping and prodding until finally the issue was found.

"A rogue interrupt on one of the communication lines (of which there were dozens) had caused the machine to trap to an uninitialised vector location," explained Neil's friend.

"However, the problem was occurring in different multiplexers each time!"

The little hand was now well past 12. It was only when one of the exhausted DEC team left the malfunctioning computer to visit the manager on the floors above that the problem was inadvertently solved.

"It was the massive elevator motors starting up, easily heard in the now empty building."

The result was a modem control change interrupt, which was the cause of the problem. The terminals had all been wired with no modem support.

And the solution? Going through multiple panels of DB-25 connectors; opening the shells, and cutting the wire to pin 20 (used for modem control).

Election night was saved.

"Later that day, a worldwide Engineering Change Order was issued to address the problem!"

Ever saved the day thanks to your trusty pair of wire-cutters, or fallen victim to the more extreme slings and arrows of RS-232 connectivity? Share your story with an email to the vultures of Who, Me? ®

Editor's note: An earlier version of this article stated the aforementioned PDP-11/70 was to be the first computer to predict the outcome of an election. That honor actually goes to UNIVAC, which predicted a landslide to Dwight Eisenhower in the 1952 US presidential race. We are happy to clarify this point.