The Glass House and the Drawbridge

One of our profs in 2nd year asked the class an intriguing question:  Why were computer rooms usually windowless? Here’s the story he told us, as best I can remember it through the usual college party fog:

Early in his career, he was working in a data center that had large windows, providing a view of the adjacent river and a drawbridge that went over it.  People enjoyed working in the data center because of the beautiful view.

Like any data center, there were occasional glitches.  One fall, the payroll run aborted at 3:15 in the afternoon.  This payroll system ran from tape and cards – one tape was the input tape containing the current payroll records, and the cards were punched with this week’s timesheet information. The program computed the payroll, banged out checks on the printer, and wrote the updated payroll records to the output tape on a second tape drive.  But on this occasion, the program crashed part way through, indicating it had hit the end of the input tape, even though it wasn’t close to the end. They called in the repair man, expecting it to be an issue with the input tape drive – specifically the End Of Tape sensor. In these tape systems, the end of a tape was marked with a silver reflective spot that an optical sensor would detect.  The technician serviced, polished, and gave lots of love the to tape drive, and all was well again. Until the next payroll run the next week, when exactly the same thing happened, although the stopping point was different.

The technician came back, but couldn’t find anything.  So the programmers dug in, searching for anything that could cause such a problem – but came up empty.  And that’s the last time it happened – for a year. The next fall, again at 3:15 in the afternoon, there were again failures of the payroll run.  And again, after two or three weekly failures, the problem went away until the following year.

By the third year, they decided to switch up the processing schedule, and run payroll at 4:15.  The failures again occurred at 3:15, but this time it was a different program running. This was the first clue that would help them isolate the problem.  

But someone did notice that the failure occurred exactly when the drawbridge over the river was in motion.  First they searched for a power relationship – was the drawbridge putting noise on the electrical grid that was upsetting the tape drive?  Nope – otherwise it would happen all summer – the drawbridge was raised and lowered on a daily schedule throughout the summer.. 

The following week, when the next failure was expected, folks gathered around the window to watch the drawbridge move, which it did precisely on schedule.  But this time, there were no failures.

Folks finally put the pieces of the puzzle together when they realized that a bunch of people blocking the window during drawbridge operation might be linked to the optical nature of the end-of-tape sensor in the tape drive.  And indeed, only at this time of year, when the sun was at a particular elevation, and only at a certain time of day, would all the perfect evil angles align. A flash of sunlight would reflect off the drawbridge as it moved, travel into the computer room via the window, and hit the tape drive in just the right manner to trigger the end-of-tape sensor.

There are lots of other reasons glass-less computer rooms (security, air-conditioning, etc.), but this is certainly the most curious I’ve heard.

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