The Write Ring

1954 · Write Enable · Work 629 of 1,871

The Write Ring, 1954, Write Enable

A plastic write-enable ring lying on a plain surface beside the hub of a magnetic tape reel, the reel's empty circular groove where the ring seats, the ring a smooth flat annulus of pale plastic, the reel of dark plastic with its tape wound tight, extreme close-up so the hub, the groove and the ring fill the frame, a single soft light from behind grazing the edge of the groove and glowing faintly through the plastic ring, the reel alone against pure black.

What it was used for
The write ring is a ring in a groove on the back of a tape reel; it decides whether the drive may write. The IBM 727 Manual of Instruction, section 4.12, describes it thus: a device is provided to prevent writing on a reel of tape if so desired; it consists of a groove cut in the rear side of the file reel and a ring that fits into that groove. If the ring is missing from the groove, writing is suppressed while reading remains allowed. The decision about write protection therefore lay with whoever mounted the reel.
What was new
What was new was where the protection sat: not in the program and not in the operating procedure, but on the object. The reel carried its own permission with it, and you could see at a glance whether it could be overwritten. That differs from the punched card, which cannot be overwritten at all, and from a program, whose effects are not visible on it. The barb: the whole safeguard hung on a loose part and on the memory of whoever had to take it out. A ring left in the groove released for overwriting a tape that nobody wanted overwritten.

What the sources leave open: The copy of the IBM 727 manual on Bitsavers (Form 22-6681-0) is marked Copyright 1956 but gives no year for the introduction of the ring; the year 1954 could not be confirmed outside Wikipedia and the IBM archive pages, which were not retrievable in this run. The sentences from the manual are translated and paraphrased, not quoted verbatim. Whether the write ring was new with the 727 or used earlier, the sources do not say.

Further reading