Magnetic Tape Drives

Do you know what this is?

It's a write-protect ring from the days when all mainframe computers would have a bank of magnetic tape drives, typically at least four. The back of each reel of tape had a groove that this ring would seat into. If the ring had been removed, then the drive would not write on the tape whatever the software told it to do. It was read-only.

Tape drives changed their specs over the years,  but many would write 6,250 bits per inch 9 tracks wide (8 data plus a parity bit). So 50,000 real data bits per inch of tape. The write speed was as high as 112 inches per second (nearly 10 feet) and the tape would accelerate up to that speed in just a couple of milliseconds. That raises the question of how those big heavy reels accelerated that fast...and the answer is that they did not. Read on to find out how that was done.

I wrote recently about sorting algorithms from before the days of disks, where the data would need to be read from two drives and written to another. See my post Supernaturally Fast Sorting. Since drives like this pre-date many of you, I thought it would make a good topic for a blog post.

This is what a tape actually looks like. It is 10.5" in diameter, made of plastic (the earliest ones were metal). The white band around the outside protects the tape when it is not in use, clamping tightly onto the reel. One of the most impressive things about using a tape drive like this is the way it loads automatically. The operator (or sometimes me) would put the tape onto the drive and press the "load" button. The drive undid the clip at the top of the picture (above "6632") so that the reel was now loose and could rotate freely within the ring. The reel spins slowly until the start of the tape is sucked out. As you can see in the picture above, there are two reels in each drive. The one on the left holds the current tape, the one on the right is permanent. It had air suction which could seal the start of the tape onto the reel, and then make a handful of turns to grip it tightly. There was actually a marker of aluminum tape that indicated to the drive how much tape should be wound onto the reel before it should be read or written. There was another one at the end to prevent the tape being read past the end of the reel.

The really clever stuff is more easily seen in the picture below with the red tape reels. Underneath the reels, you can see two columns. Once the tape is loaded and picked up by the permanent reel (using suction) then these columns are pumped out. They were colloquially called vacuum columns, but actually, it was just lower pressure. Each column would end up with a loop of a few feet of tape. To read the tape, the drive would spin up one of two lightweight capstans (the tape could be read in either direction) which would pull some tape out of one of the columns and let some tape get sucked into the other. More slowly, the servos on the reels would spin up and either release tape into the column or pull tape out, depending on the tape direction. Once everything was up to speed, the tape could be read continuously, and the reels would transfer it at the correct speed to keep everything balanced. Actually, the reels spun much faster than the tape could be read, hence the jerky spin-stop-spin behavior if you've ever seen a tape in action, or in a move, or watch the video at the end of this post.

There was also a fast rewind where the vacuum columns released the tape and the original reel pulled the tape as fast as possible so that it could be exchanged for a different tape without unnecessary delay.

There are several tape drives in the Computer History Museum in Mountain View. And this video shows one of them with the covers open so you see how it all works, not just the vacuum columns but the six motors and clutches that drive it (load motor, two capstan motors, the two reel motors, and the fast rewind motor).

Of course, every spy movie of that era had magnetic tapes in the background, since spinning reels are more photogenic than actual computers. Here, for example, The Man from U.N.C.L.E. (1966):

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