Manually Patching an 80-column Tab Card

80-column tab cards (“IBM Cards”) were the primary method of entering data into a computer from the late Fifties, when they were introduced as a major advance over manual entry using switches on the front of the computer (!), until the mid-Seventies, when they went out of use as quickly as they'd come in. They were replaced by various forms of magnetic tape, magnetic cards (briefly), and time-sharing terminals.

Tab cards were created by “tab punches”, either computer-driven (for large numbers of cards) or a machine that looked like a typewriter embedded into a desk, called a “keypunch”. In the latter, cards fed right to left under the punch head, which was a vertical bar immediately in front of the operator. Pressing a key punched that code into the card, which was advanced one column. Above the punch head was the drum, which could be programmed to do a lot of silly tricks, like skipping columns that had to be blank, tabbing over to the proper data entry column(s), and punching sequence numbers. The last eight columns of the card were usually not used, because they were reserved for sequence numbers – if you dropped the box (they came in boxes of 2000 cards, and were generally stored that way) you could sort the cards using the sequence numbers and recover your program or data. If, of course, there were sequence numbers, which there usually weren't.

During their heyday, tab cards were used for an incredible number of things, including Government paychecks (the code on the card repeated the printed data on the front), utility bills, registration cards, and anything, really, that involved input into the primitive data systems of computers of the time. Unfortunately they had to be pristine, or the reader wouldn't accept them. They were fed in using what amounts to a paper-feed system, like in a printer, sometimes lubricated by an air blast, and any distortion would jam the machine and induce the creation of foul language by the computer operator. “DO NOT BEND, FOLD, SPINDLE, STAPLE, PUNCH, CRUSH, OR MUTILATE!” the publicly-visible ones said in large, not-at-all-soothing letters. People did, anyway, thus the need to manually patch them.

There were several ways of doing that. One was to fill the hole with a bit of glue, then cut new holes for that column. You could go to the tab punch or “keypunch” and salvage a bit of chad to fill the hole if the glue you had wouldn't fill very well. The invention of Scotch Magic tape was a Godsend. The original Scotch tape was too thick – the card wouldn't feed – but Scotch Magic tape was thin enough. You had to know how the reader worked, too. If it worked by having contact “feelers” ground to a metal surface through the hole, tape was enough. If they were optical (as many high-speed ones were) you had to fill the hole with something opaque, hence fishing for chads. If the result would feed through the keypunch, you could make new holes that way; if not, or if you didn't have a keypunch available, you could do it (very carefully) with an X-Acto knife or razor blade. There were actually hand punches available.

It was also possible to feed the card into the keypunch, which had a duplication feature, and manually duplicate columns until you got to the erroneous one, then either punch the proper code or skip to leave it blank. This is what people did if they had a keypunch available and the card would feed.

A closely related, and also obsolete, skill was patching punched paper tape. The same techniques and skills were required, though the codes were different, and it was much more often necessary because the paper tape was thin and fragile. Boot loaders for early minicomputers were often on punched paper tape.