34

TL;DR; Punch card code is not binary but a collection of n out of m encodings. Long Story Yes, really a long story, so I'll only cover the main line from Hollerith to EBCDIC. there are many sidelines for special equipment, situations and as used by different manufacturers. Some covering up to 7 holes but all mostly compatible in the basic Numeric/Alpha ...


23

Uppercase text only needs six bits per character. The fundamental mistake that you are making is assuming that punch codes were binary numbers. They were not. The encodings were patterns, combinations of of zero, one, two, or three holes. This is a reference card in IBM 5081 format: The row numbering was somewhat odd, for historical reasons: 12, 11, 0, 1, ...


20

Although you have many correct answers describing the nature of the coding used in punched cards, no one has touched on the mechanical properties of the cards. Regular users of punched cards in the past would be familiar with this issue, as getting cards through the mechanics of a fast card reader regularly and repeatedly was a major issue at the time. If a ...


13

Round holes might have been 'stiffer', but rectangular holes won on packing density. When IBM invented the 80-column card (up from the previous 40-column Hollerith card of the same size), they determined you could get more columns per card by using rectangular holes. IBM's own history describes two competing designs: the 80-column rectangular-hole card we ...


9

I'd have added this in a comment but don't have enough rep. If you read far enough into the IBM history link given by another-dave in his answer, you'll find this quote that indicates the rectangular holes were in fact stronger: As well as handling more data, the unique rectangular hole was stronger [emphasis mine] and more compatible with the wire brushes ...


8

However, there were other possibilities, such as a later IBM format that used round holes. Not only later, but also previous IBM formats used round holes. Similar next to all other contemporary (1930s) manufacturers (Powell, CDC, Honeywell, etc). Intuitively it seems to me that round holes would be better from a mechanical stiffness viewpoint, making the ...


5

The code punched into a 12-row card is not a binary code, but actually a form of extended decimal coding. Rows 0-9 are used to directly encode decimal digits, while letters and symbols are encoded as one decimal row plus one "zone row" which could be the A, B or 0 rows. Within the IBM 1401 series, this was re-encoded as an extended-BCD code in six ...


1

If you punched a wrong character with a key punch - (I have used them...), You could push a rectangular "chad" into a hole (with a bit of spit) and it would be retained sufficiently well to make it through a 600 card/minute card reader. Would this be true for a circular cut-out? BTW This was on ICL (ICT) 1900 machines, so I don't think it was a ...


1

For completeness, here is an example of a punch card in the row-order byte-based Soviet GOST encoding. ,--------------------------------------------------------------------------------. 12 | X X XXXX X XX X X XXX X XX X X X X X XXXXX X X XX XXX| 11 |X X XXX XX X XX X X XX XXXX X X X XXX X XX X XX XXXX X XX X ...


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