I would appreciate any clarification of the following:
The decimal system we humans use is not readilly adapted to machine interpretative use. Machines usually work on 'binary' (two states), where as a 'bit' of information is represented by a 0 or 1, such as whether there is power or not or a hole or not. These bits are combined to form numbers. There are a variety of coding schemes to translate decimal numbers into binary machine usable format (octal, hex, etc.)
It seems that a number of applications within the Bell System used a binary coding scheme called "2 out of 5" to represent a digit. It seemed to mix well with crossbar applications. They used it to identify routnig on long distance machine-readable cards, and on trouble cards.
They would have:
0 1 2 4 7As I understood it, whichever of the above was punched out added together would indicate the digit. For example, if the 2, 4 were punched out it would mean 6.
My questions:
1) did this code go from 0 to 9 or 1 to 10? Or didn't it matter since it was representating a phone number?2) For 0 , only one item had to be punched. Since this was called 2 out 5, didn't two items have to be punched (as a kind of parity check)? (I guess for 1 they'd punch 0 and 1?)
3) Most computers used a real binary, such as 1 2 4 8, to represent a digit. I don't recall applications outside the Bell System that used the 7 as part of the representation. Now, in Bell System practice, the phone numbers could be thought of as an alpha-numeric field, that is, the phone numbers were identifiers, not used in arithemetic. Phone numbers were often translated, that is exchange LI 8 (548) might be translated as 'trunk group 3142" and the switch would go to terminals at 3142.4) Is this system still in use in telephone applications?
*Binary (and hex and octal) have never been my strong suits. (I work with hex on occassion). Years ago word-oriented architectures used octal coding. The IBM 650, a popular machine of the 1950s, used something called quinary for its magnetic drum.Thanks! [public replies please]