PDP-1 alphanumeric codes
The PDP-1 alphanumeric codes were used to encode text characters for input and output on a DEC PDP-1, a computer model that was available from 1959 to 1969. The introduction of this model predated the creation of ASCII, so (as was common in the early computer industry) a proprietary coding was used.
Several different codings were used, actually; the manuals show at least three, Friden Code, FIO-DEC Code, and Concise Code. They each assigned different code points to the various letters, numbers, punctuation, and control characters. Concise codes were two-digit octal numbers (6 bits), but were not always consecutive for letters of the alphabet; A through G were 61 through 67, but H was 70 and J was 41. Friden Codes used three digit numbers. FIO-DEC codes are the same as Concise codes, with an extra bit (octal 200) to ensure the number of 1 bits is odd. (e.g. concise code 61 is FIO-DEC 61 as that has 3 bits on, and concise code 63 is FIO-DEC 263, as octal 63 has 4 bits on). This serves as a parity bit that a program reading in FIO-DEC encoding can check.
