We have seen, in the History of the Computer series, how machine code is used to operate a computer so that it can use 1s and 0s to interpret a program's intentions. But what about a code for human communication with the computer? We can variously write and read to and from different media, such as paper tape, magnetic tape, disk or teletype, but we need some means of converting English (or another language) into a form which can be carried on these media. You may recall the old Western movies where the James gang shoot the telegraph operator so he can't call the Marshall for help. Or the movies about the '29 Wall St crash where the tycoon reads the bad news on the ticker tape and promptly leaps out of 32nd floor window. Both of these are early examples of a code being used on a medium. In this case morse code on the telegraph, and an analog code produced from a keyboard at the transmitting point, also sent by telegraph. The ticker tape, so called because of the noise made by the printing mechanism, carried the Stock Code, as used in today's stock market, and a quote. A later code used for paper tape, the baudot code, named after the French inventor, Baudot, was a five bit code, punched across the tape, giving a 1s and 0s pattern to make up the 26 letters of the alphabet. The tape was an inch wide and came on big reels. A sprocket hole was punched in the tape so that it could be moved through the reader. As we know from our look at binary codes, 5 bits will give us a total of 31 characters (1+2+4+8+16) plus zero. With 26 being used for the English alphabet, the other characters are used for Carriage Return, Line Feed, etc. These are needed to operate the typewriter used to print out the information. Another character, Bel was used to ring a bell on the typewriter to alert the operator. Later a Shift character was incorporated, similar to the Shift key on a typewriter, so that following characters would indicate numbers. Another shift code would switch back to letters. This Baudot Code was used on teletype machines to transmit data from one location to another. This was later accomplished by first punching the tape, and then transmitting it by feeding it through a 'high speed' reader. These speeds were gradually increased as the technology improved, and were generally expressed as the 'Baud Rate', roughly equivalent to bits per second, also named after Baudot. Most problems came from punching errors and synchronising transmitter and receiver. In part 2 we look at how codes are used in computers. |
