This refers to the way in which data is represented. The sender and receiver must use the same code in order to communicate properly. Here, we will briefly look at two common codes, one which was developed earlier on and was widely used in early telegraph systems, and the other, which is in widespread use today.
International Telegraph Codes
The Baudot code was used extensively in early (pre-1900) telegraph systems. It is a five bit code invented by the Frenchman Emile Baudot in 1870. Using five bits allowed 32 different characters. To accommodate all the letters of the alphabet and numerals, two of the 32 combinations were used to select alternate character sets (known as lowercase or alpha characters, and uppercase or numbers, punctuation and special symbols).
Each character is preceded by a start bit, and followed by a stop bit.
It is an asynchronous code, and thus suited for low speed data communication.
The Baudot code became know as the International Telegraph Code No. 1. Sometime around 1900 another 5-bit code called the Murray Code was invented. The Murray Code, know as the International Telegraph Code No. 2, replaced the Baudot Code. Unfortunately Murray's name has sunk into obscurity while Baudot's name is associated with every 5-bit code on the face of the planet, including the Murray Code.
The Foreign Service Communicator was most familiar with the use of the Murray Code on the perforated paper tape that was prepared with message traffic. While some later models of teletype equipment ("reperforators" or "reperfs") printed the characters on the paper tape, Communicators were capable of "reading" an unprinted plain tape and aligning it on the "TD" head ("Transmitter Distributor"). Whoops, slipping into jargon, there.
ASCII (American Standard Code for Information Interchange)
The ASCII code is the most popular code for serial data communications today. It is a seven bit code (128 combinations), and thus supports upper and lowercase characters, numeric digits, punctuation symbols, and special codes. The table below lists the values for each character in the ASCII set.
ASCII is also used as the data code for keyboards in computers. Control Codes have values between 00 and 1F (hexadecimal). Control codes are used in binary synchronous communication, and device control codes in communicating with devices such as printers or terminals.
A control code can be generated from a keyboard by holding down the
Ctrl key and pressing another key. For instance, holding down the Ctrl
key and pressing the A key generates the control code SOH.
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