terminal is smart enought to have an address and able to respond when it reads its address in a message received on the line. 3.10 TRANSMISSION CODES There are probably as many transmission codes in existence today as there are types of terminals. This proliferation exists partly be- cause computer manufacturers want to ensure the use of their own terminals, partly because of special systems requirements and partly because no standard existed 10 years ago when data communications began rapid development. Transmission codes differ from computer codes only in that certain code values are reserved for terminal con- trol and data formatting. Some transmission codes such as BCD re- semble character-oriented computer coding and employ a parity bit for error detection, as is usually done in a computer code. On the 44 TELECOMMUNICATIONS Fig. 3-6. American standard code for information interchange (ASCII). other hand, Baudot, for example which is a five-level code, has no provision for parity or the detection of errors. A chart of the code values of ASCII (Fig. 3-6) has been included since its the most widely used. ASCII represents an effort to establish a uniform coding scheme for all terminals: in practice, however, it has not been im- plemented on the scale the designers envisioned for it. Of the other existing codes, the more widely used ones are the extended binary coded decimal interchange code (EBCDIC), the five- bit Baudot code found in old teleprinter equipment, the four of eight code, the IBM punched card Hollerith code, the binary coded dec- imal (BCD) code, and the six-bit transcode. EBCDIC is an eight-level code similar to ASCII, except that while ASCII uses its eight level for parity bits, EBCDIC uses it for infor- mation bits, thereby extending the range of characters to 256. 3.11 CODE AND SPEED CONVERTERS A combination of speed-and-code converters permits the user to sat- isfy two or more dissimilar communications requirements with a sin- PROTOCOLS AND CODES 45 gle terminal, thereby reducing both the numbe