CODING SCHEME FOR SERIAL DATA COMMUNICATIONS
An encoding scheme generates an encoded nine bit code word from each input eight bit data word. The coding scheme is such that the encoded data words have advantageous properties, such as a minimum of two polarity transitions in each encoded data word, and a maximum of five bits without a polarity transition. Five of the bits from the input eight bit data word appear unchanged in the encoded data word, while the other four bits of the encoded data word are obtained by applying appropriate logical operators to the remaining three bits of the input data word in combination with two of the five bits that appear unchanged in the encoded data word. Exception codes can also be defined, that is, nine bit code words that cannot be obtained from any eight bit data word by means of the coding scheme, and can be used to embed control information into the data stream. For example, the exception codes may advantageously have six or seven bits without a polarity transition.