This International Standard specifies a method for detecting errors in information messages by attaching one block check character to the transmitted information block (or text) and checking this character when it is received. The method of correcting errors when they are detected is subject to the particular application and is performed at a higher level.
The method is applicable to systems which use the 7-bit coded character set which is the subject of ISO 646, 7-bit coded character set for information processing interchange, and the basic mode of implementing this 7-bit code in data communication systems, which is the subject of ISO 1745, Information processing— Basic mode control procedures for data communication systems.
The rules for generating the character parity bits, according to ISO 1177, Information processing — Character structure for start/stop and synchronous transmission, are that the character parity sense shall be odd in synchronous systems and even in asynchronous systems.