RS-449/442-A/423-A is

• a set of physical layer standards developed by EIA and intended to replace RS-232-C.
• a check bit appended to an array of binary digits to make the sum of the all the binary digits
• a code in which each expression conforms to specific rules of construction, so that if certain errors occur in an expression the resulting expression will not conform to the rules of construction and thus the presence of the errors is detected
• the ratio of the number of data units in error to the total number of data units
• None of the above