X.21
Sign in to savethumb|X.21 adaptor for a Japanese NEC PC-98. X.21, also referred to as X21, is an interface specification for differential communications introduced in the mid-1970s by the CCITT, now ITU-T. X.21 was the first digital signaling interface developed. The connector specification is defined by the ISO document 4903.
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thumb|X.21 adaptor for a Japanese NEC PC-98. X.21, also referred to as X21, is an interface specification for differential communications introduced in the mid-1970s by the CCITT, now ITU-T. X.21 was the first digital signaling interface developed. The connector specification is defined by the ISO document 4903.
At the time, most physical layer protocols such as RS-232-C and RS-449 use analog signaling. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications. This includes specifications for DTE/DCE physical interface elements, alignment of call control characters and error checking, elements of the call control phase for circuit switching services, and test loops.