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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Encyclopedic overview
3 sectionsContents
- Electrical characteristics
- External links
- References
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.
Excerpted from Wikipedia’s “X.21” article, available under the CC BY-SA 4.0 licence.