X.21, also referred to as X21, is an interface specification fordifferential communications introduced in the mid-1970s by theCCITT, now ITU-T. X.21 was the firstdigital signaling interface developed.^[2] The connector specification is defined by the ISO document 4903.^[3]

At the time, most physical layer protocols such asRS-232-C andRS-449 useanalog signaling. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications. This includes specifications forDTE/DCE physical interface elements, alignment ofcall control characters anderror checking, elements of the call control phase forcircuit switching services, and test loops.

X.21 normally is found on a 15-pinD-sub connector and is capable of running full-duplex data transmissions. The Signal Element Timing, or clock, is provided by the carrier (the telephone company), and is responsible for correct clocking of the data. X.21 was primarily used in Europe and Japan, for example in the ScandinavianDATEX andGermanDatex-L [de] circuit-switched networks during the 1980s.

A variant of X.21 can be found in select legacy applications as “circuit switched X.21”.

Electrical characteristics forV.10 andV.11 specifies voltage levels. V.10 is forunbalanced circuits, whereas V.11 is forbalanced circuits.

With electrical characteristicsV.11, it provides synchronous data transmission at rates from 600 bit/s to 10 Mbit/s.

With electrical characteristicsV.10, it provides transmission speeds of up to 100Kb/s.

• An X.21 brief Tutorial and overview
• X.21 pinout and some explanations

Article based onX.21 atFOLDOC, andX.21 Pinouts, used withpermission.

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