X.21, V.11, RS422 & EIA-422 Connector / Signaling Standards

An explanation of the standards, their characteristics including pinouts and how the interfaces are generally used in practice.

Interface Standards

X.21 ( occasionally written as X21 )is a specification for differential communications that that includes the definition of connector pin allocations. It is used together with V.11 to define a specification for serial synchronous communications at up to 10Mbps. A variant of X.21 called ‘circuit switched X.21’ is now no longer in use except for a few legacy systems.

V.11 is a specification for differential communications that defines signal electrical characteristics. Signals meeting V.11 requirements are used in X.21, RS-449 (EIA-449), RS-530 (EIA-530) and the balanced signal part of V.35 interfaces.

RS-422( often written as RS422 )is essentially equivalent to V.11. Note that the EIA standards have effectively replaced the RS standards.

Interface Characteristics

X.21 is a differential interface, typically limited to a maximum throughput of 10Mbps. Communications over distances exceeding 1000m is possible at low bit rates, the actual performance being mostly dependent on cable specification. A single clock signal is used for receiving and transmitting data.

Interface Applications

X.21/V.11 interfaces are used where high throughput and/or long distances are required. Commonly used bit-rates for synchronous communications are 64Kbps, 128Kbps, 256Kbps etc. The interface also offers good noise immunity enabling reliable communications in environments where there are high levels of EMI (electromagnetic interference). Typical protocols used over X.21 interfaces are HDLC, X.25, SNA and PPP.

In some common applications including industrial controls, RS-422 interfaces are used to extend the reach of asynchronous communications, with differential drivers and receivers used instead of RS-232 devices for the line interfaces. Asynchronous RS-422 applications normally use a DB9 connector although the pin assignments are not standardised.

Interface Connector Types and Pinouts

The DB15 connector is mainly used for synchronous connections. The signals used by the overwhelming majority of applications are marked in bold.


DB15 connector pinouts
Signal NameDB15 ContactSupported on FarSync cards
Transmit (A)2Yes
Control (A)3Yes
Receive (A)4Yes
Indication (A)5Yes
Signal Timing (A)6Yes
Byte Timing (A)7
Transmit (B)9Yes
Control (B)10Yes
Receive (B)11Yes
Indication (B)12Yes
Signal Timing (B)13Yes
Byte Timing (B)14

The X.21 / V.11 / RS422 standard is supported on the FarSync range of communications products

Information in this document is subject to change without notice.

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