X.21, V.11, RS422 and EIA-422 Connector / Signaling Standard

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 Name DB15 Contact Supported on FarSync cards
Shield 1 Yes
Transmit (A) 2 Yes
Control (A) 3 Yes
Receive (A) 4 Yes
Indication (A) 5 Yes
Signal Timing (A) 6 Yes
Byte Timing (A) 7
GND 8 Yes
Transmit (B) 9 Yes
Control (B) 10 Yes
Receive (B) 11 Yes
Indication (B) 12 Yes
Signal Timing (B) 13 Yes
Byte Timing (B) 14
Unassigned 15

22-Jan-2010