Everything about Rs-422 totally explained
American national standard
ANSI/TIA/EIA-422-B (formerly
RS-422) and its international equivalent
ITU-T Recommendation V.11
(also known as
X.27), are technical
standards that specify the "electrical characteristics of the balanced voltage digital interface circuit". It provides for data transmission, using
balanced or differential signaling, with unidirectional/non-reversible,
terminated or non-terminated transmission lines, point to point, or
multi-drop. In contrast to
RS-485 (which is multi-point instead of multi-drop) EIA-422/V.11 doesn't allow multiple drivers but only multiple receivers.
The current title of the ANSI standard is
TIA-422 Electrical Characteristics of Balanced Voltage Differential Interface Circuits and is now in revision B, published in May
1994, and was reaffirmed by the
Telecommunications Industry Association in
2005.
Several key advantages offered by this standard include the differential receiver, a differential driver and data rates as high as 10 mega
baud at 12 metres (40 ft). The specification itself doesn't set an upper limit on data rate, but rather shows how signal rate degrades with cable length. The figure plotting this stops at 10
Mbit/s.
EIA-422 only specifies the electrical signaling characteristics of a single balanced signal. Protocols and pin assignments are defined in other specifications. The mechanical connections for this interface are specified by
EIA-530 (
DB-25 connector) or
EIA-449 (
DC-37 connector), however devices exist which have 4 screw-posts to implement the transmit and receive pair only. The maximum cable length is 1200 m. Maximum data rates are 10 Mbit/s at 12 m or 100 kbit/s at 1200 m. EIA-422 can't implement a truly multi-point communications network (such as with EIA-485), however one driver can be connected to up to ten receivers.
A common use of EIA-422 is for
RS-232 extenders.
In video editing studios it's used to link control signals for all video and audio players/recorders to a central control board. Also, an
RS-232-compatible variant of RS-422 using a
mini-DIN-8 connector was widely used on
Macintosh hardware until it was replaced by Intel's
Universal Serial Bus on the
iMac in 1998.
EIA-422 can interoperate with interfaces designed to
MIL-STD-188-114B, but they're not identical. EIA-422 uses a nominal 0 to 5
Volt signal while MIL-STD-188-114B uses a signal symmetric about 0 V. However the tolerance for common mode voltage in both specifications allows them to interoperate. Care must be taken with the termination network.
EIA-423 is a similar specification for unbalanced signaling.
When used in relation to communications wiring, RS-422 wiring refers to cable made of 2 sets of
twisted pair, often with each pair being shielded, and a ground wire. While a double pair cable may be practical for many RS-422 applications, the RS-422 specification only defines one signal path and doesn't assign any function to it. Any complete cable assembly (for example with connectors) should be labeled with the specification that defined the signal function and mechanical layout of the connector, such as RS-449.
Further Information
Get more info on 'Rs-422'.
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