The optical terminal RS-232, RS-422, and RS-485 are all serial data interface standards that were initially developed and released by the Electronic Industries Alliance (EIA). The RS-232 was released in 1962, named EIA-232-E, as an industrial standard to ensure compatibility between products of different manufacturers. RS-422 evolved from RS-232 and was proposed to address its shortcomings. To improve the short communication distance and low speed of RS-232, RS-422 defines a balanced communication interface, increasing the transmission rate to 10Mb/s and extending the transmission distance to 4000 feet (when the rate is below 100kb/s), while allowing up to 10 receivers to be connected on one balanced bus. RS-422 is a unidirectional, balanced transmission specification for single-machine sending and multi-machine receiving, named the TIA/EIA-422-A standard. To expand its application range, the EIA formulated the RS-485 standard in 1983 based on RS-422, adding multi-point and bidirectional communication capabilities, i.e., allowing multiple transmitters to connect to the same bus, while enhancing the driver capability and collision protection characteristics of the transmitter and expanding the common mode range of the bus, which was later named the TIA/EIA-485-A standard. Since all the recommended standards proposed by the EIA use "RS" as a prefix, in the communications industry, people still habitually refer to the above standards with "RS" as a prefix.
RS-232, RS-422, and RS-485 standards only specify the electrical characteristics of the interface and do not involve connectors, cables, or protocols. Based on this, users can establish their own higher-level communication protocols. Therefore, in video applications, many manufacturers have established a set of higher-level communication protocols, either publicly available or exclusively used by the manufacturer. For example, Sony and Panasonic differ in their RS-422 control protocols for VCRs, and there are even more control protocols for video servers, such as the public Louth and Odetis protocols, while ProLINK is based on Profile.
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