In order to improve the RS-232 signal distance to be short, the speed low shortcoming, RS-422 has defined one kind of balance correspondence connection, will transmit the speed to enhance 10Mb/s, the transmitting range lengthens (speed will be lower than when 100kb/s) to 4,000 feet, and the permission will connect more than 10 receivers on a balanced main line. RS-422 is one kind of single plane transmission, many machine receives is unidirectional, the balanced transmission standard, by the naming is the TIA/EIA-422-A standard.

In order to expand the application scope, EIA has formulated the RS-485 standard in 1983 in the RS-422 foundation, increased the multi- spots, the twp-way communication ability, namely permits many transmitters to connect on the identical strip main line, simultaneously increased the transmitter driving force and the conflict protection characteristic, expanded the main line altogether mold scope, the latter naming is the TIA/EIA-485-A standard. Because EIA proposed the suggestion standard all is takes the prefix by "RS", therefore in the communication industry domain, still was used to the above standard to RS does the prefix name.

RS-232, RS-422 and the RS-485 standard only makes the stipulation to the connection electrical specification, but does not involve meets the plug-in unit, the electric cable or the agreement, the user may establish own in this foundation high level correspondence agreement. Therefore in the video frequency application, many factories has all established set of high level correspondences agreements, either is public or the factory one's own use. If in videorecorder factory Sony and Matushita have the difference to the videorecorder RS-422 controlling agreement, on the video frequency server controlling agreement then have been more, like Louth, the Odetis agreement is public, but ProLINK is based on Profile on.

RS-232 serial interface standard

At present RS-232 is in PC machine and the correspondence industry applies the most widespread one kind of serial interface. RS-232 is one kind increases by the definition in the low gear rate serial communication the communication distance 单端 the standard. RS-232 adopts not the balanced transmission mode, namely so-called 单端 communication.

Receives, the start data signal is opposite to the signal place, if the data which sends out from the DTE equipment when uses the DB25 coupling is 2 feet relative 7 feet (signal) the level, the DB25 various pins definition sees also chart 1. The typical RS-232 signal swings between the positive and negative level, when transmission data, the transmitting end driver outputs the level in +5 ~ +15V, negative level in -5 ~ -15V level. When innumerable according to transmission, on-line is TTL, from starts to transmit the data to the conclusion, on-line the level returns to the TTL level again from the TTL level to the RS-232 level. Receiver model operate level in +3 ~ +12V and -3 ~ -12V.

Because the transmission level and the receive level difference only is 2V to 3V about, therefore it mold suppression ability is altogether bad, in addition in the twisted pair line distributed capacity, its transmission is away from most greatly for approximately 15 meters, the maximum speed is 20kb/s. RS-232 is for point-to-point (namely only uses a pair to receive, to send, its driver load which equipment) the communication designs is 3 ~ 7k Omega. Therefore RS-232 suits between the local equipment the correspondence

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