RS-232 is the serial data interface standard, at
first all is (EIA) draws up and the issue by the electronic industry
association, RS-232 issued in 1962, the naming is EIA-232-E, took the
industry standard, guarantees between the different factory product
the compatibility. RS-422 comes by the RS-232 development, it is
for makes up insufficiency of the RS-232 to propose. 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
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