rs422 rs485标准

<font face="Verdana">Unbalanced Line Drivers<br/>Each signal that transmits in an RS-232 unbalanced data transmission<br/>system appears on the inteRFace connector as a voltage with reference to a<br/>signal ground. For example, the transmitted data (TD) from a DTE device<br/>appears on pin 2 with respect to pin 7 (signal ground) on a DB-25 connector.<br/>This voltage will be negative if the line is idle and alternate between that<br/>negative level and a positive level when data is sent with a magnitude of ±5 to<br/>±15 volts. The RS-232 receiver typically operates within the voltage range of<br/>+3 to +12 and -3 to<br/>-12 volts as shown in Figure 1.1.<br/>Balanced Line Drivers<br/>In a balanced differential system the voltage produced by the driver<br/>appears across a pair of signal lines that transmit only one signal. Figure 1.2<br/>shows a schematic symbol for a balanced line driver and the voltages that<br/>exist. A balanced line driver will produce a voltage from 2 to 6 volts across<br/>its A and B output terminals and will have a signal ground (C) connection.<br/>Although proper connection to the signal ground is important, it isn't used by<br/>a balanced line receiver in determining the logic state of the data line. A<br/>balanced line driver can also have an input signal calLED an “Enable” signal.<br/>The purpose of this signal is to connect the driver to its output terminals, A<br/>and B. If the “Enable” signal is OFF, one can consider the driver as<br/>disconnected from the transmission line. An RS-485 driver must have the<br/>“Enable” control signal. An RS-422 driver may have this signal, but it is not<br/>always required. The disconnected or "disabled" condition of the line driver<br/>usually is referred to as the “tristate1” condition of the driver.</font><br/>