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RS-422

RS-422 AND RS-485 APPLICATION NOTE

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Balanced Line Receivers

A balanced differential line receiver senses the voltage state of the

transmission line across two signal input lines, A and B. It will also have a

signal ground (C) that is necessary in making the proper interface

connection. Figure 1.3 is a schematic symbol for a balanced differential line

receiver. Figure 1.3 also shows the voltages that are important to the

balanced line receiver. If the differential input voltage Vab is greater than

+200 mV the receiver will have a specific logic state on its output terminal.

If the input voltage is reversed to less than -200 mV the receiver will create

the opposite logic state on its output terminal. The input voltages that a

balanced line receiver must sense are shown in Figure 1.3. The 200 mV to 6

V range is required to allow for attenuation on the transmission line.

EIA Standard RS-422 Data Transmission

The EIA Standard RS-422-A entitled “Electrical Characteristics of

Balanced Voltage Digital Interface Circuits” defines the characteristics of

RS-422 interface circuits. Figure 1.4 is a typical RS-422 four-wire interface.

Notice that five conductors are used. Each generator or driver can drive up

to ten (10) receivers. The two signaling states of the line are defined as

follows:

a. When the “A” terminal of the driver is negative with respect to the “B”

terminal, the line is in a binary 1 (MARK or OFF) state.

b. When the “A” terminal of the driver is positive with respect to the “B”

terminal, the line is in a binary 0 (SPACE or ON) state.

Figure 1.5 shows the condition of the voltage of the balanced line for an

RS-232 to RS-422 converter when the line is in the “idle” condition or OFF

state. It also shows the relationship of the “A” and “B” terminals of an RS-

422 system and the “-“ and “+” terminal markings used on many types of

equipment. The “A” terminal is equivalent to the “-“ designation, and the

“B” terminal equivalent to the “+” designation. The same relationship

shown in Figure 1.5 also applies for RS-485 systems. RS-422 can withstand

a common mode voltage (Vcm) of ±7 volts. Common mode voltage is

defined as the mean voltage of the A and B terminals with respect to signal

ground.

RS-422/485 Application Note

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