RS-422/485 Application Note 3
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax 353-91-792445
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 A and B terminals with respect to signal ground.
4 RS-422/485 Application Note
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax 353-91-792445
Figure 1.3
Figure 1.4
(96 pages)
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