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RS-422 and RS-485 Application Note
INTRODUCTION
The purpose of this application note is to attempt to describe the
main elements of an RS-422 and RS-485 system. This application
note attempts to cover enough technical details so that the system
designer will have considered all important aspects in his data system
design. Since both RS-422 and RS-485 are data transmission systems
that use balanced differential signals, it is appropriate to discuss both
systems in the same application note.
DATA TRANSMISSION WITH BALANCED
DIFFERENTIAL SIGNALS
Balanced Line Drivers
Each signal that transmits in an RS-232 unbalanced data
transmission system appears on the interface connector as a voltage with
reference to a signal ground. For example, the transmitted data (TD)
from a DTE device appears on pin 2 with respect to pin 7 (signal ground).
This voltage will be negative if the line is idle and alternate between that
negative level and a positive level when data is sent. The RS-232
receiver operates within the voltage range shown in Figure 1. The
magnitude will vary from 3 to 12 volts (see Figure 1). The RS-232 driver
produces an output voltage within the range of + or -5 to 15 volts.
In a balanced differential system the voltage produced by the driver
appears across a pair of signal lines that transmit only one signal.
Figure 2 shows a schematic symbol for a balanced line driver and the
voltages that exist. A balanced line driver will produce a voltage from 2
to 6 volts across its A and B output terminals. A balanced line driver
will have signal ground (C) connection. Although proper connection to
the signal ground is important, it isn't used by a balanced line receiver
in determining the logic state of the data line. A balanced line driver
can also have an input signal called an "Enable" signal. The purpose
of this signal is to connect the driver to its output terminals, A and B. If
the "Enable" signal is OFF, one can consider the driver as
disconnected from the transmission line. An RS-485 driver must have
the "Enable" control signal. An RS-422 driver may have this signal, but
it is not always required. The disconnected or "disabled" condition of
the line driver usually is referred to as the "tristate" condition of the
driver.
RS-422/485 Application Note
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1992 B&B Electronics • PO Box 1040 • Ottawa, IL 61350
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Figure 1
Figure 2
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RS-422/485 Application Note
1992 B&B Electronics • PO Box 1040 • Ottawa, IL 61350
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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 3 is a schematic symbol for a balanced
differential line receiver. Figure 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 -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 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 4 is a typical RS-422 four-wire
interface between Data Terminal Equipment (DTE) and Data Circuit -
Terminating Equipment (DCE). 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 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 same relationship shown in Figure 5
also applies for RS-485 systems.
RS-422/485 Application Note
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1992 B&B Electronics • PO Box 1040 • Ottawa, IL 61350
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Figure 3
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RS-422/485 Application Note
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Figure 4
RS-422/485 Application Note
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1992 B&B Electronics • PO Box 1040 • Ottawa, IL 61350
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