ADM3485E
Rev. D | Page 12 of 16
APPLICATIONS INFORMATION
DIFFERENTIAL DATA TRANSMISSION
Differential data transmission is used to reliably transmit data
at high rates over long distances and through noisy environ-
ments. Differential transmission nullifies the effects of ground
shifts and noise signals that appear as common-mode voltages
on the line.
Two main standards that specify the electrical characteristics of
transceivers used in differential data transmission are approved
by the Electronics Industries Association (EIA). The RS-422
standard specifies data rates up to 10 Mbps and line lengths up
to 4000 feet. A single driver can drive a transmission line with
up to 10 receivers. The RS-485 standard was defined to cater to
true multipoint communications. This standard meets or
exceeds all the requirements of RS-422 but also allows multiple
drivers and receivers to be connected to a single bus. An
extended common-mode range of −7 V to +12 V is defined.
The most significant difference between RS-422 and RS-485 is
the fact that under the RS-485 standard the drivers may be
disabled, thereby allowing more than one to be connected to a
single line. Only one driver should be enabled at a time, but the
RS-485 standard contains additional specifications to guarantee
device safety in the event of line contention.
CABLE AND DATA RATE
The transmission line of choice for RS-485 communications is a
twisted pair. Twisted-pair cable tends to cancel common-mode
noise and also causes cancellation of the magnetic fields
generated by the current flowing through each wire, thereby
reducing the effective inductance of the pair.
The ADM3485E is designed for bidirectional data communi-
cations on multipoint transmission lines. A typical application
showing a multipoint transmission network is illustrated in
Figure 25. Only one driver can transmit at a particular time,
but multiple receivers may be enabled simultaneously.
As with any transmission line, it is important that reflections are
minimized. This can be achieved by terminating the extreme
ends of the line using resistors equal to the characteristic impe-
dance of the line. Stub lengths off the main line must also be
kept as short as possible. A properly terminated transmission
line appears purely resistive to the driver.
RECEIVER OPEN-CIRCUIT FAIL-SAFE
The receiver input includes a fail-safe feature that guarantees
a logic high on the receiver when the inputs are open circuit
or floating.
Table 10. RS-422 and RS-485 Interface Standards
Specification RS-422 RS-485
Transmission Type Differential Differential
Maximum Cable Length 4000 ft 4000 ft
Minimum Driver Output Voltage ±2 V ±1.5 V
Driver Load Impedance 100 Ω 54 Ω
Receiver Input Resistance 4 kΩ min 12 kΩ min
Receiver Input Sensitivity ±200 mV ±200 mV
Receiver Input Voltage Range
−7 V to +7 V −7 V to +12 V
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MAXIMUM NUMBER OF TRANSCEIVERS ON BUS: 50
03338-027
Figure 25. Multipoint Transmission Network
