ADM483
Rev. 0 | Page 11 of 16
APPLICATIONS
DIFFERENTIAL DATA TRANSMISSION
Differential data transmission is used to reliably transmit
data at high rates over long distances and through noisy
environments. Differential transmission nullifies the effects
of ground shifts and noise signals that appear as common-
mode voltages on the line. There are two main standards
approved by the Electronics Industries Association (EIA),
which specify the electrical characteristics of transceivers
used in differential data transmission.
The RS-422 standard specifies data rates up to 10 Mbaud
and line lengths up to 4000 ft. A single driver can drive a
transmission line with up to 10 receivers.
To achieve true multipoint communications, the RS-485
standard was defined. This standard meets or exceeds all the
requirements of RS-422, but also allows up to 32 drivers and 32
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 that the drivers may
be disabled, allowing up to 32 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.
Table 5. Comparison of 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
Drivers/Receivers per Line 1/10 32/32
CABLE AND DATA RATE
The preferred transmission line for RS-485 communications
is a twisted pair. Twisted pair cable tends to cancel common-
mode noise and the magnetic fields generated by the current
flowing through each wire, thereby reducing the effective
inductance of the pair.
The ADM483 is designed for bidirectional data communica-
tions on multipoint transmission lines. A typical application
showing a multipoint transmission network is shown in
Figure 24. An RS-485 transmission line can have as many as
32 transceivers on the bus. Only one driver can transmit at a
particular time, but multiple receivers can be enabled simul-
taneously. As with any transmission line, it is important to
minimize reflections. This can be done by terminating the
extreme ends of the line by using resistors equal to the char-
acteristic impedance of the line. Stub lengths of the main
line should also be kept as short as possible. A properly
terminated transmission line appears purely resistive to
the driver.
05079-024
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MAXIMUM NUMBER OF TRANSCEIVERS ON BUS: 32
ADM483
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ADM483
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Figure 24. Typical Half-Duplex RS-485 Network Topology
