ADM485
Rev. F | 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 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 EIA that
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 cater to true multipoint communications, the RS-485
standard was defined. This standard meets or exceeds all the
requirements of RS-422 but also allows for 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 the RS-422 standard and the RS-485 standard is
the fact that the drivers can be disabled, thereby allowing more
than one (32 in fact) 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 7. 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
No. of Drivers/Receivers per Line 1/10 32/32
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 causes cancellation of the magnetic fields generated
by the current flowing through each wire, thereby reducing the
effective inductance of the pair.
The ADM485 is designed for bidirectional data communications
on multipoint transmission lines. A typical application showing
a multipoint transmission network is illustrated in
Figure 30.
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 simultaneously.
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Figure 30. Typical RS-485 Network
As with any transmission line, it is important that reflections be
minimized. This can be achieved by terminating the extreme ends
of the line using resistors equal to the characteristic 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.
THERMAL SHUTDOWN
The ADM485 contains thermal shutdown circuitry that protects
the part from excessive power dissipation during fault conditions.
Shorting the driver outputs to a low impedance source can result
in high driver currents. The thermal sensing circuitry detects
the increase in die temperature and disables the driver outputs.
The thermal sensing circuitry is designed to disable the driver
outputs when a die temperature of 150°C is reached. As the
device cools, the drivers are re-enabled at 140°C.
PROPAGATION DELAY
The ADM485 features very low propagation delay, ensuring
maximum baud rate operation. The driver is well balanced,
ensuring distortion free transmission.
Another important specification is a measure of the skew
between the complementary outputs. Excessive skew impairs
the noise immunity of the system and increases the amount of
electromagnetic interference (EMI).
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.
