REV. –8–
ADM1485
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 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.
RT
D
R
DD
R
R
D
R
Figure 5. Typical RS-485 Network
Thermal Shutdown
The ADM1485 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 ADM1485 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 electro-
magnetic 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.
APPLICATION 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 Electronics
Industries Association (EIA) that specify the electrical charac-
teristics 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.
In order to cater to true multipoint communications, the RS-485
standard was defined. This standard meets or exceeds all the
requirements of the RS-422 but also allows for up to 32 drivers
and 32 receivers to be connected to a single bus. An extended com-
mon-mode range of –7 V to +12 V is defined. The most
significant difference between the RS-422 and the RS-485 is the
fact that the drivers may 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 III. 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 also causes cancellation of the magnetic fields generated
by the current flowing through each wire, thereby reducing the
effective inductance of the pair.
The ADM1485 is designed for bidirectional data communications
on multipoint transmission lines. A typical application showing
a multipoint transmission network is illustrated in Figure 5.
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 may be enabled simultaneously.
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