ADM1486
Rev. A | Page 13 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 Electronics
Industries Association (EIA) that specify the electrical char-
acteristics of transceivers used in differential data transmission.
The RS-422 standard specifies data rates up to 10 MBaud and
line lengths up to 4,000 feet. A single driver can drive a trans-
mission line with up to 10 receivers.
In order to address true multipoint communications, the RS-485
standard was defined. This standard meets or exceeds all of the
requirements of RS-422, and it allows up to 32 drivers and
32 receivers to connect 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 and the RS-485 is that the drivers
with RS-485 can be disabled, allowing more than one driver to
be connected to a single line; in fact, 32 drivers can be
connected to a single line. Only one driver should be enabled at
a time, but the RS-485 standard contains additional specifica-
tions to guarantee device safety in the event of line contention.
CABLE AND DATA RATE
Twisted pair is the transmission line of choice for RS-485
communications. 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 ADM1486 is designed for bidirectional data com-
munications on multipoint transmission lines. A typical
application showing a multipoint transmission network is
shown in Figure 33.
An RS-485 transmission line can have as many as 32 trans-
ceivers on the bus. Only one driver can transmit at a time, but
multiple receivers may be enabled simultaneously.
As with any transmission line, it is important to minimize
reflections. 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 ADM1486 contains thermal shutdown circuitry that pro-
tects the part from excessive power dissipation during fault
conditions. Shorting the driver outputs to a low impedance
source can result in high driver currents. Thermal sensing
circuitry detects the increase in die temperature and disables
the driver outputs. Thermal sensing circuitry is designed to
disable the driver outputs when a die temperature reaches
150°C. As the device cools, the drivers are re-enabled at 140°C.
PROPAGATION DELAY
The ADM1486 features very low propagation delay, ensuring
maximum baud rate operation. The well-balanced driver
ensures 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.
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02603-033
Figure 33. Typical RS-485 Network
