ADM485
APPLICATION INFORMATION
Differential Data Transmission
on the bus. Only one driver can transmit at a particular time, but
multiple receivers may be enabled simultaneously.
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.
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.
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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.
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In order to cater for 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 RS-422 and 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.
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Figure 5. Typical RS-485 Network
Thermal Shutdown
Table III. Comparison of RS-422 and RS-485 Interface Standards
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.
Specification
RS-422
RS-485
Transmission Type
Maximum Cable Length
Differential
4000 ft.
Differential
4000 ft.
Minimum Driver Output Voltage Ϯ2 V
Ϯ1.5 V
54 Ω
12 kΩ min
Ϯ200 mV
Driver Load Impedance
Receiver Input Resistance
Receiver Input Sensitivity
Receiver Input Voltage Range
100 Ω
4 kΩ min
Ϯ200 mV
Propagation Delay
–7 V to +7 V –7 V to +12 V
The ADM485 features very low propagation delay, ensuring
maximum baud rate operation. The driver is well balanced, ensuring
distortion free transmission.
No. of Drivers/Receivers per Line 1/10
32/32
Cable and Data Rate
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).
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 gener-
ated by the current flowing through each wire, thereby reducing
the effective inductance of the pair.
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.
The ADM485 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
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REV. E
ROCHESTER [ Rochester Electronics ]
