ADM485
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 which appear as common-mode voltages
on the line. There are two main standards approved by the
Electronics Industries Association (EIA) which specify the elec-
trical 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 transmis-
sion line with up to 10 receivers.
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 time, but the RS-485 standard contains
additional specifications to guarantee device safety in the event
of line contention.
Cable and Data Rate
ure 26. An RS-485 transmission line can have as many as 32 trans-
ceivers on the bus. Only one driver can transmit at a particular time
but multiple receivers may be enabled simultaneously.
As with any transmission line, it is important that reflections are
minimized. This may be achieved by terminating the extreme
ends of the line using resistors equal to the characteristic imped-
ance 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 which pro-
tects the part from excessive power dissipation during fault con-
ditions. 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 reenabled at 140°C.
Propagation Delay
The ADM485 features very low propagation delay ensuring
maximum baud rate operation. The driver is well balanced en-
suring distortion free transmission.
Another important specification is a measure of the skew be-
tween the complementary outputs. Excessive skew impairs the
noise immunity of the system and increases the amount of elec-
tromagnetic interference (EMI).
Receiver Open-Circuit Fail Safe
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 gener-
ated by the current flowing through each wire, thereby, reducing
the effective inductance of the pair.
The ADM485 is designed for bidirectional data communica-
tions on multipoint transmission lines. A typical application
showing a multipoint transmission network is illustrated in Fig-
The receiver input includes a fail-safe feature which guarantees
a logic high on the receiver when the inputs are open circuit or
floating.
Table III. Comparison of RS-422 and RS-485 Interface Standards
Specification
Transmission Type
Maximum Cable Length
Minimum Driver Output Voltage
Driver Load Impedance
Receiver Input Resistance
Receiver Input Sensitivity
Receiver Input Voltage Range
No of Drivers/Receivers Per Line
RS-422
Differential
4000 ft.
±
2 V
100
Ω
4 kΩ min
±
200 mV
–7 V to +7 V
1/10
RS-485
Differential
4000 ft.
±
1.5 V
54
Ω
12 kΩ min
±
200 mV
–7 V to +12 V
32/32
REV. A
–7–
AD [ ANALOG DEVICES ]
