Data Sheet
ADM3491-1
THEORY OF OPERATION
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.
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.
The two main standards approved by the EIA specify the
electrical characteristics of transceivers used in differential
data transmission:
RECEIVER OPEN-CIRCUIT FAIL-SAFE FEATURE
The receiver input includes a fail-safe feature that guarantees a
logic high on the receiver when the inputs are open circuit or
floating.
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.
3.3V
3.3V
0.1µF
0.1µF
RS-485 standard was defined to cater to true multipoint
communications. This standard meets or exceeds all the
requirements of RS-422, but also allows multiple drivers
and receivers to be connected to a single bus. An extended
common-mode range of −7 V to +12 V is defined.
V
DE
DI
V
CC
CC
R
RE
RO
A
Y
Z
D
B
RS-485/RS-422 LINK
ADM3491-1
ADM3491-1
The most significant differentiator of the RS-485 standard is
that the drivers can be disabled, thereby allowing more than one
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.
Z
B
A
RO
R
DI
D
Y
RE
DE
GND
GND
Figure 26. ADM3491-1 Full-Duplex Data Link
Table 6. Comparison of RS-422 and RS-485 Interface Standards
Specification
RS-422
Differential
4000 ft.
2 V
100 Ω
4 kΩ min
200 mV
RS-485
Differential
4000 ft.
1.5 V
54 Ω
12 kΩ min
200 mV
Transmission Type
Table 7. Transmitting Truth Table
Maximum Cable Length
Minimum Driver Output Voltage
Driver Load Impedance
Receiver Input Resistance
Receiver Input Sensitivity
Receiver Input Voltage Range
Inputs
Outputs
Y
RE
DE
DI
Z
X
X
0
1
1
1
0
0
1
0
X
X
0
1
Hi-Z
Hi-Z
1
0
Hi-Z
−7 V to +7 V −7 V to +12 V
Hi-Z
CABLE AND DATA RATE
Table 8. Receiving Truth Table
Inputs
Outputs
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.
RE
DE
A–B
RO
0
0
0
1
X
X
X
X
≥ +0.2 V
≤ −0.2 V
Inputs O/C
X
0
0
1
Hi-Z
The ADM3491-1 is designed for bidirectional data communica-
tions on multipoint transmission lines. A typical application
showing a multipoint transmission network is illustrated in
Figure 26. Only one driver can transmit at a particular time, but
multiple receivers can be enabled simultaneously.
Rev. C | Page 11 of 16
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