Data Sheet ADM3491-1
Rev. C | Page 11 of 16
THEORY OF OPERATION
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:
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.
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 dened.
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.
Table 6. 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
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 gener-
ated by the current flowing through each wire, thereby reducing
the effective inductance of the pair.
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.
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.
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.
RE
V
CC
RO
R
A
B
Z
Y
GND
DE
DI
ADM3491-1
RS-485/RS-422 LINK
ADM3491-1
GND
RE
RO
DI
DE
V
CC
Y
Z
A
3.3
0.1µF
0.1µF
3.3
B
D
D
R
05234-026
Figure 26. ADM3491-1 Full-Duplex Data Link
Table 7. Transmitting Truth Table
Inputs Outputs
RE
DE DI Z Y
X 1 1 0 1
X 1 0 1 0
0 0 X Hi-Z Hi-Z
1 0 X Hi-Z Hi-Z
Table 8. Receiving Truth Table
Inputs Outputs
RE
DE A–B RO
0 X ≥ +0.2 V 0
0 X ≤ −0.2 V 0
0 X Inputs O/C 1
1 X X Hi-Z
