ADM485E/ADM487E/ADM1487E
Rev. B | Page 13 of 16
THEORY OF OPERATION
ADM485E/
ADM487E/
ADM1487E
ADM485E/
ADM487E/
ADM1487E
RS-485/RS-422 LINK
RO
DI
DE
DI
DE
B
A
GND
B
A
GND
RO
0.1µF0.1µF
5
5
V
CC
V
CC
RE
RE
06356-012
The ADM485E/ADM487E/ADM1487E are ruggedized RS-485
transceivers that operate from a single 5 V supply. They contain
protection against high levels of electrostatic discharge and are
ideally suited for operation in electrically harsh environments
or where cables can be plugged or unplugged. These devices are
intended for balanced data transmission and comply with TIA/
EIA standards RS-485 and RS-422. They contain a differential
line driver and a differential line receiver, and are suitable for
half-duplex data transmission because the driver and receiver
share the same differential pins.
The input impedance on the ADM485E is 12 kΩ, allowing up
to 32 transceivers on the differential bus. The ADM487E/
ADM1487E are 48 kΩ, allowing up to 128 transceivers on the
differential bus.
Figure 26. Typical Half-Duplex Link Application
CIRCUIT DESCRIPTION
Table 7 and Table 8 show the truth tables for transmitting and
receiving.
The ADM485E/ADM487E/ADM1487E are operated from
a single 5 V ± 10% power supply. Excessive power dissipation
caused by bus contention or output shorting is prevented by
a thermal shutdown circuit. If, during fault conditions, a sig-
nificant temperature increase is detected in the internal driver
circuitry, this feature forces the driver output into a high
impedance state.
Table 7. Transmitting Truth Table
Transmitting Inputs Transmitting Outputs
RE
DE DI B A
X
1
1 1 0 1
X
1
1 0 1 0
The receiver contains a fail-safe feature that results in a logic
high output state if the inputs are unconnected (floating).
0 0 X
1
High-Z High-Z
1 0 X
1
High-Z High-Z
A high level of robustness is achieved using internal protection
circuitry, eliminating the need for external protection compo-
nents, such as tranzorbs or surge suppressors.
1
X = don’t care.
Table 8. Receiving Truth Table
Receiving Inputs Receiving Outputs
Low electromagnetic emissions are achieved using slew rate-
limited drivers, minimizing both conducted and radiated
interference.
RE
DE A to B RO
0 0 ≥+0.2 V 1
0 0 ≤−0.2 V 0
The ADM485E/ADM487E/ADM1487E can transmit at data
rates up to 250 kbps.
0 0 Inputs open circuit 1
1 0 X
1
High-Z
A typical application for the ADM485E/ADM487E/ADM1487E
is illustrated in
Figure 26, which shows a half-duplex link where
data can be transferred at rates up to 250 kbps. A terminating
resistor is shown at both ends of the link. This termination is
not critical because the slew rate is controlled by the ADM485E/
ADM487E/ADM1487E and reflections are minimized.
1
X = don’t care.
ESD Transient Protection Scheme
The ADM485E/ADM487E/ADM1487E use protective clamping
structures on their inputs and outputs that clamp the voltage to
a safe level and dissipate the energy present in ESD.
The protection structure achieves ESD protection up to ±15 kV
human body model (HBM).
The communications network can be extended to include
multipoint connections, as shown in Figure 29. As many as
32 ADM485E transceivers or 128 ADM487E/ADM1487E
transceivers can be connected to the bus.
