Datasheet

ADM4850–ADM4857
Rev. 0 | Page 12 of 16
CIRCUIT DESCRIPTION
The ADM4850–ADM4857 are high speed RS-485/RS-422
transceivers offering enhanced performance over industry-
standard devices. All devices in the family contain one driver
and one receiver, but offer a choice of performance options. The
devices feature true fail-safe operation, which means that a logic
high receiver output is guaranteed when the receiver inputs are
open-circuit or short-circuit, or when they are connected to a
terminated transmission line with all drivers disabled (see the
Fail-Safe Operation section).
SLEW-RATE CONTROL
The ADM4850/ADM4854 feature a controlled slew-rate driver
that minimizes electromagnetic interference (EMI) and reduces
reflections caused by incorrectly terminated cables, allowing
error-free data transmission rates up to 115 kbps. The ADM4851/
ADM4855 offer a higher limit on driver output slew rate, allowing
data transmission rates up to 500 kbps. The driver slew rates of
the ADM4852/ADM4856 and the ADM4853/ADM4857 are not
limited, offering data transmission rates up to 2.5 Mbps and
10 Mbps, respectively.
RECEIVER INPUT FILTERING
The receivers of all the devices incorporate input hysteresis. In
addition, the receivers of the 115 kbps ADM4850/ADM485 and
the 500 kbps ADM4851/ADM4855 incorporate input filtering.
This enhances noise immunity with differential signals that have
very slow rise and fall times. However, it causes the propagation
delay to increase by 20%.
HALF-/FULL-DUPLEX OPERATION
Half-duplex operation implies that the transceiver can transmit
and receive, but it can only do one of these at any given time.
However, with full-duplex operation, the transceiver can
transmit and receive simultaneously. The ADM4850–ADM4853
are half-duplex devices in which the driver and receiver share
differential bus terminals. The ADM4854–ADM4857 are full-
duplex devices, which have dedicated driver output and receiver
input pins. Figure 26
and Figure 27 show typical half- and full-
duplex topologies.
04931-026
RO
RE
DE
DI
D
R
A
B
MAXIMUM NUMBER OF TRANSCEIVERS ON BUS: 256
RO
RE
DE
DI
D
R
A
B
R
D
RO RE DE DI
AB
R
D
RO RE DE DI
AB
ADM4850/ADM4851/
ADM4852/ADM4853
ADM4850/ADM4851/
ADM4852/ADM4853
ADM4850/ADM4851/
ADM4852/ADM4853
ADM4850/ADM4851/
ADM4852/ADM4853
Figure 26. Typical Half-Duplex RS-485 Network Topology
04931-027
GND
RO
DI
D
R
A
B
RO
DI
D
A
B
Z
Y
R
GND
V
DD
V
DD
Z
Y
ADM4854/ADM4855/
ADM4856/ADM4857
ADM4854/ADM4855/
ADM4856/ADM4857
Figure 27. Typical Full-Duplex Point-to-Point RS-485 Network Topology