ADM4850/ADM4851/ADM4852/ADM4853/ADM4854/ADM4855/ADM4856/ADM4857

Rev. D | Page 13 of 16

CIRCUIT DESCRIPTION

The ADM4850/ADM4851/ADM4852/ADM4853/ADM4854/

ADM4855/ADM4856/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 and 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 and 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 and ADM4856 and the

ADM4853 and 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 and ADM4854

and the 500 kbps ADM4851 and 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 do only one of these at any given time. How-

ever, with full-duplex operation, the transceiver can transmit and

receive simultaneously. The ADM4850/ADM4851/ADM4852/

ADM4853 are half-duplex devices in which the driver and the

receiver share differential bus terminals. The ADM4854/

ADM4855/ADM4856/ADM4857 are full-duplex devices that

have dedicated driver output and receiver input pins. Figure 28

and Figure 29 show typical half- and full-duplex topologies.

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

Figure 28. Typical Half-Duplex RS-485 Network Topology

GND

RO

DI

D

R

A

B

RO

DI

D

A

B

Z

Y

R

GND

Z

Y

ADM4854/ADM4855/

ADM4856/ADM4857

ADM4854/ADM4855/

ADM4856/ADM4857

Figure 29. Typical Full-Duplex Point-to-Point RS-485 Network Topology