Data Sheet

ADM4850 to ADM4857

Rev. F | Page 13 of 16

THEORY OF OPERATION

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 guarantees a logic

high receiver output 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 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/

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 and ADM4854

and the 500 kbps ADM4851 and ADM4855 incorporate input

filtering, which 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-DUPLEX/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. However,

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 29

and Figure 30 show typical half-duplex and full duplex topologies.

Figure 29. Typical Half-Duplex RS-485 Network Topology

Figure 30. Typical Full Duplex Point-to-Point RS-485 Network Topology

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

A

B

R

D

RO

RE DE

DI

A

B

ADM4850

ADM4852/ADM4853

ADM4850/

ADM4852/ADM4853

NOTES

1. THE ADM4851 IS A HALF-DUPLEX RS-485 TRANSCEIVER, BUT IT DOES NOT HAVE THE DRIVER ENABLE (DE) AND THE RECEIVER ENABLE (RE) PINS.

GND

RO

DI

D

R

A

B

RO

DI

D

A

B

Z

Y

R

GND

VCC

VCC

Z

Y

ADM4854/ADM4855/

ADM4856/ADM4857

ADM4854/ADM4855/

ADM4856/ADM4857