ADM3082/ADM3085/ADM3088

–12 –

PRELIMINARY TECHNICAL DATA

DETAILED

DESCRIPTION

The ADM3082, ADM3085 and ADM3088 are high-

speed RS-485/RS-422 transceivers offering enhanced per-

formance over industry-standard devices All devices in the

family contain one driver and one receiver, but there is a

choice of performance options. The devices feature fail-

safe operation, which means that a logic-high receiver out-

put is guaranteed when the receiver inputs are open-circuit

or short-circuit, or when they are connected to a termi-

nated transmission line with all drivers disabled (see the

section on Fail-Safe Operation).

SLEW RATE CONTROL

The ADM3082 features a controlled slew-rate driver that

minimize electromagnetic interference (EMI) and reduce

reflections caused by incorrectly terminated cables, allow-

ing error-free data transmission rates up to 115kbps (see

the section on Reduced EMI and Reflections).

The ADM3085 offers a higher limit on driver output

slew-rate, allowing data transmission rates up to 500kbps.

The driver slew rate of the ADM3088 is not limited, of-

fering data transmission rates up to 10Mbps.

RECEIVER INPUT FILTERING

The receivers of all devices incorporate input hysteresis.

In addition, when operating in 115kbps or 500kbps mode,

the receivers of the ADM3082 and ADM3085 incorporate

input filtering. This enhances noise immunity with differ-

ential signals that have very slow rise and fall times, but it

does increase propagation delay by 20%.

HALF-/FULL-DUPLEX

OPERATION

The ADM3082, ADM3085, and ADM3088 are dedicated

half-duplex devices (driver outputs internally linked to re-

ceiver inputs). Figure 29 shows a typical half-duplex con-

nection between two devices.

THREE-STATE BUS CONNECTION

All the devices have a Driver Enable pin (DE) that en-

ables the driver outputs when taken high or puts the driver

outputs into a high-impedance state when taken low. This

allows several driver outputs to be connected to an

RS-422/RS-485 bus.

Similarly, all the devices have a (active-low) Receiver En-

able pin (

RE). Taking this low enables the receiver, while

taking it high puts the receiver outputs into a high-imped-

ance state. This allows several receiver outputs to be con-

nected to a serial data bus.

HIGH INPUT IMPEDANCE

The input impedance of the devices is 96k , which is 8

times higher than the standard RS-485 load of 12k . A

standard driver can driver 32 standard loads, so up to 256

ADM308X receivers, or a combination of ADM308X and

other devices up to 32 unit loads, may be connected to an

RS-422/RS485 bus driven by a single driver.

SHUTDOWN

MODE

All the devices have a low power shutdown mode that is

enabled by taking

RE high and DE low.

If shutdown mode is not used, the fact that DE is active-

high and

RE is active-low offers a convenient way of

switching the device between transmit and receive, by ty-

ing DE and

RE together. This is useful, for example, in

applications using half duplex operation and where several

receiver outputs are connected to a serial bus.

The device is guaranteed not to enter shutdown mode if

DE and

RE are driven in this way. If DE is low and RE is

high for less than 50ns the device will not enter shutdown.

If DE is low and

RE is high for greater than 600ns, the

device is guaranteed to enter shutdown.

FAIL-SAFE OPERATION

The ADM3082/ADM3085/ADM3088 offer true fail-safe

operation while remaining fully compliant with the

±200mV EIA/TIA-485 standard. A logic-high receiver

output is guaranteed when the receiver inputs are shorted

together or open-circuit, or when they are connected to a

terminated transmission line with all drivers disabled.

This is done by setting the receiver threshold between

-50mV and –200mV. If the differential receiver input volt-

age (A–B) is greater than or equal to –50mV, RO is logic

high. If A–B is less than or equal to –200mV, RO is logic

low. In the case of a terminated bus with all transmitters

disabled, the receiver’s differential input voltage is pulled

to 0V by ADM308X family, which results in a logic high

with a 50mV minimum noise margin.

ENHANCED ESD PROTECTION

All Analog Devices parts incorporate protection against

electrostatic discharge (ESD) to protect the devices dur-

ing handling, assembly and normal operation. In addition,

the ADM308X family has enhanced ESD protection up to

±15kV on the receiver inputs and driver outputs (A, B) to

protect against severe operational conditions such as line

transients, connection and disconnection.

100n

+5V

+5V

100n

R

T

R

T

DA T A

IN (D I)

REC EIVER

OUT P UT (RO)

DA T A

IN (D I)

REC EIVER

OUT P UT (RO)

1

2

3

4

8

7

6

5

D

R

D

R

REC EIVER

ENA BL E (

RE)

DR IVER

ENA BL E (DE)

5

6

7

8

4

3

2

1

REC EIVER

ENA BL E (

RE)

DR IVER

ENA BL E (DE)

AD M 308 2/5/8

AD M 308 2/5/8

Figure 29. Half-Duplex Configuration for ADM3082, ADM3085 or ADM3088

REV. PrA 02/02