ADM4850–ADM4857

Rev. 0 | Page 13 of 16

HIGH RECEIVER INPUT IMPEDANCE

The input impedance of the ADM485x receivers is 96 kΩ,

which is 8 times higher than the standard RS-485 unit load of

12 kΩ. This 96 kΩ impedance, enables a standard driver to

drive 32 unit loads or be connected to 256 ADM485x receivers.

An RS-485 bus, driven by a single standard driver, can be

connected to a combination of ADM485x and standard unit

load receivers, up to an equivalent of 32 standard unit loads.

THREE-STATE BUS CONNECTION

The half-duplex parts have a driver enable (DE) pin that enables

the driver outputs when taken high, or puts the driver outputs

into a high impedance state when taken low. Similarly, the half-

duplex devices have an active-low receiver enable (RE) pin.

Taking this pin low enables the receiver, while taking it high

puts the receiver outputs into a high impedance state. This

allows several driver outputs to be connected to an RS-485 bus.

Note that only one driver should be enabled at a time, while

many receivers can be enabled.

SHUTDOWN MODE

The ADM4850–ADM4853 have a low power shutdown mode,

which 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 tying DE and RE together.

The devices are 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 50 ns, the device does not enter shutdown mode. If DE

is low and RE is high for less than 3000 ns, the device is

guaranteed to enter shutdown mode.

FAIL-SAFE OPERATION

The ADM4850–ADM4857 offer true fail-safe operation while

remaining fully compliant with the ±200 mV EIA/TIA-485

standard. A logic-high receiver output is generated 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 −30 mV and −200 mV. If the differential receiver input

voltage (A-B) is greater than or equal to −30 mV, RO is logic

high. If A-B is less than or equal to −200 mV, RO is logic low. In

the case of a terminated bus with all transmitters disabled, the

receiver’s differential input voltage is pulled to 0 V by the

ADM485x’s internal circuitry, which results in a logic high with

30 mV minimum noise margin.

CURRENT LIMIT AND THERMAL SHUTDOWN

The ADM485x incorporates two protection mechanisms to

guard the drivers against short circuits, bus contention, or other

fault conditions. The first is a current-limiting output stage,

which protects the driver against short circuits over the entire

common-mode voltage range by limiting the output current to

approximately 70 mA. Under extreme fault conditions where

the current limit is not effective, a thermal shutdown circuit

puts the driver outputs into a high impedance state if the die

temperature exceeds 150°C, and does not turn them back on

until the temperature falls to 130°C.