ADM4850 to ADM4857

Data Sheet

Rev. F | Page 14 of 16

HIGH RECEIVER INPUT IMPEDANCE

The input impedance of the ADM4850/ADM4851/ADM4852/

ADM4853/ADM4854/ADM4855/ADM4856/ADM4857 receivers

is 96 kΩ, which is eight 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 to be connected to 256 ADM4850/

ADM4851/ADM4852/ADM4853/ADM4854/ADM4855/

ADM4856/ADM4857 receivers. An RS-485 bus, driven by a

single standard driver, can be connected to a combination of

ADM4850/ADM4851/ADM4852/ADM4853/ADM4854/

ADM4855/ADM4856/ADM4857 devices and standard unit

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

THREE-STATE BUS CONNECTION

The half-duplex devices (ADM4850/ADM4852/ADM4853)

have a driver enable pin (DE) 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 pin (RE). Taking this pin low

enables the receiver, whereas taking it high puts the receiver

outputs into a high impedance state, which allows several driver

outputs to be connected to an RS-485 bus. Note that only one

driver must be enabled at a time, but that many receivers can be

enabled.

SHUTDOWN MODE

The ADM4850/ADM4852/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.

If DE is driven low and RE is driven high for less than 50 ns, the

devices are guaranteed not to enter shutdown mode. If DE is

driven low and RE is driven high for at least 3000 ns, the devices

are guaranteed to enter shutdown mode.

FAIL-SAFE OPERATION

The ADM4850/ADM4851/ADM4852/ADM4853/ADM4854/

ADM4855/ADM4856/ADM4857 offer true fail-safe operation

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

standard. A logic high receiver output generates 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 logic high 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 differential input voltage

of the receiver is pulled to 0 V by the internal circuitry of

the ADM4850/ADM4851/ADM4852/ADM4853/ADM4854/

ADM4855/ADM4856/ADM4857, which results in a logic high

with 30 mV minimum noise margin.

CURRENT LIMIT AND THERMAL SHUTDOWN

The ADM4850/ADM4851/ADM4852/ADM4853/ADM4854/

ADM4855/ADM4856/ADM4857 incorporate 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.