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.
