adm1487ar Analog Devices, Inc., adm1487ar Datasheet - Page 7

• Manufacturer Part Number: adm1487ar
• Description: Ultra-low Power Rs-485/rs-422 Transceiver With Low Emi And +/-10kv Esd Protection
• Manufacturer: Analog Devices, Inc.

TRANSMITTING
X = Don’t care
CIRCUIT DESCRIPTION
The ADM1487 is a differential input, differential output
transceiver designed RS-485/RS-422 serial data communi-
cations over twisted-pair cables. It is a half-duplex device
intended for alternate transmission and reception of data
over a single twisted-pair cable, and the driver outputs and
receiver inputs are internally linked and brought out to a
single pair of pins.
Its principal features include ultra-low power operation, a
controlled slew-rate driver that minimizes electromagnetic
interference (EMI) and signals reflections due to incor-
rectly terminated lines, and high input impedance that al-
lows up to 256 receivers to be connected to a bus.
THREE-STATE BUS CONNECTION
The ADM1487 has a Driver Enable pin (DE) that enables
the driver outputs when taken high or puts the driver out-
puts into a high-impedance state when taken low. This al-
lows several driver outputs to be connected to an RS-422/
RS-485 bus.
Similarly, the device has a (active-low) Receiver Enable
pin (RE). Taking this low enables the receiver, while tak-
ing it high puts the receiver outputs into a high-impedance
state. This allows several receiver outputs to be connected
to a serial data bus.
The input impedance of the device 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
ADM1487 receivers, or a combination of ADM1487 and
other devices up to 32 unit loads, may be connected to an
RS-422/RS485 bus driven by a single driver.
LOW POWER OPERATION
The ADM1487 draws very little supply current (typically
80µA) when the driver outputs are disabled or unloaded.
This will increase by up to 72µA if the driver outputs are
enabled (but unloaded) and applying the maximum output
swing of 5V across the receiver input resistors.
However, in practical applications with the device driving
and RS-485/RS422 cable or bus, the current drain is
dominated by the current sourced into the line termination
resistors and any other devices on the bus. With a half-ter-
mination (120
current can be in excess of 13mA as the drivers force a
minimum of 1.5V across the line, and can be greater than
25mA with a fully terminated 60
Rev. PrC
R E
X
X
0
1
INPUTS
D E
1
1
0
0
load) between the driver outputs, the load
DI
X
X
1
0
PRELIMINARY TECHNICAL DATA
High-Z
line attached.
Z
0
1
Shutdown*
OUTPUTS
DEVICE TRUTH TABLES
High-Z
Y
1
0
–7–
SHUTDOWN MODE
If RE is taken high while DE is taken low, the device will
enter a low-power shutdown mode, in which the supply
current is typically 1µA.
If the driver outputs are active immediately before shut-
down (DE high), the supply current will not drop to 1µ
until the driver outputs have settled to their three-state
mode, which can take up to 2.6µs worst-case. If DE is al-
ready low and shutdown is entered by taking RE high, the
supply current will drop to 1µA very quickly.
When RE is taken low and/or DE is takem high, the de-
vice will wake up within 3.5µs.
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.
RECEIVER INPUTS
The ADM1487 receiver has an input common-mode
range covering the entire RS-485 range of –7V to 12V.
Internal 96k
ground provide a 1/8 unit load to the RS485 bus.
Differential signals greater than ±200mV within the speci-
fied input common-mode range are converted to a TTL-
compatible logic signal at the receiver output. A small
amount of input hysteresis is included to minimize the ef-
fects of noise on the line signals. If the line is terminated
or the receiver inputs are shorted together, the receiver
output will retain the last valid line signal due to the
45mV of hysteresis incorporated in the receiver circuit. If
the receiver inputs are left floating (unterminated), an in-
ternal pull-up of 10µA at the A input will force the re-
ceiver output to a guranteed high output state.
DRIVER OUTPUT STAGE
The RS-485 specification requires that a transceiver with-
stand common-mode voltages of up to 12V above or –7V
below system ground at the RS-485 line connections.
In addition, the transceiver must be protected against both
ESD and latch-up. This precludes the use of conventional
RECEIVING
*In shutdown mode, driver and receiver outputs high impedance
R E
0
0
0
1
input resistors from each line terminal to
INPUTS
D E
X
X
X
0
Open/shorted
A-B
-0.2V
0.2V
X
ADM1487
Shutdown*
OUTPUT
R O
1
0
1