RS-422 interface to a microcontroller.
The interface chip is a MAX3081.
A 100 ohm resistor at each receiver
provides a termination that absorbs
voltage reflections when the outputs
switch. The termination resistor
should match the cable’s characteristic
Unlike other RS-422 chips which
can handle bit rates as fast as 10
Mbps, the MAX3081 has a limited
slew rate that reduces the maximum
bit rate to 115,200 bps. An
advantage to the chip is that cables
of 220 feet or less can eliminate the
terminating resistors. The cable can
be as long as 4,000 feet at bit rates
up to around 90 kbps. At 115,200
bps, the maximum cable length drops
to around 3,000 feet.
On a PC, a quick way to obtain
an RS-422 port is to attach a USB/
RS-422 adapter. An example is
USBGear’s USB-COMi-M, available
from Saelig Company. Other options
are RS-422 expansion cards and
To use hardware flow control
with RS-422, add another MAX3081
or similar driver/receiver pair and a
twisted pair for each. RS-485 is a
similar interface that also supports
serial networks that share a single
data path. The MAX3081 is
compatible with both RS-422 and
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Maxim Integrated Products
RS-232 and RS-485 interface chips
microEngineering Labs, Inc.
Saelig Company, Inc.
Contact the Author
Jan Axelson is the author of
Serial Port Complete, now in its
second edition. Jan’s website is
May 2008 71