Specifications

Communications 3-6
RS-485
RS-485 is a recently developed communications standard to satisfy the
need for multidropped systems that can communicate at high data rates
over long distances. RS-485 is similar to RS-422 in that it uses a balanced
differential pair of wires switching from 0 to 5V to communicate data. RS-485
receivers can handle common mode voltages from -7V to +12V without loss
of data, making them ideal for transmission over great distances. RS-485
differs from RS-422 by using one balanced pair of wires for both transmitting
and receiving. Since an RS-485 system cannot transmit and receive at the
same time it is inherently a half-duplex system. RS-485 offers many
advantages over RS-232C:
1) balanced line gives excellent noise immunity
2) can communicate with D1000 modules at 115200 baud
3) communications distances up to 4,000 feet.
4) true multidrop; modules are connected in parallel
5) can disconnect modules without losing communications
6) up to 32 modules on one line; 122 with repeaters
7) no communications delay due to multiple modules
8) simplified wiring using standard telephone cable
RS-485 does have disadvantages. Very few computers or terminals have
built-in support for this new standard. Interface boards are available for the
IBM PC and compatibles and other RS-485 equipment will become avail-
able as the standard gains popularity. An RS-485 system usually requires
an interface.
We offer the A1000 and A2000 interface converters that will convert RS-232
signals to RS-485 or repeat RS-485 signals. The A1000 converters also
include a +24Vdc, one amp power supply for powering D1000 series
modules. The A1000 or A2000 connected as an RS-485 repeater can be
used to extend an existing RS-485 network or connect up to 122 modules
on one serial communications port.