3
Introduction:
Most persons that work with computers and/or communications equipment are at least
somewhat familiar with the terms RS-232, USB, Fire-wire, and LAN to name a few.
RS-485 is less commonly known however, but serves well in applications where high
data rates, excellent noise immunity, and long cable runs are necessary. As is often the
case, there are both advantages and downsides to RS-485 when compared against other
systems. It is the purpose of this manual to help develop a basic understanding of the
RS-485 data transfer system, as well as providing specific details related to installation of
the RS-485 adaptor board.
Overview of RS-232:
RS-232 (also known as V.24) is a relatively aged serial data transfer topology (1969), but
still finds wide acceptance due to the large installed base, simplicity, and proven
performance. Initially introduced with a 25-pin D-Sub style connector, the 9-pin variety
is far more common at the present. Serial data is transferred over TXD and RXD lines
with respect to a signal common. Transition levels are specified at a maximum of +15
volts to a minimum of -15 volts, although in practice it is more common to find swings in
the range of approxi8 to -8 volt levels. Data is transferred as a series of high and
low signal levels, using a serial data transfer protocol with start bits, data bits, stop bits,
and parity. A typical setting might be 1 start bit, 8 data bits, 1 stop bit, and no parity to
transfer a single byte (8-bits) of useful information.
Maximum practical data transfer rate is 19.2k bits per second (bps), with cable lengths at
50 feet, and several times faster with shorter cable runs. It is only possible to have a
single pair of devices connected; multi-drop capability does not exist. Immunity to
electrical noise is rather poor when operating in harsh environments such as factory floor
sites, traffic light controllers, refineries, etc.
Overview of RS-485:
As previously alluded to, RS-232 has some definite limitations for applications requiring
longer cable runs and higher data transfer rates. RS-485 (EIA-485 standard) uses
balanced transmission lines (differential signaling) to transfer serial data bytes. This
method has inherently higher noise immunity, thereby permitting high data rates and long
cable runs. Theoretical data transfer rates are on the order of 10M bps and 1200m
(approx. 4000 ft) cable length, although it is often necessary to scale back the speed at
such extreme lengths.
In the case of the RS-485 adaptor board coupled with the CNI, the limiting factor is the
CNI board itself, which only supports data transfers up to 38,400 bps. Taking into
consideration the intended applications for the CNI product, higher data transfer rates are
not relevant, and it is the potential long cable runs that are beneficial.
RS-485 specifications state that ‘multi-drops’ are supported for additional transmitters and
receivers sharing the same cable. Receiver impedance is specified to be 12k ohms for a
single unit load. Given this impedance, up to 32 unit loads can be supported on the bus.