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Receivers with a higher input impedance will have a fractional unit load value assigned,
since the circuit loading effect is correspondingly less.
To obtain optimal performance with long cable runs and high data rates, it is necessary to
carefully select the cable type that is to be used. A typical cable will have twisted pairs of
24awg. wires, be fully shielded, and have a nominal impedance of 120 ohms. Alpha Wire
Company type 5473C, or equivalent is suitable for most applications. Additional
recommendations for wiring practices are detailed in a later section of this document.
While it is beyond the scope of this document to elaborate on transmission line theory, it
is not beyond the scope of the field installed device to demand proper termination of the
signal line. Long cable runs and high data rates will require that a 120 ohm termination
resistor is at the end of the cable, near the last receiver. This prevents reflections from
occurring at the end of the cable, resulting in signal distortion. Referring again to the
CNI product, leisurely data transfer rates of 38.4k bps (or slower) do not require
as much concern about termination resistors. As a matter of practice, it is still a good idea
however, and the RS-485 board can have the 120 ohm termination added by
simply placing a jumper on JP1.
RS-485 differential signaling levels are specified to not exceed a ±6 volt swing for an
unloaded transmitter, and the receiver sensitivity must be at least ±0.200 volts. Since it is
possible to have multiple transmitters on a bus, the data transmission protocol must
ensure that 'line contention' events do not take place. Specifics of signaling protocols and
data transmission are not defined within the framework of the EIA-485 standard
however; only the physical interface is detailed.
If the preceding description appears to suggest that the installer must possess an
Engineering degree to understand and setup a system, then it must be admitted that this
feeling is at least partially justified. If the goal was to operate at maximum transfer rates
with lengthy cable runs, and numerous nodes, then it is true that the installation is likely
to be a bit of a challenge. Fortunately enough, most applications for the CNI /
RS-485 combination are relatively straightforward, and the following guidelines are
intended to make the process as simple as possible.
RS-485 Board Installation:
If the RS-485 adaptor board is not already installed into the enclosure, then Figures 1 & 2
should be referenced for assembly. Two small #4 size self-threading type screws and a
cable assembly will have been included with each kit.
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