User's Manual

October 15 2009 – Installation Guide Draft – Edit Purposes Only Page 29
Installation Guide –April 2010
2010
RS-485 communications. The cable connection includes a differential line (+ and -) and
a GND connection.
Table 2-2 Category 5 Cable Characteristics
Specification Recommendation
Capacitance (conductor to conductor) <20 pF/ft.
Characteristic Impedance 100 - 120 ohms
Nominal DC resistance <100 ohms/1000 ft.
Wire gauge 24 AWG stranded
Conductors/Shielding >2 pair (shielding is recommended)
2.1.6.2.2 RS-485 CABLE LENGTHS
As outlined in the RS-485 specification, the total length of the communication cable
(adding up all of the segments of the run) should not exceed 1200 meters (4000 feet).
Although the RS-485 specification calls for a maximum cable length of 1200 meters and
provides a maximum baud rate well above that of the 4G device, a more conservative
system should be configured to no more than 1000 meters and run at a baud rate of
9600 bits per second. After the network is configured and is running in a stable manner,
the baud rate can be increased if faster network communications are desired.
Drops (down-leads, stubs, T-connections, etc.) to equipment are not recommended, but
if required, should not exceed one foot) and should use the same cable recommended
above. On a long stub, a signal that travels down the wire reflects to the main line after
hitting the input impedance of the device at the end. This impedance is high compared
with that of the cable and the net
effect is degradation of signal quality on the bus.
2.1.6.2.3 RS-485 NETWORK TOPOLOGY
Communication cables for RS-485 should be laid out in a daisy chain configuration (See
Figure 2-3 below). Long stubs or drop downs and the star configuration should be
avoided because they create discontinuities and degrade signal quality. The star
configuration usually does not provide a clean signaling environment even if the cable
runs are all of equal length. The star configuration also presents a termination problem,
because terminating every endpoint overloads the driver. Terminating only two
endpoints solves the loading problem, but creates transmission line problems at the
unterminated ends. A true daisy chain configuration avoids these problems.