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APPENDIX T – ADDITIONAL RS-485 CONNECTION INFORMATION
Cable type:
Bidirectional communication possible over a single pair of twisted cables (AWG 16-26).
EOL resistor sizing criteria:
A standard RS-485 node has a rated input impedance of 12Kohms. A standard RS-485 transmitter can't drive more
than 32 devices and one pair of 120 ohm termination resistors.
Termination:
In a transmission line, there are two wires, one to carry the currents from the driver to the receiver and another to
provide the return path back to the driver. RS-485 links are a little more complicated because of the fact that they have
two signal wires that share a termination as well as a ground return path. However, the basic principles of transmission
lines are the same.
For reliable RS-485 communications, it is essential that the reflections in the transmission line be kept as small as
possible. This can only be done by proper cable termination.
Reflections happen very quickly during and just after signal transitions. On a long line, the reflections are more likely
to continue long enough to cause the receiver to misread logic levels. On short lines, the reflections occur much sooner
and have no effect on the received logic levels.
In RS-485 applications require termination at the master node and the slave node furthest from the master.
Maximum run distances
When high data rates are used, the application is limited to a shorter cable. It is possible to use longer cables when low
data rates are used. The dc resistance of the cable limits the length of the cable for low data rate applications by
increasing the noise margin as the voltage drop in the cable increases. The ac effects of the cable limit the quality of
the signal and limit the cable length to short distances when high data rates are used.
The maximum distance of RS-485 without using a repeater is 4000 feet (1220 meters) at baud rates up to 90Kbps.
Examples of data rate and cable length combinations vary from 90 kbps at 4000 feet to 10 Mbps at 15 feet.
RemotEye 4 User Manual – 90988-007
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