VersaTRAK RTU User M anual
Page 25 of 31
Last Revised: 05/23/02
SIXNET
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Box 767
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Clifton Park, NY 12065 USA
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+1 (518) 877-5173
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FAX +1 (518) 877-8346
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Termination
Resistor
The RS422/RS485 main port has a built-in Receive Data line termination resistor. This
resistor is software enabled from within the VersaTRAK or Mini-VersaTRAK Main Port
Setup window of the SIXNET I/O Tool Kit. Here are guidelines for termination:
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Termination should be enabled only at both ends of an RS422 communication
cable.
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Termination should be enabled only at the two end stations on an RS485 network.
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No more than two stations should be terminated on an RS485 network.
Termination components are usually built into other RS422/RS485 devices, and are
typically enabled by setting a jumper or DIP switch on the device. If these components are
not populated in the unit, then add them to the RS485 wiring as shown below. The
components consist of a 150 Ohm resistor and a 0.1 uF capacitor connected in series.
Bias
Resistors
On an RS485 2 or 4 wire network there should be one pair of bias resistors acting upon the
receive or transmit/receive wires. If bias resistors are not present, the receive inputs on some
RS485 devices may react to noise on the floating wires. The bias resistors will force the
receive or transmit/receive wires to a known (non-floating) state when none of the RS485
devices are transmitting data. There should be only one pair of these resistors on the
network.
Bias resistors are typically installed at the host RS485 device. Some host devices have bias
resistors built-in, and are enabled through DIP switch or jumper settings. If the host does not
provide bias resistors, they can be installed at any one of the VersaTRAK RS422 RTUs.
Note:
If your RS485 network is made up exclusively of VersaTRAK and Mini-
VersaTRAK RTUs, these bias resistors are not necessary.
