VersaTRAK RTU User M anual
Page 27 of 31
Last Revised: 05/23/02
SIXNET
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Termination
Resistor
The RS485 User 1 port has a built-in Receive Data line termination resistor. This resistor is
enabled by installing a jumper between the “ T1” and “ T2” terminals. Here are guidelines for
termination:
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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 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
(VT-M2-224-xxP)
On an RS485 2 wire network there should be one pair of bias resistors acting upon the
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
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 provided in the VT-M2-224-xxP and are permanently enabled. If any other
RS485 devices on the network have bias resistors, they should be disabled or removed from
the circuit. They are often enabled or disabled through DIP switch or jumper settings.
