Ametek CEM/Humox, User Manual

Page: 41 / 136

Installation
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3-21
4-Wire connections
The connections used for the RS-485 4-wire communications on the wiring card
are labeled as follows:
4WTX+/2W+
4WTX-/2W-
4WRX+
4WRX-
Connect all control unit 4WTX+, 4WTX-, and all 4WRX+ and 4WRX- connections
in parallel in a daisy chain fashion. Make the following connections between the
host computer and the controller in a daisy chain fashion (see Figure 3-18):
Host Computer TX- to Controller 4WRX-
Host Computer TX+ to Controller 4WRX+
Host Computer RX- to Controller 4WTX-
Host Computer RX+ to Controller 4WTX+
Be sure to use twisted-pair cable for all connections.
4-Wire termination resistor
The Series 2000 Control Unit is equipped with termination resistors that can be used
for the last control unit on the network. The switch settings are:
SW1-1 = ON
SW1-2 = ON
SW1-3 = ON
This provides 120 ohm termination resistors in the RS-485 circuit.
See the “Option Card Installation” section at the end of this Installa -
tion chapter for help on how to remove and access the display module.
You may need to experiment to get optimum performance over long lengths of
communications wiring to see if having a termination resistor on the last control
unit of the network improves communications. Remember, though, only to insert
a termination resistor on the last control unit on the network, and not on all control
units on the network. The factory default setting is to have Switch 3 OFF with no
termination resistor in the circuit.
NOTE
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