Ametek Thermox 2000 ATEX Series User Manual page 64

3-40 | Thermox Series 2000 WDG-HPII / HPIIC - ATEX
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- con-
nections in parallel in a daisy chain fashion. Make the following connec-
tions between the host computer and the controller in a daisy chain fash-
ion (see Figure 3-23):
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.
NOTE
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 net-
work, 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.