14
Fastcom
®
: 422/4-PCI-335
Termination Resistance
In both the RS-422 and the RS-485 mode, the receiver end of the cable between two stations must be terminated
with a resistor equal to the characteristic impedance of the wire. This is to prevent signal reflections in the wire
and to improve noise rejection. However,
you do not need to add a terminator resistor to your cables when
you use the Fastcom: 422/4-PCI-335. The termination resistance is built in.
We have installed a terminator
resistor for each receiver: between RX+ and RX- and between CTS+ and CTS- for each channel.
If you are using the Fastcom: 422/4-PCI-335 in a multi-drop network, the termination resistor should be removed
from all units except the first and last (see the RS-485 illustration below). Call for technical support if you need to
modify the resistor. You may also order the Fastcom: 422/4-PCI-335 without the termination resistor installed (it
is easier to add the resistor than to remove it). Observe the resistors in the following drawings and remember that
they are built into the Fastcom: 422/4-PCI-335 (shown below):
Typical RS-422 Installation
Typical RS-485 Installation
1
2
TX
R1 & R2 - Line Termination (100 ohms)
TX+
TX+
TX-
TX-
TX
RX
RX+
RX+
R2
RX-
RX-
RX
R1
1
2
4
3
TX
TX
R1 & R2 - Line Termination (100 ohms)
TX+
TX+
TX+
TX+
TX-
TX-
TX-
TX-
TX
TX
RX
RX
RX+
RX+
RX+
RX+
R2
RX-
RX-
RX-
RX-
RX
RX
R1
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