Termination Resistance - CommTech FASTCOM SuperFASTCOM Hardware Reference Manual

Four channel high-speed synchronous serial adapter for pci bus

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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 SuperFastcom. The

termination resistance is built in. We have installed a terminator resistor for each receiver:

between each RD+ and RD- and between CTS+ and CTS- for each channel.

If you are using the SuperFastcom 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 SuperFastcom

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

SuperFastcom:

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