Each RS485 interface has specific connection and
termination biasing requirements which will be
detailed in their installation instructions. The general
principals are as follows.
Terminations Because each wire is a transmission line,
it must be properly terminated to prevent reflections.
Where multiple instruments are daisy-chained
together, a 120 ohm terminating resistor should be
fitted at the connection to the PC and to the last
instrument in the chain.
Bias Resistors When transmission lines are not
transmitting, they remain in an indeterminate state
which can allow receivers to receive invalid data bits
due to electrical noise on the cable. To prevent this,
the lines should be forced into a known state by
fitting two 620 ohm bias resistors to one point
(node).
If an RS485 interface card is being fitted to the PC,
separate bias resistors may not be needed because
they may already be fitted to the card. Check the
manufacturers specification.
As mentioned earlier RS485 cards and converters
differ greatly in their requirements and therefore the
installation instructions supplied with the interface
should be read carefully.
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