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In RS-485 communications, the choice of cables and components can mean the
difference between systems that communicate flawlessly every time and systems
that fail either immediately or at seemingly random and thus unpredictable
times.

The following guidelines will help in designing RS-485 circuits that work with-
out problems:

1. Use the slowest drivers possible for the bit rate.


2. Terminate long lines with their characteristic impedance.


3. Wire the nodes in a bus topology.


4. Bias inactive links.


5. Use twisted-pair cable.


6. Limit common-mode voltages.

This chapter explains the reasons behind the guidelines and shows how to fol-
low the guidelines in your designs.