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Old August 5th, 2008   #4
Task Group Member
Join Date: Aug 2008
Posts: 373

In your initial post, you described the problem at 5v. Are you seeing the problem with with 5v line drivers, or only under common mode conditions?

One option might be to build a separate "termination power" supply on your design. This would prevent line conditions from coupling back into your logic supply.

Under high common mode situations, the bias and termination resistors will dissipate a non-trivial amount of power. Certainly enough to damage 1206 and smaller resistors.

The line biasing network is pretty tightly constrained. It has to match the 120ohm impedance of the line, but still operate across the full common mode range. In an idea world, you wouldn't need it (485 drivers are supposed to maintain state when there's no input), but real-word tests have shown it's required.
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