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Old August 5th, 2008   #6
Senior Member
Join Date: Jan 2008
Posts: 102

Hi Eric,

I did say 5V originally and I have seen the problem with a 5V external driver driving into my 3V3 master when the master power is dropped suddely during communication. The 5V is then able to couple from the data line through the pullup into my power and the circuit does not die off properly and interesting things happen at that moment.

As for the common mode and 12V issues, we have to cater for all conditions possible and still remain reliable.
The unit I am designing only has a 3V3 power supply and I can't add anything more because of other constraints.

I don't think this pullup method is the best solution. It seems more like an afterthought.

Kind regards

Originally Posted by ericthegeek
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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