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Old February 23rd, 2015   #2
ericthegeek
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Join Date: Aug 2008
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The pull-apart resistor values were chosen solely to maintain the minimum differential voltage with an idle bus (no active line drivers) and 32 unit loads.

Indeed, E1.20 imposes driver current handling requirements that go beyond that required by 485. A line driver that can only handle the drive current given in 485 will not qualify for RDM. To address this, the standard provides a number of tests to establish if your chosen line driver will work (Refer to E1.20-2010 Appendix F).

Fortunately, it's been our experience that most line drivers do meet the requirements and can handle the extra current. If you need to conduct the tests on your chosen line driver, Goddard Design sells a test jig for that purpose, or you can build your own.

http://goddarddesign.com/driver_gts.html

(In the interest of full disclosure: I have worked for Goddard design)
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