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RDM Physical Layer/Hardware Discussion Discussion and questions relating to the physical or hardware layer of RDM.

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Old February 18th, 2015   #1
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Default RDM Line Bias - bus loading ambiguous

ANSI E1-20 2010 specifies line bias network for RDM. It indicates that the network will maintain bias when link is loaded with equivalent of 32 UL. Per Ti RS485 design guidelines http://www.ti.com/lit/an/slla272b/slla272b.pdf the bias network impacts the maximum number of unit loads. The recommended bias network reduces the maximum number of loads to 12. This is not clear in the standard and may imply that 32 ULs can be connected to the bus.

Fortunately many of today's transceivers are 1/4 or even 1/8 unit loads. With an 1/8 UL, up to 96 devices can be connected to the bus.

Perhaps this section of the RDM standard should be updated.
markktoo is offline   Reply With Quote
Old February 23rd, 2015   #2
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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.


(In the interest of full disclosure: I have worked for Goddard design)
ericthegeek is offline   Reply With Quote


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