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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.
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