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RDM General Implementation Discussion General Discussion and questions relating to implementing RDM in a product.

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Old November 19th, 2010   #1
DavidR
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Default RDM & DMX frames multiplexing

Hi All,

I'm new in this forum and although I have browsed the FAC, I didn't find an answer to my question.

RDM specification mention a lot of timing values for RDM frames, but doesn't really explain how to multiplex DRM & DMX frames.

Suppose the controller is sending DMX (typically during a crossfade) at a rate of 25 (up to ~48 according to DMX512A specs) cycles/second. How the controller shall insert RDM frames and manage reception of anwsers (and possibly timeouts)?

In other words I'm wondering if there is some gidelines or whitepaper about the rules on this matter?

Thanks,

David
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Old November 19th, 2010   #2
sblair
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David,

Good question! Section 3.1.2 specifies the timing values for packet spacing between Null Start Code frames and RDM frames as well as RDM Discovery frames. These timings are the only requirements in how you handle interleaving between the packets.

As long as you follow these requirements you are then free to interleave in whatever makes the most sense for your product.

In general, a number of products will do a 1-1 interleave of DMX NSC, RDM Request/RDM Response, DMX NSC. With that type of 1-1 interleave you can still maintain about 26HZ refresh rate of the NSC packets.

There are many tricks and optimizations you can do based on your application and need. For example, if you are in the middle of doing an active crossfade, you may choose to reduce or eliminate the frequency you send RDM messages in the background in order to maintain a higher refresh rate during the crossfade.

At the same time, if you are doing something more intensive with RDM and your DMX values aren't changing then you can dramatically throttle back the number of DMX NSC packets you are sending and mostly just send RDM packets. Your only real requirement is to send an NSC packet at least 1/second to keep the signal alive so the devices don't go into a "loss of signal" scenario. The rest of the messages could all be RDM traffic at that point.

Hope this helps shine some light on it for you.

Scott
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Old November 20th, 2010   #3
DavidR
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Hi Scott,

thank you for this quick answer, this is exactly what I expected.

Best Regards,

David
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