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

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Old January 21st, 2015   #1
RobbG
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Default RDM Implementation Requirements

Hi there,

I'm in the process of designing a new implementation of RDM responder functionality but there's one thing that I can't seem to find anywhere. What's the maximum possible length of any single RDM packet going to be? There doesn't seem to be much mention of it in the standard and I'm aware that there may not be one set but having to compensate for the possibility of receiving Megabytes long packets over RDM is quite difficult on an embedded platform.

Thanks in advance,

// Robb
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Old February 18th, 2015   #2
ericthegeek
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257 bytes.

The maximum packet size is determined by the "Message Length" field that is an 8 bit value between 24 and 255. This length includes the Startcode, but does not include the checksum.

Adding a 2-byte checksum to the 255 maximum for the "Message Length" field gives you 257 bytes. You will never see a megabyte long packet.

See E1.20-2010 section 6.2.3 for details.

Any individual packet will be 257 bytes max, but responders can generate an arbitrary number of ACK-Overflow responses to return data of any size by breaking it into smaller chunks. Due to practical constraints, responders should limit ACK-Overflow sequences to a few packets, I typically say 5 or 6 maximum. That's because any error in the ACK-Overflow sequence (lost or corrupt packet) effectively requires the controller to start over from the beginning.

From the responder's point of view, it only has to implement ACK-Overflow if it needs it. Most responders don't need to use the feature.

Controllers on the other hand need to aware that responders may generate long ACK-Overflow sequences.
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