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-   -   NACK Code to use for Subdevice that has no data for requested PID? (http://www.rdmprotocol.org/forums/showthread.php?t=1247)

prwatE120 February 24th, 2016 05:23 PM

NACK Code to use for Subdevice that has no data for requested PID?
A further complication in the use of NACK reason codes applies to the case whereby not all subdevices can meaningfully reply to all of the PIDS listed as "supported" by subdevices.

For example, I have multiple subdevices, only some of which can support CURVE. But as stated in the standard, there is only one list of supported PIDs for subdevices. (essentially the superset of all PIDS supported by all subdevices).

So it follows that a message such as GET:CURVE sent to Subdevice 001 could respond with NACK: Unknown PID and the same message sent to SubDevice 002 could respond with the CURVE detail.

I cannot use the NACK : ACTION NOT SUPPORTED from E1.37-2 since it specifically refers to use with SET commands.

Is there another or better NACK reason code to use ?

So for a controller to build a complete picture of which messages can be sent to which subdevices requires trial and error of each PID with each Subdevice.

Which is likely to be slower than fetching a list of suported PIDS for each subdevice and allowing them to be different.

ericthegeek February 24th, 2016 09:26 PM

Asymmetrical Sub-devices have long been a grey area. The problem you're raising is similar to when you have a dimmer rack with both dimmer and relay modules installed. There are PIDs that make sense for some modules, but not for other modules.

I think the best you can do is declare the superset of all PIDs in the Supported Parameters list, and when an inapplicable PID is sent, NACK it with Unknown PID.

Sub optimal to be sure, but the best you can do with the current doc. In a future rev, perhaps allow each sub-device to declare a different list of PIDs?

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