RDM discovery trouble - E1.20 RDM (Remote Device Management) Protocol Forums

ericthegeek · October 4th, 2010

It sounds like you are running into the phantom device problem. There are several things that can cause this to occur:

Responder Firmware Errors:
Make sure that your firmware is correctly handling the upper and lower bounds for the DISC_UNIQUE_BRANCH message. An off-by-one error where your device responds when it shouldn't (or doesn't respond when it should) can appear as a phantom device. A common mistake is to use less-than/greater than comparisons rather than less-than-equal-to/greater-than-equal-to.

I don't think this is what's happening to you. Typically this results in a phantom device with the last digit being one more or one less than the real responder's UID.

Collisions During the Discovery Response:
During the 5.8ms discovery response period, it's common to get collisions on the DMX wire when multiple responders reply simultaneously. Sometimes these collisions can appear to have a valid checksum. A properly written controller will attempt to mute the phantom device, get no response, and continue further down the binary tree.

However, some RDM controllers do this wrong, and add the phantom device to the Table of Devices even if the device does not respond to the DISC_MUTE message. This is a common bug in RDM controller software, and I suspect this is what is happening to you. You'll need to contact them and ask how their discovery routine works. Better RDM controllers will not have this problem.

I have not used an "ELC checker" or the "TBox RDM" so I can't tell you how they behave. To know for sure what's happening you'll need an RDM packet analyzer.

October 4th, 2010	#2
ericthegeek Task Group Member Join Date: Aug 2008 Posts: 390	It sounds like you are running into the phantom device problem. There are several things that can cause this to occur: Responder Firmware Errors: Make sure that your firmware is correctly handling the upper and lower bounds for the DISC_UNIQUE_BRANCH message. An off-by-one error where your device responds when it shouldn't (or doesn't respond when it should) can appear as a phantom device. A common mistake is to use less-than/greater than comparisons rather than less-than-equal-to/greater-than-equal-to. I don't think this is what's happening to you. Typically this results in a phantom device with the last digit being one more or one less than the real responder's UID. Collisions During the Discovery Response: During the 5.8ms discovery response period, it's common to get collisions on the DMX wire when multiple responders reply simultaneously. Sometimes these collisions can appear to have a valid checksum. A properly written controller will attempt to mute the phantom device, get no response, and continue further down the binary tree. However, some RDM controllers do this wrong, and add the phantom device to the Table of Devices even if the device does not respond to the DISC_MUTE message. This is a common bug in RDM controller software, and I suspect this is what is happening to you. You'll need to contact them and ask how their discovery routine works. Better RDM controllers will not have this problem. I have not used an "ELC checker" or the "TBox RDM" so I can't tell you how they behave. To know for sure what's happening you'll need an RDM packet analyzer.