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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 October 20th, 2006   #1
Andy Macdonald
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Join Date: Sep 2006
Location: London
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Default Line biasing


I'm sure you're busy at LDI, but when you're back in the office and have a moment, could you explain a little of the background to line biasing please. What is it used for/why does it need to be there? Is it just to maintain the marking state between packets as per section 2.4 or is there more going on?

Sections 2.3 and 2.4.1 of the spec says that you need to have it, and 2.6 says it "shall be enabled prior to RDM communication beginning", but I'm not sure what that means in practice. In particular:

- Should a piece of RDM distribution equipment (eg a DMX/RDM splitter or merger) put line bias on its output ports if it does not detect any on an input port?
- If a fixture/device receives a valid RDM packet, but does not detect line bias, should it obey the packet or ignore it?

The best reason I can come up with for having line bias is that it indicates to a fixture that the controller would receive any RDM response messages it send, and therefore would know that it exists as an RDM fixture. This depends on the behaviour of the fixture and any RDM distro equipment as per my questions though.

For example, if an RDM console was connected to an RDM fixture via a DMX only (non RDM) splitter, the fixture would receive any RDM messages the console sent, but the DMX splitter would not pass any responses back to the console. The console would not therefore be able to detect the fixture, so would not realise there was a fixture it did not know about. It might then send out a command aimed at the RDM fixtures it knew about, but which might be obeyed by the RDM fixture in the non RDM portion of the network. The fixture would not have been detected, so the console would not know its UID, but it could still be something a broadcast message like Set Lamp State where it would have an unintended effect on those fixtures. Maybe the issue of undetected fixtures obeying broadcast messages is just too insignificant a problem to worry about.


Andy Macdonald
Carallon Ltd
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Old October 26th, 2006   #2
Join Date: Feb 2006
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Sorry for the delay...but as you guessed I was at LDI!

The Line Biasing is just to maintain the marking state. There shouldn't be any other logic or decisions based around message handling placed around it.

The "shall be enabled prior to RDM communication beginning" is a fancy way of saying it should be enabled on power up. There are some implementations where for one reason or another the line biasing my be switchable or may not be active at the instant of powerup. The main issue is that it is enabled before you start turning transceivers around and doing RDM communication, so that is why we worded it that way.

If a device receives a valid packet, it should act on it regardless of what is going on with line bias. It was never our intention that any logic be placed around sensing line bias states..

For distribution equipment I would say it is always a good idea to have the line bias in place regardless of what is going on elsewhere in the network. If you are doing RDM communication then you should have line bias regardless of what others are doing.

Also keep in mind that there is a lot of existing products out there that will be adding RDM support, but may not fully have the proper line biasing network. Odds are decent that in most cases everything will still function, so you don't want to have gear disabling communication only because it didn't properly detect a line biasing.

Nothing wrong with reporting a Status Message Warning in RDM that proper line biasing wasn't detected though.

As far as the case with legacy DMX splitters, even with proper line biasing, you may have some non-RDM devices on the same link as RDM devices, so you couldn't make decisions about line bias detection there.

Scott M. Blair
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