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Old March 24th, 2015   #2
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Join Date: Aug 2008
Posts: 369

Because I don't know the details of that specific RDM controller it's hard to say for sure. What is the frequency of the high-low sequence you are seeing, and what are the voltage levels?

Most RDM controllers will drive the 485 line all of the time, except when they are expecting a response from a responder. But, it's possible that the controller is going into some kind of power saving mode where it disables the line driver when it's not actively transmitting.

For example, the following would be valid behavior (especially for a battery powered device):
1: Enable the line driver for TX
2: Send a discovery request
3: Enable the line driver for RX
(Nothing responds to the discovery request)
4: Disable the line driver
5: Sleep for some period of time
6: Repeat

To an oscilloscope, this might look like a relatively low frequency square wave.

This is just one hypothesis, there are many other possibilities.

That being said, I looked at the Euchips RDM-100 and I question how well implemented it is. It uses a 3-pin XLR connector, which is explicitly dis-allowed by E1.11 (on which RDM is based). If the developer didn't pay attention to the most basic physical requirements of the standard, what other critical elements were ignored?
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