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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 April 5th, 2008   #1
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Default AC coupled termination

I'm updating some of our hardware and figured it's time to add more support.

A draft copy of DMX512-A that I have suggests using AC coupled termination (0.047uF in series with 120 ohm). That draft also has an RDM section and which only shows the normal 3 resistor termination/bias.

The final specification only says the termination should be 120 ohm impedance which would allow the capacitor but the RDM termination was removed from the specification.

Is it ok to use the capacitor coupled termination and use the standard bias resistors (about 560 ohm pull up and down) as with other RS-485 systems or does RDM really insist on the three resistor version with the specified values?

I also noticed a timing error in one of charts in the DMX spec., but that doesn't apply here. I did report the error, but it's a bit late since it's been published.
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