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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 August 4th, 2008   #1
berntd
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Join Date: Jan 2008
Posts: 102
Default Line biasing Flaws - need solutions

Hello,

The line biasing method in the RDM spec is flawed in my opinion.
The reason is that the voltages on the DMX lines are allowed to rise way further than the power supply that biases the lines.

When something drives the line to a higher level, current starts flowing back onto the power supply of the master and depending on th capabilities of the driver, this may cause the supply to rise above its VCC level and damage can occur to the electronics on that line.

I am currently using a 3V3 low power system system and I just noticed that the data resoponses from a responder are quite capable of driving up the voltage on the Vcc towards 5V. this will result in damage.

In fact, data on the line (not normal operation) is even able to power the master through the biasing network, resulting in complete malfunction.

I am now scratching my head on how to solve this problem and still operate within the specification.

I am considering adding a diode into the biasing network. tjhis will upset the impedance quite badly when the diode become reverse biased.

Another option is to clamp the supply rail to 3.3V but there seem no Tranzorbs on the market for this voltage. Maybe a zener but that is not great either.

The diode may well be the only answer but I would love to hear other suggesstions.

Perhaps I am missing something?

Kind regards
Bernt
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