Re: DMX optoisolator

On Sun, 28 Aug 2005 08:11:10 +0100, E. Lee Dickinson <lee@xxxxxxxxxxxxxxxxxxxxx> wrote:


"Frank Wood" <frankwood95@xxxxxxx> wrote:

Differential signals need something to be different from. Think of a
centre-tapped transformer. Leave the centre-tap alone, and you have a
balanced output: ground it, amd you have a differential output.



I can see the sin waves perfeclty in my head. Thanks for the transformer
bit, Frank.

I understand balanced signals discriminating against common mode noise.
Ballanced makes perfect sense to me. What I don't understand is what good a
differential signal is.

In DMX, we've got Data+, which (as you guys are teaching me), references
ground. We've got Data-, which also references ground. Why do we need both
of them, if they're not going to be subtracted from eachother (as they would
in a balanced line)?




They are subtracted from each other :-) Which is why it's fairly good at rejecting noise. If you want to get your head round this then you need to read the TI datasheets for RS485 control signals, it's not specifically a DMX issue. Go to TIs website for more guff
http://www-s.ti.com/sc/psheets/slla036b/slla036b.pdf
.

Quantcast