Re: DMX optoisolator

E. Lee Dickinson wrote:
Differential, according to your post, requires a ground reference. 

Seems like I was wrong talking about "ground reference" - see below.

But I fail to see why it's needed. Can't the inverting and non-inverting signals reference off eachother? That's what they do in, for repeating example, Krystof Nys's design. Data+ and Data- drive the input optoisolator, and ground appears unconnected.

Ok, I took a look at the design, and AFAIK it is bad practice just to use two LEDs (one seperate, one in the optocoupler) instead of a proper line driver.

What's the point of ground in a differential signal?

From some googling:
- (Line Driver) Although proper connection to the signal ground is important, it isn't used by a balanced line receiver in determining the logic state of the data line.
- (Line Receiver) It will also have a signal ground (C) that is necessary in making the proper interface connection.
- The signal ground line is also recommended in an RS-485 system to keep the common mode voltage that the receiver must accept within the -7 to +12 volt range
- Note that the signal ground line should also be connected in the system. This connection is necessary to keep the Vcm common mode voltage at the receiver within a safe range. The interface circuit may operate without the signal ground connection, but may sacrifice reliability and noise immunity.

So, from what I read from http://www.bb-elec.com/tech_articles/rs422_485_app_note/table_of_contents.asp and many other sources, it is considered good engineering to connect a ground line. And (at least with DMX) it is also considered good engineering not to connect pin 1 with case because otherwise someone might get zapped by common mode voltage - that (according to some designs i just saw) normally should be connected to Device grounding via some resistor...

bye
Christian
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