Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Please help again -- more DMX trouble!
References:  <[email protected]> 
Tim Mitchell  writes:

> In article <[email protected]>, Frank Wood
>  writes
> >>The shield is connected to SIGNAL GOUND *not* MAINS GROUND.
> >>There is a difference. The screen is the reference as I've said before.
> >
> >In a proper installation, there is no such thing as signal ground. Signals are
> >sent around balanced, with no ground reference.
> 
> No no no, that might be true in a balanced line audio system, but DMX
> is NOT a balanced line system. 

DMX512 uses EIA-485 which is a balanced system.

It carries the characteristics of balanced system:
- information is transmitted differentially to to wires
- information is received as voltage difference between wires
- system is impedance balanced (same impedance from both wires to ground)

There is a limitation of allowed common voltage on the signal
wires (something like +-7 volts). 

The RS485 transmitter which is always on (like in DMX-512 system 
the transmitter it) can be very much compared to an electronic 
balanced output found in audio mixers (now juts optimzed for 
high speed digital signals instead of analogue audio).

The typical RS485 receiver can be compared quite well to 
an electronic balanced input (without transformer isolation) 
as found in most modern mixers. It amplifies the difference 
between those two wires to signal which is used inside mixer 
(usually unbalanced line level signals in cheap mixers, 
TTL level digital signal in typical DMX512 equipment).
Both the analogue mixer electronic input and RS485 digital 
receiver has a limited common mode voltage range they have.

An optically isolated RS485 input circuit would have 
pretty much similar performance characteristics as a 
transformer isolated balanced input in audio world would 
have.

> It's a differential voltage system which is not the same thing at all. 

Differential voltage system is one part of the balanced system 
characteristics, but the RS-485 had also othber parts in it.

Differential signaling and impedance balance makes a balanced system...

> And the cable screen is signal
> ground in a DMX system. The two signal conductors carry +5V or 0V, and
> the reference point is pin 1.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/