Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: length of DMX 512 cable
References: <[email protected]>    <[email protected]>
Ben  writes:

> Cat 5 is the wrong impedance and isn't shielded.  In a noisy
> environment that'll be a problem.  But if you don't push the envelope
> it'll work. Regular mic cable is the wrong impedance, but at least
> it's shielded.

CAT5 cable is 100 ohms twisted pair cable. 
It can be unshielded (UTP) or shielded (STP, FTP).

Typical DMX512 systems lines are terminated to 120 ohm 
impedance and tha cable used has been something with  
impedance soewhat close to 120 ohms. 
DMX512 system uses RS-485 electrical communication 
interface, that works well on both 120 ohms and 100 ohms cable.

Basically RS-485 should work on good environment with 
unshielded cable, nut on noisy environment a shielded cable 
or cable intealled to grounded metal conduit is recommended.


Refences:

DMX512A public review say:

"A data link shall consist of a single active differential line driver,
a terminated transmission line and one or more differential line
receivers. The transmission line shall be a data cable with a nominal
characteristic impedance of 100 to 120 ohms."

"Cabling systems shall provide a balanced, nominal 120 ohm terminated
transmission line, and be made of
cables with a characteristic impedance in the range 100-120 ohms."

"Note: Mixing cables of different impedances and other characteristics
not isolated by buffers or other
processing devices may affect system reliability."

"Cable for general application shall be shielded twisted pair approved
by its manufacturer for EIA-422/EIA-485-A use"

"Limited use of 4-twisted pair cabling systems as described in ISO/IEC
11801-2000 (and ANSI/TIA/EIA-568-B)
 Category 5 or higher  shall be permitted in accordance with clause
7.5 of this Standard."

"Table 5 is the connection schedule for the use of ISO/IEC 11801
Category 5 or higher Cable."



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