Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Proper DMX termination
References:  <[email protected]>    <[email protected]>
Dale Farmer  writes:

> > > Cat 5e's unshielded, install grade AES/EBU 110Ohm digital mic cable/DMX
> > > cables about the same price as screened Cat 5 & thinner
> >
> >         Good point. I haven't priced ScTP. Best price for DMX cable I've
> > gotten is $0.49/ft.  I get UTP Cat 5e for < 14 cents/ft. Obviously not the
> > right cable for this situation.
> 
>     Cat 5 cable is the wrong cable for the situation.  Wrong impedance.  

120 ohm resistance is recommended for RS-485 / DMX-512 termination.
The wiring for what DMX-512 is designed to is 120 ohm shielded twisted 
pair wiring. 
With 120 ohm termination resistor the RS-485 / RS-485 system works well 
with twisted pair cables that has impedance of 100-120 ohms. 
CAT5 cable falls in this impedance range and is useable. 
For best reliabity you might want to take a look at using 
shielded cable type instead for normal CAT5 UTP, although it 
should work pretty well in "not so noisy" environments at least.

In response to a perceived industry requirement for lower cost DMX512
cable installations, the DMX-over- Category 5 Cable Task Group was
formed by ESTA's Control Protocols Working Group (CPWG) at the January
1998 TSP meetings. The goal of this testing was to establish whether
Category 5 cable, or "generic premises cable" as it has become known,
could be used as a low cost substitute in permanently wired DMX512
installations. The result was:

"Data obtained from all three of these test sessions confirms that, in
most respects, UTP and STP Category 5 cable can be expected to perform 
at least as well as EIA-485 rated data cable for DMX512 applications."

"A separate ESTA standard (BSR E1.11, Entertainment Technology . USITT
DMX512-A Asynchronous Serial Digital Data Transmission Standard for 
Controlling Lighting Equipment and Accessories) is in development 
which addresses implementation of DMX512 over Category 5 cable."

The newest DMX512 standard public review version I read (it was
available for download at ESTA page one day) seemed to also mention 
CAT5 UTP cables and usign RJ-45 patch panels in DMX-512 connections. 

Link: Report on DMX512 over Cat 5 Cable
http://www.esta.org/tsp/DMXoverCat5.htm

> Look in the cable catalogs for RS-485 serial cable.  
> Two twisted pairs in an overall shield.   

That is a good recommendation.
DMX512/1990 suggests Belden 9841/2 and Alpha 5271/2 cables, 
depending on if you want one or two pairs. 
There are many other cables around which work well with DMX512 system. 
You need a shielded cable approved for EIA-485 use.

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