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

> Tomi Holger Engdahl wrote:
> 
> > 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.
> 
>     A very interesting report.  Since I can buy a 100 foot Cat 5e patchcord for
> less than ten bucks, this is far less expensive than a real DMX cable.   Just
> need to make up some eight position modular connector to 5 pin XLR adapters.
> has anyone come up with a standard pinout for that yet?
> 

DMX512A public rewiev document Revision 5.2 
describes the following pinout:

RJ-45   DMX512 XLR5          Signal

1       3                    Data1 +
2       2                    Data1 -
3       5                    Data2 +
6       4                    Data2 -
4       not conntected
5       not connected
7       1                    Signal common
8       1                    Signal common

I don't know if it did get to the final standard or nor or 
is it udes anywhere yet.

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