Re: DMX Branch Lines

On 25/04/2008 Duncan Wood wrote:

> On Fri, 25 Apr 2008 12:54:24 +0100, Torrance Bell
> <starlite528@[EMAIL PROTECTED]
> wrote:
> 
> > As far as unterminated branch lines no longer than a certain
> > distance, I   think the idea there is that you don't want the total
> > combined length of   wire longer than the specification states
> > (4000ft), or else the signal   will have degraded so much from
> > voltage drop to be unrecognizable.
> > 
> > 
> > Torrance
> 
> 
> You won't get a dmx signal down 4000' of wire though, it's more than
> 100KHz. You don't want branch lines as the reflected signal degrades
> the  original. Terminating them won't help as every termination gives
> you at  least a 3dB drop in signal.

RS485 is unlikely to go 4000' for Baud rates higher than 19200. DMX is
250kBaud and both ends of a DMX line should be terminated. This
prevents reflections from the unterminated end interfering with the
outgoing data. It is not generally recognised that the source end
should be terminated, but this is im****tant and should be internal to
the master device. Measuring with a meter across the signal pins of the
source when the source is unpowered will tell you if it is correctly
terminated.

For all the information you never thought you'ld need about RS485
networks go here:

http://www.bb-elec.com/technical_library.asp


and download the first paper "Rs-422/485 Application Note".

-- 
John B