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XYWE > Theatre > DMX Branch Lines 29 April 2008 02:52:02

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DMX Branch Lines

Christian Marg 29 April 2008 02:52:02
 Hello,

of course cabling DMX in a 'Y' shape is a big no, no, but since DMX is
"just" RS485 and RS485 allows for unterminated branch lines up to 5
Meters, maybe I could use 1m or so to connect a dimmer and save some
connectors I'd need to use for daisy chaining. Couldn't I?

Or am I misreading RS485 and missing the note about "branch lines are
only allowed for up to 1200 Baud"?

bye
Christian
Add comment
Torrance Bell 19 April 2008 08:37:28 permanent link ]
 Christian Marg wrote:
Hello,
of course cabling DMX in a 'Y' shape is a big no, no, but since DMX is
"just" RS485 and RS485 allows for unterminated branch lines up to 5
Meters, maybe I could use 1m or so to connect a dimmer and save some
connectors I'd need to use for daisy chaining. Couldn't I?
Or am I misreading RS485 and missing the note about "branch lines are
only allowed for up to 1200 Baud"?
bye
Christian

I thought at least /some/ dimmer racks usually included screw terminals
so you wouldn't have to go through all of that.

Torrance
Add comment
Duncan Wood 19 April 2008 14:47:22 permanent link ]
 On Sat, 19 Apr 2008 02:51:44 +0100, Christian Marg
<christian.marg@tu-­clausthal.de> wrote:

Hello,
of course cabling DMX in a 'Y' shape is a big no, no, but since DMX is
"just" RS485 and RS485 allows for unterminated branch lines up to 5
Meters, maybe I could use 1m or so to connect a dimmer and save some
connectors I'd need to use for daisy chaining. Couldn't I?
Or am I misreading RS485 and missing the note about "branch lines are
only allowed for up to 1200 Baud"?
bye
Christian


You're misreading it, the length of a branch depends on the data rate. FOr
those odd occasions where you've got a house dimmer with only a plug or
the like then you can get away with short (10m) branches, doing it to save
on plugs though would be idiotic, they're a couple of quid, compared with
the length of time it takes to wire a Y split & the first time you have to
fix a fault Macdonalds would pay better. Far better.
Add comment
Duncan Wood 25 April 2008 15:21:02 permanent link ]
 On Fri, 25 Apr 2008 08:18:51 +0100, <whodewawhybot@gmai­l.com> wrote:

On Apr 19, 10:51am, Christian Marg <christian.m...@tu-­clausthal.de>
wrote:
Hello,
of course cabling DMX in a 'Y' shape is a big no, no, but since DMX is
"just" RS485 and RS485 allows for unterminated branch lines up to 5
Meters, maybe I could use 1m or so to connect a dimmer and save some
connectors I'd need to use for daisy chaining. Couldn't I?
Or am I misreading RS485 and missing the note about "branch lines are
only allowed for up to 1200 Baud"?
bye
Christian
I once opened an LSC Tour dimmer. To find the In and Out DMX
connections just wired together, and then from the In connector to a
circuit board, total lenght of cable to the circuit board was under 2
inches. I would not want to risk it in a production environment.
Mike!


That's how most dimmers do it. Nobody y splits RS485 at the chip, it would
be pointless.
Add comment
Torrance Bell 25 April 2008 15:52:05 permanent link ]
 
That's how most dimmers do it. Nobody y splits RS485 at the chip, it
would be pointless.

RS-485, from what I understand, is a very reliable protocol. Wikipedia
shows that its range is up to four-thousand feet. Daisy chaining from
one dimmer rack to another is AFAIK a pretty standard practice.

http://en.wikipedia­.org/wiki/RS-485

"The recommended arrangement of the wires is as a connected series of
point-to-point (multidropped) nodes, a line or bus, not a star, ring, or
multiply-connected network. Ideally, the two ends of the cable will have
a termination resistor connected across the two wires."

Torrance
Add comment
Torrance Bell 25 April 2008 15:54:24 permanent link ]
 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
Add comment
Duncan Wood 26 April 2008 01:42:52 permanent link ]
 On Fri, 25 Apr 2008 12:54:24 +0100, Torrance Bell
<starlite528@yahooo­oooooooooooooooooooo­ooooooooooo.com> 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.
Add comment
John B 26 April 2008 20:09:47 permanent link ]
 On 25/04/2008 Duncan Wood wrote:

On Fri, 25 Apr 2008 12:54:24 +0100, Torrance Bell
<starlite528@yahooo­oooooooooooooooooooo­ooooooooooo.com> 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 important 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
Add comment
Torrance Bell 27 April 2008 01:46:00 permanent link ]
 
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 important 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:
and download the first paper "Rs-422/485 Application Note".

Heh, I never would expect it to go that far anyways.

Torrance
Add comment
Torrance Bell 29 April 2008 01:00:38 permanent link ]
 Of course, DMX is only a protocol that runs on top of a specification.
It can most likely work with any digital signaling system that supports
at least the minimum data rate. I can only guess right now that they
chose 485 as the 'norm' is because of its simplicity, reliability, and
cheapness.

Torrance
Add comment
Duncan Wood 29 April 2008 01:31:53 permanent link ]
 On Mon, 28 Apr 2008 22:00:38 +0100, Torrance Bell
<starlite528@yahooo­oooooooooooooooooooo­ooooooooooo.com> wrote:

Of course, DMX is only a protocol that runs on top of a specification.

It was both.

It can most likely work with any digital signaling system that supports
at least the minimum data rate. I can only guess right now that they
chose 485 as the 'norm' is because of its simplicity, reliability, and
cheapness.
Torrance


Add comment
Torrance Bell 29 April 2008 02:52:02 permanent link ]
 Duncan Wood wrote:
On Mon, 28 Apr 2008 22:00:38 +0100, Torrance Bell
<starlite528@yahooo­oooooooooooooooooooo­ooooooooooo.com> wrote:
Of course, DMX is only a protocol that runs on top of a specification.
It was both.

I just can't get things right this week! Maybe I've been getting too
much sleep.

Torrance
Add comment
 

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XYWE > Theatre > DMX Branch Lines 29 April 2008 02:52:02

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