Newsgroups: alt.engineering.electrical
Subject: Re: cable question :)
References:  
"operator jay"  writes:

> "Ed"  wrote in message 
> news:[email protected]...
> > Hi All,
> > I do pyrotechnics and I am looking to build myself a new firing system. 
> > Basically I plan to build an RS485 network of modular boxes with n 
> > channels per box each addressable from the 'desk' end.
> >
> > Cables are obviously an issue and RS485 requiring twisted pair, but I also 
> > need to carry a supply to each box of up to 24volts at a reasonable 
> > current (say a conductor capable of 10-15A) due the the long runs of wire, 
> > and the ability to fire many channels at the same time without causing a 
> > system brown out.
> >
> > So my question does anyone here know of any cables that are ideally 4 core 
> > with twisted pair data lines and two extra current carrying conductors 
> > that finally doesn't cost the earth? Even better if you know of any 
> > weather resistant type of connectors to terminate the ends of the cables 
> > with!
> >
> > Thanks :)
> 
> Would DMX be suitable for this?

DMX is a lighting industry standard way of controlling lighting
equipment. The whole idea behind DMX as a standard is that it allows
equipment from one manufacturer to talk to that made by
another. DMX-512 allows for one controller (like a lighting desk)to
control many lights. DMX is limited to controlling 512
separateparameters, that's where the name DMX-512 comes from. DMX-512
is a protocol for controlling (at a basic level) the brightness
oflights in a theatre (it is vary capable and can be used for
controlling moving lights, smoke machines, strobe lights etc.). It
basically consists of one 8-bit (on or off) signal for each light
whichsets the brightness level (0-100%) of the light to one of 256
levels.

DMX512 is connected using a daisy-chain methodology where the source
connects to the input of the first device, the output of the first
device connects to the input of the next device, and so on. The
standard allows for up to 32 devices on a single DMX link. The DMX512
communications protocol is very simple and robust. DMX512 uses
EIA-485-A (commonly refered to as RS485) which is abalanced
system. Normative references for electrical specifications at
ANSI/TIA/EIA-485-A-1998 Electrical Characteristics of Generators
&Receivers for Use in Balanced Digital Multipoint Systems. 
The DMX-512 interface is electrically RS-485 bus where there is one
transmitter all the time transmitting and multiple receives along the bus.

Officially DMX is carried on 5 pin connectors, which carry the
data(RS485) on pins 2 & 3, screen (and common mode reference) on
1. Thereis a second data link on pins 4 & 5 the format of which has
never been well defined and it is seldom used in practice. 

The DMX512 communications protocol is very simple and robust. The
protocol used in DMX-512 bus is similar to normal serial
communications(like RS-232 with 8 data bits + 1 stop bit) and operates
at 250 kbps speed. 

The communications path for DMX-512 system shoudl be well built to
guarantee error free data transmission. Error free data transmission
is needed for reliable operation, because DMX-512 does not include any
error detection or error correction capabilities in it. This means
that if some error happens on the signal on the line, then one dimmer
channel some dimmer channels receive incorrect values. Random errors
that happen very rarely do not cause too much problem, because the
data sent to dimmers is repeated very often (typically tens of times
in second). If the dimmer gets wrong data at one data packet, the next
packet with correct value will make the dimmer setting right again. So
the transmission errors typically do not cause any noticeable problems
in light controlling, usually at works cases maybe some random
flashing of lights. DMX controlled dimmers either hold the last value
or go to zero when a control signal is lost (i.e. when someone turns
off the board with the dimmer packs still on). Because DMX-512 does
not have any error control or detection capabilities, it should not be
used in any application where the reception of wrong control value can
cause permanent damage or danger (for example you should not use
DMX-512 to control mechanical movemement of any heavy objects).

So normal DMX-512 as is used widely nowadays might not be as 
such a good option for pyrotechnics. I think the transmission 
should include some form of error checking so that 
random transmission errors caused by noise etc.. 
do no cause accidential fring of pyrotechnics. 

Some newest standard version also standard also has provision 
for optional data transmission checksum and also option 
for other new data formats. 

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