cable question :)

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 :)
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Ed wrote:

Just like to add that I don't really know anything about serial comms so please feel free to laugh if what I have written makes no sense as yet. However it has just crossed my mind that a small battery within each module may far outweigh the costs of expensive power/data cable..
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I suspect that all you need to fire a pyro fuse is a short pulse.
I that is true, you can use a much smaller data cable, such as CAT5 network cable, and charge a large capacitor in each addressable unit. Use 1 pair for data, all the others for dc power. You may have to experiment to determine the capacitor value. Use at least double the minimum value capacitor to assure firing. Capacitor charge time will vary with wire length, but should be a fraction of a minute. Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz
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Bill Kaszeta / Photovoltaic Resources wrote:

I freely admit that I know absolutely nothing whatsoever about explosives and pyrotechnics and how they are detonated - other than seeing news clips of it being done.
It struck me that they always appeared to be using a device with zero stored energy. The operator had to crank a hand generator immediately before ignition, in order to initiate an explosion. Of had to raise and then push down on a plunger - which I assume was also geared to a hand generator and had no stored energy.
This all seemed perfect sense - if the unit has no stored energy, indeed no method of storing energy, that energy cannot be released accidently to initiate an explosion.
I am not sure how a capacitor system, even one charged just a fraction of a second before ignition, would be made as inherently safe as a system which never stores energy - other than in a mechanical system which will always run to a stop. I suppose capacitors could be produced which are inherently leaky..
--
Sue





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never overlook the murphy factor.

Sue, the smaller systems (that i have used) use a spring loaded keyswitch in series with the power supply and momentary contact toggles for each effect. it takes 2 hands to fire a show, one holds ths safety "master" and the other fires the individual effect. the key stays in your pocket until just before showtime.
the electric match that fires the concussion, gerbs, and flash powder will work just fine on a standard 9V battery
the saddest part is that for all your work in setting up and stress of getting the timing right the guy the fires the pyro usually does not even get to see the show much less enjoy it. you are usually at the side of the stage or behind the orchestra somewhere. even the outdoor guys are much closer then the crowd is allowed and more or less have to look straight up... if you have time to look :)
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Would DMX be suitable for this?
j
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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
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Ed wrote:

There is a cable used in theater lighting for powering and controlling color scrollers. One pair is data, the other pair is power for the motors and fans. Uses a 4 pin XLR connector. The connector is not weatherproof, and the cable is not armored against ground bursts. Don't know the specs offhand, but it's probably made by Belden. It's been a few months since I've had one open, but I think the power pair are 14AWG.
You could separate the power and data cabling. If you are going to the expense of building a local box with RS485 in it, adding a small power supply and just running an extension cord out to it may be an easier and more cost effective approach.
The fireworks shows I've seen just used old 25 pair phone cables that he had picked up at salvage from the phone company. Thus a direct wire pair from each thing back to the control point. Simple, reliable, and nearly bulletproof.
--Dale
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