Ujjal's DMX512 Pages....DMX512 Physical properties

DMX512(1990)PHYSICALS



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The format in which DMX512 signals travel between transmitting & receiving devices and how that format is interpreted by them has been explained in DMX512 PACKET.

The actual electrical method by which the digital 1s' & 0s' are spit out by your console and then received by all the gear on stage is also important to understand, in view of the fact that 80% of your problems are caused by distortion, in some way, of the physical DMX512 signal.

The DMX512 signal is transmitted via the industry standard interface known as the EIA485, more familiar as the RS485. This is not dissimilar to the RS232 port behind your standard desktop, but is not the same stuff exactly.

Specialised instrumentation desktops have these RS485 ports for process control jobs, but the format for communication is quite different. It's like using the same English alphabets to write two different languages.

The RS485 standard uses two/three wires to transmit the digital HIs & LOs:
1) The +signal wire (+s)
2) The -signal wire (-s)
3) The 0 wire or ground wire (0v)

A digital 1 is sent out when the +s wire is at a higher potential then the -s wire.

A digital 0 is sent out when the +s wire is at a lower potential then the -s wire.

The difference between these two data wires is what's IMPORTANT ; NOT the difference between EACH of them and the ground wire which is simply a reference point. The ground wire may not be present at all in some EIA485 installations.

The high or low potential levels are defined clearly by the RS485 interface standard.
Any of the two wires can go upto +12volts (measured to ground) or down to -7volts (measured to ground).

e.g. To transmit a digital 1 ,the +s wire is held at +5volts and the -s wire is held at -5 volts.

To transmit a digital 0 ,the +s wire is now taken to -5volts and the -s wire to +5 volts.

LIKE THIS:

RS485/DMX input action

How much differential shall the receiver detect in order to declare a change of state ?

EIA485 says that it should be AT LEAST 200 milivolts or 1/5th of a volt. The good thing about common mode operation (that's what it's called) is that the RELATIVE difference BETWEEN the two wires is important. The ground wire is often used only as a shield in the shielded cables used for DMX512 transmission. In simple terms, this yields two major benefits:

1) Noise (and there is plenty of that all around in a gig), is picked up equally by both wires and in the SAME PHASE....which means it goes up and down the same way in both wires.Result: When it reaches the receiver there is NO differential in it as far as the receiver is concerned (remember the 200 millivolt requirement for detection!). Therefore it is ignored.

2) If the signal starts out from the console at ,say, +/- 5 volts and passes down a bad ,high resistive cable then BOTH wires of the cable will offer EQUAL resistance to the two signals in each. This means that even if it came down to +/-100 millivolt, the receiver will still detect a difference and pass on information. That's because the 200 millivolt difference was still there.

Well it could be bad but never as bad as that !!!

USING GOOD CABLES & CONNECTORS

The single most important thing to ensure that the scanner mirror does'nt quiver at you or the pars don't wink at you, out of turn, is to have good cables & connectors.

The cable should be of instrumentation grade as suggested for EIA485 use, with TWISTED pairs (if you want them to catch the noise equally, they should entwine each other !!) and a good shielding braid and foil. I use BELDEN 9829 at the moment but that's personal choice. Actually, I bought three rolls of unused cable real cheap from a guy in Florida who had gone bust. However, anything complying will do! See bottom of this page for a complying cable chart.

Never,never borrow a mike cable from the sound guys in case you have a 3 pin XLR system in your DMX equipment. It may let you down in the finale.

Those cables arn't twisted and, worse, they generally have their ground wire connected to both connector bodies. Many control consoles have their chassis connected to mains ground and this can cause data error at times.
Another important thing to remember is that audio cables (very good ones) can carry a signal of upto 80KHz nicely enough without harmonic distortion. That means , in simple terms, a square wave remains a square wave at the end of the cable. DMX512 is a square wave at a frequency of 250KHz. With a audio grade cable, you are likely to land up with a triangular (or God knows what!) shaped DMX signal. The dimmers & intelligent lights receiving the DMX512 signal, are not going to like that one bit !!

Connection is thru 5 pin XLR connectors:

Connection:
Pin 1 to PIn 1 = shield (do NOT connect to body of connector)
Pin 2 to Pin 2 = -s
Pin 3 to Pin 3 = +s
Pin 4 to Pin 4 = -spare ,often used for dimmer status return data,TB or scroller power.
Pin 5 to Pin 5 = +spare ,as above

Some manufacturers use 3 pin XLRs to allow use of mike cables, but that's non-standard as far as USITT DMX512 is concerned.

NETWORKING LOADS

A DMX512 output from a USITT DMX512 complying transmitter (console, PC or whatever) has the capacity to drive upto 32 units (scanners, dimmers, scrollers or whatever). These are connected serially since most of them have a DMX512 IN and a DMX512 OUT. The convention is opposite to that of sound equipment --- a DMX OUT connector is always FEMALE and a DMX IN connector is always MALE.In order to drive more than 32 units , a DMX SPLITTER is required. A DMX splitter consists of a DMX IN connector and several DMX OUT connectors. Each of the DMX OUT connectors can now drive 32 units each. The 32 units should all comply with EIA485 receivers specs. If some units load-up the line more than specified , lesser units will have to be used in that line.

Networking DMX

IMPORTANT:

Like all data communication equipment, a termination resistor needs to be fitted at the end of a DMX512 output, even if 1 or all 32 units are used. This is generally a 120 ohms/ .25watt resistor soldered between pins 2 and 3 of a male 5 pin XLR connecter. This is then inserted into the DMX OUT female connector in the LAST unit on that line (Yes , one in each line ,in case of splitters!!). Not doing this will cause data on that line to reflect back down the line. The reflected data will cause other units on that line to detect a DMX format error or to sense wrong data and cause a wrong movement or flicker. The resistor eats up the data at the end, not allowing it have the strength to reflect back.

So much for DMX physicals!!


DMX cable selection chart

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