5 pin DMX to 3 pin DMX

jfitzpat

Member
Joined
Nov 22, 2005
Odd notes from an old and fuddled brain.

The Standard: Actually, I'd have to argue that the standard was/is kind of crappy. I know I howled about no checksum or CRC in the packets. The counter argument was that dimmer packs should be able to use shift registers instead of UARTS and microcontrollers. Several folks showed you could do a checksum in a GAL (even then, about $1 in COG), but to no avail. Ironically, almost no 'dumb' implementations every shipped in products.

The electrical stuff was pretty much borrowed directly from international standards on RS485 and 422. Some additional measurement and work went into RDM (a talk-back for configuration on the main transmission pair), but very little into the original standard.

That said, it could be worse - we could all be running AMX 192 (which used to be printed in the same USITT standard as DMX-512).

5 vs. 3: 5 is the standard, 3 is even prohibited. Some 3 pin 'swapped' fixtures make the situation suckier still - but 3 pin fixtures and control outputs are here to stay. I'm with the purists, but even I had to cave on our entry level interface.

On the flipside, abuse of the extra two pins (4 & 5) has made the 5 pin world muckier too.

Cable: Mic cable is not twisted, and is a different impedence. So the digital signal is 'rounded' and longer runs are more succeptible to external noise. Termination causes the transmitter to produce more current, which sharpens the edges somewhat, but mic cable is still less than ideal. DMX cable should be slightly noisier in high end audio applications since it removes a natural beyond-range, low pass filter from the system.

Termination: From an engineering standpoint, it is always a good idea. A lot of folks who think they never have problems without it would be surprised to see how many bit errors can be occuring at individual fixtures. I used to always recommend building your own terminators with an LED in it (so you get used to having a signal indicator and don't fall back out of the habit of terminating), but if RDM ever really catches on, the simplest LED designs might be a problem.

-jjf
 

Thomas

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Joined
Nov 2, 2005
Location
Durban, South Africa
As far as using terminators with LEDs on them, will the LED not affect signal in some way? What type of LED and resistor should be used?

I'm just toying the idea with a little flashing LED in my mind right now...
 

jfitzpat

Member
Joined
Nov 22, 2005
Any small indicator LED should do. You would still terminate normally (perhaps erroring towards a slightly higher ohm value than normal), then put an LED and current limiting resistor in parallel to it.

Driving the LED is roughly the same as driving an opto isolator (opto splitter) and you should not see any adverse effects. I've seen a few problems with RDM, but only with a fairly high current running through the LED. The problem is the impact on settle time as the lines turn around for the bi-directional communcation required.

-jjf
 

GeneralDoom

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Joined
Feb 10, 2005
Location
Lafayette, LA
ok, odd/ignorant question here, what exactly is "termination" and how do you implament it and in what senarios? sorry I've had little experiance with DMX.
 

soundlight

Well-Known Member
Joined
Oct 27, 2005
Location
NJ & NYC
Termination prevents the signal from being reflected back down the DMX line and causing problems. This is normally not an issue, but it has thrown a number of people for a loop back before terminators were common, and still now when they are. It can feed a signal back to a fixture that is not what the console is feeding it, and this can be a BIG problem, especially for moving lights.

For a pratical overview of DMX, check this out:

http://www.dmx512-online.com/
 

SHARYNF

Well-Known Member
Joined
Sep 3, 2006
Here is a different spin on the cable issue:

IMO it is not the impedance of the cable, in fact I believe that terminating the cable has the effect of resolving this to a large extent, it is the capacitance of the mic cable, which is typically higher in mic cable, this is what causes the alteration of the waveform, but again unless you are runnning really long lengths I have seen termination resolve the issues with Mic cable. The DMX chip sets that are used for the rs422 analysis are typically set up so that they are not all that sensitive to the rounded edges. Again, just personal observation

Since the cable is shielded IMO the twisting is not a factor, twisting works for noise rejection, but since the cable is shielded, it is not a factor.

From a practical point of view, the cheapest alternative is to use Cat5 utp

Sharyn
 

JD

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Joined
Jan 1, 2005
Location
North Wales PA
Since the cable is shielded IMO the twisting is not a factor, twisting works for noise rejection, but since the cable is shielded, it is not a factor.
Shielding takes care of electrostatic noise problems (voltages in proximity to the cable) but does not reject electromagnetic induced noise (produced by transformers, ballasts and light dimmers) which can be an even bigger problem. The twisting of the pair greatly increases electromagnetic rejection. Always best to use a cable that is both.
 

jfitzpat

Member
Joined
Nov 22, 2005
Since the cable is shielded IMO the twisting is not a factor, twisting works for noise rejection, but since the cable is shielded, it is not a factor.
JD is correct, shielding does not resolve several noise sources. Ideally, you want twisted pair (so noise is distributed evenly) and shielding braid and foil. However, in this case there is a bigger problem still - A lot of existing DMX devices bind chassis to earth ground, so with a mic cable, where the ground is typically attached to both connector bodies, the shield can become a source of noise, not a sink for it.

I'm not sure I understand your point about capacitance. A good audio cable IS designed and built to minimize harmonic distortion, but with a roll off of 70-80 kHz. DMX is inherently an assymetric square wave up at 250 kHz. In demonstrating the signal distortion, most testers examine the signal at the 120 ohm termination resistor. Are you saying that the principle problem is some form of capacitive coupling when the termination resistor is not present? If so, I'm not following you. Look at the schematic of a typical tranceiver input:

http://rocky.digikey.com/WebLib/Texas Instruments/Web data/SN65176B,SN75176B.pdf

I'm not trying to argue, I'm just trying to better understand your point.

-jjf

Edit: In re-reading, it seems you are talking about cable capacitance (n pico farads per foot), but I'm still confused. How does the terminator, which is in series with the two cable 'capacitors', help?

Put really simply, error prone reflections are a product of impedence miss match, the transmission signal is not completely 'absorbed' by the receiver. We terminate so that the transmitter, cable, and our dummy receiver (the termination resistor) can all work at a matched impedence, thus minimizing reflections. The reason that cable length is an issue it changes propogation delay for the signal. IE, length times .66 to .75 the speed of light. In UART communications the typical rule of thumb is that when total propogation delay of all 'bounces' is signficantly less than a single bit width, termination is not required. Because the UART receiving the signal will typically sample in the middle of the bit interval, after the line has settled.

Cable capacitance can be a communications issue, particularly as data speeds go up, but the couple of audio cables I checked all still seem to be within the EIA limit for RS-485 at 250 kbit. Remember, it is a differential signal.
 
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SHARYNF

Well-Known Member
Joined
Sep 3, 2006
My points were:
Digital Cable tends to have a lower cable pico farad rating, and that in my experience, it is this that will alter the shape of the wave form, rounding off the edges, bit the differences are slight as you mentioned, so, again in my experience,

Most of the problem again in my experience comes from reflection problems, and that the termination, corrects this by adjusting the impedence as you say.

While I agree that twisting on a "balanced" signal can improve immunity to Electromagnetic noise, but again, the shield can operate as a faraday shield. As for example PC designs with higher and higher ghz ratings have become more common, design practice has shown that Faraday "cage" effect has been effective from both Electromagnetic and Electrostatic noise. Since there is normally not any current on the shield, it is hard to tell how effective this is, or if in fact there is sufficient current from the grounding to create this effect.

Currently good practice is NOT to connect the shield in an audio cable to the connector, as this has been shown to introduce multiple grounds in the signal based on what the connector is resting on.

I guess my point was more that from practical experience, again not scientific, I have found that if you use mic cable, and you terminate you will probably be ok, if you use mic cable and do NOT TERMINATE you are more likely to have problems. My feeling on this is that the most critical problem that you are then minimizing is the reflections. But if you want an inexpensive alternative to digital cable, UTP, again from practical not scientific stand point seems to be working well IF you make sure that you put the signal thru a pair, NOT as some people can mistakenly do simply use three pairs as conductors.

Sharyn
 
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SHARYNF

Well-Known Member
Joined
Sep 3, 2006
This whole area has been the subject to lots of discussion in the audio industry typically referred to as pin 1 problems. Basically what is best practice today is to INSIDE the equipment, connect the pin 1 to the chassis ground, and do that directly at the connector. At one time when connectors were all metal the thinking was that this was accomplished by having the body of the connector be wired to pin 1 and then the metal in the connector would be in contact with the metal in the chassis mount socket etc. Problem with this is then all the connectors in the path were also connected to pin 1 and if they were in contact with other metal or the earth or what ever, another path to ground was created with potential for ground loops. Some of the newer combo trs/xlr sockets are not even made of metal any more.

So now post Muncy http://www.audiodesignline.com/howto/196900858
the practice is to not wire the connector body to pin 1 but to wire pin 1 directly to the chassis of the equipment NOTE THIS is NOT connecting shield to Audio signal ground, which was the source of endless Pin 1 problems. It was this connection to audio signal ground that caused a number of folks to recommend disconnecting (lifting) the ground on one end of the cable, which of course worked in some cases but caused other problems (shield becoming an antenna in others)

The practice of connecting the shield to audio signal ground in the equipment is far more common that people might think especially with older equipment.

SO now you can actually build a tester
http://www.tvtechnology.com/features/audio_by_design/f_Gruszka-02.16.05.shtml
http://www.tvtechnology.com/features/audio_by_design/f_Gruszka.shtml
http://www.tvtechnology.com/pages/s.0071/t.1587.html

As Mary shows in her articles, resolving the problems in improperly designed equipment can be done, phantom power for mics is a special problematic case.

Here is a link to the new Neutrik EMC-xlr that Mary refers to
http://www.neutrik.com/content/products/detail.aspx?id=210_1822126843&catId=CatMSDE_audio


Mary does a good job explaining lots of audio topics
http://www.tvtechnology.com/pages/s.0071/t.p0001.html

Here is a link for the dmx over cat 5 testing that was done
http://www.esta.org/tsp/working_groups/CP/docs/DMXoverCat5_P1.pdf

For clarification on what I was saying, the shield of the audio cable is connected to pin 1, but the shield is not connected to the casing of the connector (xlr) BUT inside the equipment, pin one is connected to chassis ground. This whole area can be very confusing to say the least. If anyone has the time look at the various links for a lot more information

The mods of the forum might move some of this discussion to an new thread, since I sort of got off the DMX topic
I might add that Pin 1 problems are a lot more common than you might think and are the source of a significant amount of noise problems in audio systems

Sharyn
 
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jfitzpat

Member
Joined
Nov 22, 2005
This whole area has been the subject to lots of discussion in the audio industry typically referred to as pin 1 problems.
Makes sense, balanced shield to earth ground on both ends - but by strapping 1 inside the equipment.

The mic cables I pulled out of the milk crate here had body<>body conductivity, but who knows how old they are or where they came from.

My one note would be, don't be fooled by plastic XLR panel mount connector bodies. If you look closely most still have a metal contact point, which is brought out to a tab or pin (PCB mount). This may or may not be attached to chassis ground inside the gear.

Thanks for the info.
-jjf
 

SHARYNF

Well-Known Member
Joined
Sep 3, 2006
You are absolutely correct, there is an option usually on the plastic connectors or part of the design, but again from the practical side of things the old idea of using the connector body as the link has proven even with metal connectors to be unreliable, and of course if inside the equipment pin one is connected to audio ground you are going to have problems.

It really is something of a nightmare in audio these days, and from an audio stand point 1993+ for its recognition is considered "recent" but typically is not widely discussed outside of pro audio circles.

From a practical point of view, if you have a newer under warranty piece of equipment with the problem it is certainly worth while contacting the company and they probably have a mod. If the equipment is a bit older, but a well know manufacturer again it is worth contacting them and they probably have a fix.

Anyway good discussion
Sharyn