Ground (pin 1) lifting DMX

[david.elsbury]

Hi guys

Just a quick one, wondering what if any would be the possible consequences for lifting pin 1 on a DMX line (was trying to troubleshoot a problematic system the other day and tried this to see if it would help.)

 

In my case when I did it everything still seemed to work fine. I assume that because DMX is a balanced pair it shouldn't cause any problems if the ground is connected or not... but I could be wrong :** laughs out loud **:

 

Cheers

[Brian]

It's one of those 'it depends.'

 

I can see scenarios where downstream kit, say a splitter/isolator, is relying on the incoming pin 1 to provide its 'earth' and to give the outgoing feed an earth reference. A lack of earth could well cause problems with RF susceptibility.

 

 

Also, whilst the data is balanced it might not be floating and with no earth then the input stage's common mode range may well be exceeded. You've normally only got about 7v to play with. A difference in earth potentials could easily exceed this. It may manifest itself as the system working fine with no load but with data errors showing as lamps come on.

[David Duffy]

Lifting pin 1 is not recommended. The signals on the data pair are referenced to pin 1. As Brian said, without it the common mode range of the input can be exceeded. This may result in malfunction or damage.

 

As for a splitter/isolator, the input pin 1 and output pin 1('s) are not connected anyway. That's the idea of it being isolated. This prevents bad (fault) voltages from propagating to other DMX segments.

 

All that said, if you left pin 1 disconnected, your system may continue to work using the mains earth as the common reference.

[david.elsbury]

Ah I see. So if I were chasing a possible ground loop, an isolating splitter at the source/ on the same power as the console would definately be the way to go then?

 

I have another question for you good folk, please, too... if I were borrowing a line on the sound company's snake, and the snake happened to have a transformer split on it, what would the transformer do to the DMX signal?

 

Cheers :** laughs out loud **:

[Don Allen]

The transformer will cause deterioration of the data square wave, depending on its frequency response, so it will not work.

 

Everything in a DMX signal chain is either digital such as microcontrollers (nowadays, discrete logic in the old days) and opto couplers or fast slew rate balanced drivers.

 

I have never thought to try this, has anyone ?

[David Duffy]

The transformer will completely destroy the DMX signal IMO. Due to the high speed, DMX is fairly sensitive to waveform distortion.

 

I was reminded of this recently when designing a optically coupled splitter. A single transistor in the data path skewed the signal just enough to cause grief (data errors). I had to change the design to keep the edges of the data nice and clean.

[timmath]

We used to regularly 'lift' pin one when running systems on multiple generators across large sites - buffers isolate the +/- data pair but not the earth. when doing that kind of thing these days we use fibre and artnet!

[KevinE]

I suppose it's worth a try on a get-you-going basis, howeever the screen is there on DMX to ensure continuity of the electrostatic screen around the cables, rather than being part of the RS485 signal itself. So it wouldnt increase its susceptibility to electromagnetic induced interference (eg common mode mains hum or otherwise) but would leave the equipment susceptible to electrostatic discharge because the DMX cable insulation isnt rated to ESD levels.

 

This is why I say that proponents of using CAT5 UTP for DMX ought to consider FTP instead.

[Dmills]

The real issue tends to be that RS485 (which is what DMX is electrically speaking) specifies a limited common mode range (+12, -7 IIRC), and keeping the screen connected helps to ensure this is met at the receiver.

 

Best practise when designing DMX receivers is to run the RS485 receiver chip from an isolated supply (DC-DC is common) and to opto isolate between the receiver and the downstream micro controller. This ensures that the line is effectively floating with respect to the local metalwork and that the common mode reference can be set by pin one without risking major current flow in a fault situation.

 

Push comes to shove you can lift pin one, but there are more reliable ways to skin this cat.

 

Regards, Dan.

[David Duffy]

We used to regularly 'lift' pin one when running systems on multiple generators across large sites - buffers isolate the +/- data pair but not the earth.

Really? Every DMX isolator (not a plain distributor) I've seen isolated the incoming and outgoing lines completely. Maybe we're talking about different items?

 

... however the screen is there on DMX to ensure continuity of the electrostatic screen around the cables, rather than being part of the RS485 signal itself.

But the +/- (A/B) voltages are referenced to 0V and the common (shield) maintains that reference between two systems.

 

This is why I say that proponents of using CAT5 UTP for DMX ought to consider FTP instead.

I assume you mean STP? :P DMX over CAT5 uses another pair as the common reference. STP is not required (but it won't hurt) as long as the data wires are run in a twisted pair.

 

Best practise when designing DMX receivers is to run the RS485 receiver chip from an isolated supply (DC-DC is common) and to opto isolate between the receiver and the downstream micro controller. This ensures that the line is effectively floating with respect to the local metalwork and that the common mode reference can be set by pin one without risking major current flow in a fault situation.

This is very good advice.

[KevinE]

But the +/- (A/B) voltages are referenced to 0V

 

Not as far as I know, since RS485 is a balanced system and is only comparing the hot and cold signals with respect to each other. DMX will happily work without pin 1 connected at either (both) ends, it doesn't need pin 1 at all to work.. However it is part of the spec. as Dmills has mentioned. The OV rails in fixtures and desks should be floating to avoid hum loops between each other, they sometimes get grounded and under certain fault-conditions can lead to daisy-chain blow ups and other nasty effects.

 

I assume you mean STP?

No, I mean FTP...foil-screened twisted pair. This uses a drain wire which makes it easier to neatly terminate into an XLR when 30 feet up a ladder with a gas soldering iron in the semi-darkness, the foil rips easily and cleanly and you dont have to worry about loose strands of braid getting loose. FTP is the norm in the UK for screened CAT5.

[David Duffy]

Not as far as I know, since RS485 is a balanced system and is only comparing the hot and cold signals with respect to each other.

While that is true, the data pair still needs to be kept within the common mode range (0V reference). That's the job of pin 1. If you leave it disconnected, it can still work due to the mains earth providing a path. With truly isolated inputs and outputs though, you may run into trouble.

[KevinE]

Fair comment, I suppose most DMX use of RS485 will be from one isolated transmitter to another isolated receiver or receiver string...where theres no path at all from the tranceiver ICs' OV to anywhere else. I suppose the same pathways (which may be tortuous) that enable 2-wire-only DMX to work are the same pathways that sometimes blow tranceiver chips!

[David Duffy]

Yeah, that's what I was trying to get at. RS485 really is a 3 wire transmission, unlike transformer balanced audio where the pair can function by itself.

[KevinE]

So how do virtual earths work? Ive read somewhere that you can get receiver circuits that create their own virtual earth, is this anything like DC restoration in TVs where the decoder has to ac-couple the video then re-insert the dc component to get the right brightness level? Just a thought.