Clarification on ILDA voltages

[harrissanford]

I bought a chinese galvo set and showcard and being the curious individual I am decided to probe the outputs. On my kit I read the following:
x (there is a three prong connector labeled "+, gnd, -". "gnd and -" are tied together): -5v to +5v
y (there is a three prong connector labeled "+, gnd, -". "gnd and -" are tied together): -5v to +5v
red (two prong connector labeled (+ and gnd): 0v to +5v
green (two prong connector labeled (+ and gnd): 0v to +5v
blue (two prong connector labeled (+ and gnd): 0v to +5v
shutter (two prong connector labeled (+ and gnd): 0v to +5v


After probing these, I looked up the ILDA standard online and found this:
http://www.laserfx.com/Backstage.Las.../Pinouts9.html

After reading this, I'm rather confused!

For x and y signals:
I see they are are able to vary a total of 10v (in other words, 10v peak to peak) and gnd is supposed to be in the middle of the range. This leads me to believe the max voltage in relation to ground should be +5V and the min should be -5V... so in otherwise, this is a bipolar analog signal.

1. If this is truly the case, how can this chinese board get away with tying GND to the - pin? It works.. but it doesn't seem proper? What is the downside to this setup?

2. I have searched the forums regarding this before posting and some people say you should see an overall 20v swing. Are they confused themselves, or am I missing something important?


For r, g, b, and shutter signals:
3. It says it is a unipolar signal that is able to vary 5v (in other words, 5v peak to peak).. but if it is a unipolar signal, why is gnd expected to be in the middle of the range (thus the max signal voltage should be 2.5v and the min be -2.5v)? I just don't get this, especially given all lasers and shutters I have been tinkering with expect 0-5v ranges where 0v = gnd.

Can someone clarify this for me. I'm getting a headache just thinking about this

[taggalucci]

My attempt to answer your questions... (other more knowledgeable electronics-gurus feel free to correct as required )

X and Y signals are best handled as differential (bipolar) signals to eliminate noise in the lines which would affect scanner precision, whereas the colour and shutter signals have been deemed to not be as critical/subject to noise issues and are generally unipolar.

I can't fully answer your questions about the Chinese scanners and the show card as their electrical characteristics, requirement for bipolar/unipolar signals may differ to what I'm using. In fact it may well be that because each galvo driver or show card doesn't have differential inputs (or that the show card doesn't have differential outputs), the ILDA signal wires negative and ground are bridged to make them unipolar. The downside may be that these galvos are more prone to noise, but given they're Chinese (and likely Phenix Technology or similar), they may well be prone to a lot more than just noise!

There shouldn't be a 20v swing. +5v and -5v = 10v difference.

As for the colour and shutter signals, one could use differential drivers to eliminate noise etc but this is not expected, then you would need the difference to = 5v which would logically place ground half way and make pos = +2.5v and neg = -2.5v. Most DACs are not wired this way. The modulation inputs on lasers use only positive and negative, so even if driven differentially, the result would still be the same i.e. a difference of 5v from the bottom of the signal range to the top.

Does that make sense?

[taggalucci]

...and if you haven't already, please read this great projector wiring guide http://www.pangolin.com/resguide12.htm

[harrissanford]

Ah, I didn't think about it for noise reasons. This makes sense, especially after I looked at this picture:
http://en.wikipedia.org/wiki/FileiffSignaling.png

You won't notice small fluctuations in color, but you would notice movement.. and according to the ilda specs at least, the x and y outputs have more bits thus they would be more susceptible to noise.


If I have a dac that I can attach to an arduino that outputs 0-5v, what would be the best way to make a proper output? Looks like I have more searching to do.

[taggalucci]

If I have a dac that I can attach to an arduino that outputs 0-5v, what would be the best way to make a proper output?

Please can you explain what you mean in more detail?

[Stoney3K]

As for the colour and shutter signals, one could use differential drivers to eliminate noise etc but this is not expected, then you would need the difference to = 5v which would logically place ground half way and make pos = +2.5v and neg = -2.5v. Most DACs are not wired this way. The modulation inputs on lasers use only positive and negative, so even if driven differentially, the result would still be the same i.e. a difference of 5v from the bottom of the signal range to the top.

Does that make sense?

There are some advantages to driving the shutter and colour signals differentially as well.

One of the main things is to eliminate ground loops and hum if the projector is connected to a different power phase/breaker as the DAC and controlling system. If the projector is not galvanically separated from the source, it can introduce some nasty problems with long ILDA cables.

On galvos, ground loop problems are more visible since hum from the grid (a few dozen mV at 50-60Hz) will cause the projected image to warp and wobble. Differential driving is more important on galvos than it is on other signals, but on the colour/shutter signals, it's possible to notice hum as a slight drift in intensity with analog modulated lasers. If you were to project a flat 'liquid sky' image with the lasers full on, waves will appear in the pattern in mid-air because the laser's intensity changes as the image progresses.

The proper way to drive ILDA signals is differentially with a 100-120Ohm matching resistor. Scanners are driven in a bipolar mode (voltage swing of -10v to +10v differential), whereas shutter and colour should be driven in a unipolar (0v to 5v, still differential!) fashion.

As for the modulation inputs, only positive and negative are required to get a good differential signal input. Ground is not required, although it may be useful for potential levelling if more than 1 device are connected together. On a good laser driver, both the Mod+ and Mod- inputs are irrespective of ground (also called: have their ground lifted) so any grounding issues can't have a serious effect on the signal.

[mixedgas]

Summing op-amp inverting op amp

10 turn potentiometer.

Steve

[harrissanford]

The proper way to drive ILDA signals is differentially with a 100-120Ohm matching resistor. Scanners are driven in a bipolar mode (voltage swing of -10v to +10v differential), whereas shutter and colour should be driven in a unipolar (0v to 5v, still differential!) fashion.

Thanks for your reply

-10 to +10v is 20v peak to peak. I'm having great difficulty understanding why you said this. I would understand if you said -5v to +5v .. or +2.5v to +12.5v (both 10v differential peak to peak)... but I don't understand this 20v swing

[harrissanford]

Here is what I have tried doing before posting this thread. I used two i/o pins on an arduino. and toggled them high and low in different patterns as shown in the picture.

It appears to work. The dot moves to the far left, far right, or center of the output. I don't see the point jitter at all.





Stoney3K, from your description it sounds like I really need to send out + or - 10v (a 20v peak to peak differential signal) instead of a + or - 5v (10v peak to peak differential signal).


Does this help explain what's confusing me? Is what I did right? Is there a better way to do it? From my understanding, the only thing I need to do with this setup to make it work over longer distances is to add two voltage followers (non inverting op-amps with no gain).

Thanks again for the help sofar!

[taggalucci]

Thanks for your reply

-10 to +10v is 20v peak to peak. I'm having great difficulty understanding why you said this. I would understand if you said -5v to +5v .. or +2.5v to +12.5v (both 10v differential peak to peak)... but I don't understand this 20v swing

Have you read the ILDA spec? http://www.laserist.org/StandardsDoc...finaldraft.pdf

Have a look at page 7. Because the signal is being driven differentially, the signal at each extent will either be +10v or -10v with respect to the "normal signal line (+)". So this this would never reach 20v. the +/-5v is with respect to ground, not the opposing pole. The +/-10v is with respect to the opposing pole.

I hope I got that right!