15kpps scanner signal, what are the values?

[buffo]

Correct.

All scanners are designed to accept -5 to +5 signals. That's the ILDA standard.

Adam

[JohnYayas]

The delta is actually +10V to - 10V. With differential inputs, one pin can be +5V while the other is at -5V. So, relative to gnd it is +-10V. If you hook up these pins to a 15K scanner you will overdrive it because it only uses +-5V. You'll need to hook these scanners up to only one of the ILDA XY pins and to pin 1(gnd). Almost all other scanners expect to be hooked up to both XY (+ and -) pins. If you hook them up as described for the 15K scanners your image size will only be 1/2 of what it should be. This is one of the first things I found out when I got into this hobby.

[buffo]

The delta is actually +10V to - 10V. With differential inputs, one pin can be +5V while the other is at -5V. So, relative to gnd it is +-10V.

I think you're confusing terms, Gary.

If you convert the ILDA standard +/- 5 volt differential signal to a single-ended signal that is referenced to ground, you will get zero to +20 volts. And if you use a differential receiver (like most scanners do on the input), then the signal varies above and below ground by 10 volts (so +/- 10 volts).

In both cases you still have the same peak-to-peak voltage (20 volts), but one alternates above and below ground potential and the other is single-ended, so positive voltage only.

But *both* of these outputs are a result of the same ILDA standard input, which is +/- 5 volt differential. That's what the scanners are designed to accept on the input. When you see an X+ and an X- pin on the input to the amp, you know it's looking for a differential input. (Admittedly, many amps also include a signal ground pin for single-ended input sources, but it should not be used for differential inputs.)

If you hook up these pins to a 15K scanner you will overdrive it because it only uses +-5V.

The circuit above generates a +/- 5 volt differential output from a 0-5 volt single-ended input. The scanners are designed to accept a +/- 5 volt differential input. Where are you saying that they will be over-driven? They are designed for the ILDA standard input, which is what the circuit produces. (It's no different than connecting the scanners to a standard DAC...)

You'll need to hook these scanners up to only one of the ILDA XY pins and to pin 1(gnd).

If he wanted to do that, he could do that directly with the 0-5 volt output he has. He wouldn't need the circuit above.

And as you pointed out, doing this will reduce his image size. Although it will reduce it to 25% of max, because it's a single-ended source, so the x+ can't go below zero. Only if the X+ pin went negative would he be able to get to 50% of size.

But the whole point of the circuit is to produce a true differential output from his single ended input so that he can get the full size from the scanners.

Adam

[dchammonds]

The delta is actually +10V to - 10V. With differential inputs, one pin can be +5V while the other is at -5V. So, relative to gnd it is +-10V. If you hook up these pins to a 15K scanner you will overdrive it because it only uses +-5V. You'll need to hook these scanners up to only one of the ILDA XY pins and to pin 1(gnd). Almost all other scanners expect to be hooked up to both XY (+ and -) pins. If you hook them up as described for the 15K scanners your image size will only be 1/2 of what it should be. This is one of the first things I found out when I got into this hobby.

I'm a bit confused- are you saying that typical 15K scanners have a different gain than others, or have single ended inputs?

[dchammonds]

In both cases you still have the same peak-to-peak voltage (20 volts), but one alternates above and below ground potential and the other is single-ended, so positive voltage only.

Adam

In single ended systems that use +/- split supplies you can deal with both positive and negative...

[JohnYayas]

I'm a bit confused- are you saying that typical 15K scanners have a different gain than others, or have single ended inputs?

15K scanners that I used only had one input that accepted a +/- 5V input. Of course there is a ground that is connected to pin 1 of the ILDA cable. This +-5V will give full deflection on the galvos. (10v p-p)
Other scanners have two inputs, to allow for differential inputs. These are for the Ilda + and - pins. Because the + and - pins are opposite this will give a +-10 differential voltage so (20v p-p).
In other words, 15K scanners only need half the voltage of other scanners for full deflection. I think what Adam and I are saying are the same thing but you can't supply that 20V p-p to a 15K scanner or else it will be overdriven by 10V!!

[SaltyRobot]

I thought the reason why ILDA standard is +/-5V is so that you still have an additional +/-5V for offsetting the ILDA pattern around the screen. If your DAC was allowed to make +/-10 there would be no room left for offsetting your pattern. When I first started in lasers I was using function generators with 15K scanners and at times had signals more than 5vp-p. Never had an issue but also didn't use the scanner for more than a couple years. Maybe I was damaging them? Didn't seem like it when the analog pattern was huge. Once I moved to DAC output the first question I asked was why is the pattern so small. At the time it was explained this was so the pattern could be offset around the screen and still maintain the ILDA test pattern.

Have I been thinking about this wrong all these year?

[buffo]

15K scanners that I used only had one input that accepted a +/- 5V input.

Ah... I see. This is where the confusion is coming from. I have a few sets of 15K scanners in my collection as well, but they all have differential inputs (so both an X+ and an X- pin).

But yes, I can see if you had a set of scanners that was not designed to accept the standard ILDA X/Y signals, then sending the full output would over-drive them.

In other words, 15K scanners only need half the voltage of other scanners for full deflection.

I would amend that to say that *some* 15K scanners only need half the voltage to get full deflection. It comes down to how the inputs are labeled on the amps.

If you have X+ and X- (or more commonly X+, gnd, and X-), then they are designed to the ILDA standard and will accept normal differential signaling.

But if you only have an X+ and gnd pin on the input, then the half-voltage statement applies.

I thought the reason why ILDA standard is +/-5V is so that you still have an additional +/-5V for offsetting the ILDA pattern around the screen. If your DAC was allowed to make +/-10 there would be no room left for offsetting your pattern.

Well, yes and no. Ideally if you are displaying graphics you won't be scanning at full angle anyway. (After all, the scanners are only rated for 8 degrees, at least when displaying the ILDA test pattern, which is arguably one of the most challenging patterns.) So in that case, having the extra headroom on the signal input does allow you to offset the pattern to position it where you want it on the display surface.

However, when you are scanning beams the pattern is relatively simple. In that case it's often desirable to scan as wide as possible. So in that case you will be using the full bandwidth of the signal input and you won't have any headroom left to adjust the absolute position.

When I first started in lasers I was using function generators with 15K scanners and at times had signals more than 5vp-p. Never had an issue.

This is a common source of confusion, and it's what Gary was referring to earlier when he said that it was one of his early learning experiences.

So we all know that the ILDA standard for X/Y signals is "+/- 5 volt differential". But what does that mean? It means that you have two wires labeled X+ and X-. Each of these wires (or pins, if you will) can be at any voltage between -5 volts and + 5 volts. So if you take the *difference* between the X+ and X- wires, you end up with the following maximum possible values:

Voltage on X+	Voltage on X-	Difference
+5 volts	-5 volts	+10 volts
-5 volts	+5 volts	-10 volts

As you can see, the total swing between these two maximums is +10 volts to -10 volts, or a total peak-to-peak voltage of 20 volts.

Thus, any differential receiver that is designed to accept an ILDA standard position input (which, remember, is a +/- 5 volt differential signal) will generate a 20 volt peak-to-peak output.

When you were sending just 5 volts peak-to-peak to your scanners, you were only driving them at roughly 1/4th their maximum. So yeah, no risk of overdrive there.

The confusion results when one person is talking about the voltage limit on the signal (+/- 5 volts) while the other person is talking about the output of the receiver stage, which is 20 volts peak-to-peak. They are really just two different ways of saying the same thing.

Now, in Gary's case, he has a set of scanners that only have inputs for X+ and ground. In this case, they are not designed to accept a full ILDA signal, and he is correct in saying that they can't handle more than 10 volts peak-to-peak. Thinking back, I'm pretty sure I've seen one or two amps like this before myself, although they were always combo-boards that had both X and Y amps on a single card. (Maybe the old "Big Dipper 2000" Chinese projectors that were so popular back around 2005 or so?)

The 15K scanners I have now all support normal ILDA inputs with differential signalling. But it's possible that you might still find some non-standard amps that don't.

Adam

[SaltyRobot]

A couple follow up questions.

Is it noticeable when you over drive them. I am thinking clipping most likely? Like what would you see if you put differential on 15k that only had +/-5 and GND pins.

Second question. My dad uses G124. Any idea what the proper signals for those are? I want to say they are single ended and not looking for a differential signal.

This is very helpful information because while building my analog console I have been thinking about things incorrectly.

[buffo]

Is it noticeable when you over drive them?

Yes. You just hit a wall and they won't scan any further in that direction. Whether this damages anything depends on how the scanner amps were designed, but most amps will simply limit the output and you end up with a squished looking pattern.

what would you see if you put differential on 15k that only had +/-5 and GND pins.

Let me re-phrase your question first. If the amp had an X+ and Gnd input (and no X- input), and you put a standard ILDA +/- 5 V differential signal on it (with X- connected to the gnd pin), THEN if you started by displaying a spiral at a small size and gradually increased the size, you would see the pattern expand as you increased the size until you reached the limits of the amp, at which point the outer curves would begin to squish together to form a square (that doesn't get any bigger) while the center of the spiral continued to expand towards those hard outer edge limits. And because the amp wasn't designed for a differential input, you would hit this "wall" much sooner than you otherwise would expect if you were using an amp designed for ILDA standard differential signal input.

Saying that the amp had +/- 5 and GND could be construed as +X, -X, and Gnd, which is actually the standard you'll see on most ILDA scan amps. Just trying to avoid confusion.

My dad uses G124. Any idea what the proper signals for those are?

If he is using the G124 accelerator amps (they come on a PCB card with 10 contacts on the card-edge, and you need 1 card for each scanner), then I believe they only use the X+ and Gnd connections for signal input. I don't know what the voltage limits are though. Aron Bacs (Lumia) can definitely comment more extensively on this...

I want to say they are single ended and not looking for a differential signal.

I believe you are correct.

Adam