CX-75 light pen oddities

[phaeron]

I recently acquired a CX-75 light pen to check actual hardware behavior. It seems to work, and confirms one thing I had suspected, that the light pen has a significantly different offset than a light gun. However, there are a couple of odd behaviors that don't seem to be related to the pen degrading from age. Wanted ask if anyone else had a light pen and CRT and could check if they were seeing similar things on their setup.

 

The first issue is that there is quite a bit of noise in the horizontal position. This is from drawing vertically slowly in Atari Graphics with smoothing set to the lowest setting:

 

 

However, it's not random noise, because if you hold the light pen steady in one spot, the position is stable. It seems to be position-dependent noise. My theory is that it's picking up the phosphor pattern on the CRT, although the range of noise is a bit high for that. The other theory I have is that the synchronization on ANTIC's light pen input is not very good and causes errors when latching the horizontal position counter. Either way, the noise is pretty bad at times, it's enough that I have trouble hitting the right palette color area when using the color mixing panel to change a color to white.

 

The second and more surprising issue is that pressing or releasing the switch on the pen tip also seems to fire the light pen trigger (!). This causes bogus positions to be latched into ANTIC. I tapped the light trigger and pen switch signals and the scope confirmed that the switch is triggering the light pen trigger, sometimes even multiple times (CH1 yellow is light sensor connection to joystick trigger, CH2 cyan is pen tip switch connected to joystick up):

 

 

In fact, the CX-75 seems a bit prone to noise in general, if I hover my hand near the joystick connector I can make the light pen pick up the end of horizontal blank signal. It's pretty amusing, I can move my hands closer or farther and change the triggering rate like a Theremin.

 

So... anyone else got a light pen and a CRT who could verify? Also, anyone got an XG-1 light gun and a scope? I'd love to see measurements of the width of the triggering pulse for an XG-1. Wanted to pick one up but they seem to be very expensive these days.

 

[Rybags]

Fairly sure Antic latches PENH on both up and down transitions on the TRIG input.

 

Could it be some passive component/s in either or both the computer or pen that have aged (think caps or resistors maybe) and cause excessive jitter?  Sort of similar to what we get from PIA PORTA inputs (especially 0->1 transitions having that shallow rise slope)

Maybe also the whole thing is prone to interference - how about a ferrite core around the controller cord or something?

 

I've got a CRO and XE light gun but won't likely have spare time to look at it in the next week.

[+Stephen]

I have a light pen (not sure the model - I've never used it).  I'll try to fire it up this evening.

[gilsaluki]

11 hours ago, phaeron said:

I recently acquired a CX-75 light pen to check actual hardware behavior. It seems to work, and confirms one thing I had suspected, that the light pen has a significantly different offset than a light gun. However, there are a couple of odd behaviors that don't seem to be related to the pen degrading from age. Wanted ask if anyone else had a light pen and CRT and could check if they were seeing similar things on their setup.

 

The first issue is that there is quite a bit of noise in the horizontal position. This is from drawing vertically slowly in Atari Graphics with smoothing set to the lowest setting:

 

 

However, it's not random noise, because if you hold the light pen steady in one spot, the position is stable. It seems to be position-dependent noise. My theory is that it's picking up the phosphor pattern on the CRT, although the range of noise is a bit high for that. The other theory I have is that the synchronization on ANTIC's light pen input is not very good and causes errors when latching the horizontal position counter. Either way, the noise is pretty bad at times, it's enough that I have trouble hitting the right palette color area when using the color mixing panel to change a color to white.

 

The second and more surprising issue is that pressing or releasing the switch on the pen tip also seems to fire the light pen trigger (!). This causes bogus positions to be latched into ANTIC. I tapped the light trigger and pen switch signals and the scope confirmed that the switch is triggering the light pen trigger, sometimes even multiple times (CH1 yellow is light sensor connection to joystick trigger, CH2 cyan is pen tip switch connected to joystick up):

 

 

In fact, the CX-75 seems a bit prone to noise in general, if I hover my hand near the joystick connector I can make the light pen pick up the end of horizontal blank signal. It's pretty amusing, I can move my hands closer or farther and change the triggering rate like a Theremin.

 

So... anyone else got a light pen and a CRT who could verify? Also, anyone got an XG-1 light gun and a scope? I'd love to see measurements of the width of the triggering pulse for an XG-1. Wanted to pick one up but they seem to be very expensive these days.

 

I am a prolific Atari light pen user.  Posted of few works here on site.  One thing to ensure is to check and ensure the intensity, brightness is set correctly. Too bright a screen, the pen won't track.  Just play around with it.  Make sure you are indeed on the finest pencil/pen width setting.  Play with that.  I always wipe my screen with my hand, discharge the static, that helps a lot.  Don't do this while wearing headsets listening to UFO or Rush, the static will shock your ears (really).  

[phaeron]

9 hours ago, Rybags said:

Fairly sure Antic latches PENH on both up and down transitions on the TRIG input.

 

Could it be some passive component/s in either or both the computer or pen that have aged (think caps or resistors maybe) and cause excessive jitter?  Sort of similar to what we get from PIA PORTA inputs (especially 0->1 transitions having that shallow rise slope)

Maybe also the whole thing is prone to interference - how about a ferrite core around the controller cord or something?

 

I've got a CRO and XE light gun but won't likely have spare time to look at it in the next week.

I don't think ANTIC latches on both edges, ijor's reverse engineered schematic shows what looks like a falling edge detector on the LP pin:

 

That matches the description in the hardware manual and the behavior I see with a joystick trigger, though it does frequently update on release anyway due to switch bounce. (Need to clean these joysticks, it's not like the fire buttons were ever that great 30 years ago, much less now.)

 

Ferrite coil's an interesting idea, I think I have a snap on one for HDMI that might work. Probably completely inappropriate for the signals, but what's the worst that could happen. I took scope measurements on the controller port side of the inductor (L30 on an 800XL), but maybe I should also probe farther into the computer like on the ANTIC side of the AND gate. I'm also curious what's in the connector as the CX-75 has got to have the chonkiest joystick port connector I've ever seen for an Atari peripheral, it barely fits into a 130XE. But I haven't seen a schematic for the CX-75 anywhere.

 

Appreciate any time that you, Stephen, or anyone else can provide here -- no rush, just trying to collect data to improve emulation. This kind of data is hard to get without actual equipment measurements. When I saw that the CX-75 was relatively cheap I pounced on it.

 

1 hour ago, gilsaluki said:

I am a prolific Atari light pen user.  Posted of few works here on site.  One thing to ensure is to check and ensure the intensity, brightness is set correctly. Too bright a screen, the pen won't track.  Just play around with it.  Make sure you are indeed on the finest pencil/pen width setting.  Play with that.  I always wipe my screen with my hand, discharge the static, that helps a lot.  Don't do this while wearing headsets listening to UFO or Rush, the static will shock your ears (really). 

Thanks, I did clean the screen and take measurements regarding the brightness threshold for the light pen -- it starts dropping out around luma 2-4 and the effects above are with colors well above that threshold. Found a video on YouTube that shows the same effects, you can see the horizontal jittering of the Atari Graphics cursor if you look closely:

 

 

Although, reading the description, it sounds like he got the same model from the same source as me, so possibly from the same batch of light pens.

 

[phaeron]

Tapped a couple of points in the /LP path, and turns out that the ringing on the trigger line wasn't as innocuous as it looked. CH1 (yellow) is controller trigger line, CH3 (purple) is after filter on input to 74LS08, CH2 (cyan) is output of 74LS08 to ANTIC /LP. The light pen is glitching the /LP input and that's why the errors are occurring -- I'm guessing that the phase relative to the system clock matters as to how this affects the latching.

 

 

After this, I had a suspicion that the XL/XE design might be part of the issue and tried the light pen on an Atari 800. Sure enough, the horizontal position is much more stable on it, which means that both the specific model of light pen and computer matter. Looking at the schematics, the 800 differs quite a bit from the XL/XE models in the way that it generates the /LP input from the controller triggers.

 

[ascrnet]

interesting research, I once had a light pencil based on this design for XL/XE I think it is simpler than the CX-70. anyone made it? ?

[Irgendwer]

I own a (quite sophisticated) "Turbo Computer Light Pen" and while developing the CLM17 a noticed that the light pen is a much more "direct" device contrary to a light gun which has AFAIR a delay of about a half scan line until reporting the beam back to the machine.

 

IMHO an important thing, often overlooked, is that a light pen/gun is much more a pointing device than usable for delivering constant positions. (I regard "Atari Graphics " not as ideal in this regard.)

Contrast to background was already mentioned, so it's impossible to draw a continuous black line on a white background while crossing the just created line during draw would mean loosing track of the position estimation.

So the obligatory "flash" (like in light-gun applications) is also of great use in light pen applications (sticking to e.g. poly-line control point creation).

The jitters can also be reasoned by not delivering enough light to the sensor (blue background), making triggering harder, or reaching light from the line above partially the sensor.

But as we all know, precise horizontal estimation is always more difficult timing wise and more critical by design.

Edited May 1, 2022 by Irgendwer

[Mclaneinc]

We at Maplin did a light pen kit, which I built for fun, found it to be extremely jittery as you can see from Paul's video, perhaps worse than the Atari own pen. As said by others, the crt has to be at a certain level of brightness or the control just fails of is extremely random. I think they were a fun toy but a real pain for an actual artist to use without constantly correcting stuff. Interesting that the floating electricity of the human body actually has an effect on the pen. I'm one of those people who can actually not need to press the screen on my phone to make it work and the phone does not have a fancy height setting like some do where you just need to hover near the screen. The number of falsely made calls I do is annoying to say the least.

 

Interesting to have light pen usage in Altirra, as always, appreciated Avery..