How does the Asus Pen work, technically?

Question

I've received the Asus Pen for use with my Asus computer.

First, note the official Asus page about the pen says the battery supplied is rechargeable, but it isn't. Asus Customer Support has confirmed, the spec has changed, and the webpage is out of date.

How To Geek has an article about stylus technologies that lists three kinds:

• Capacitive: Just a high-tech version of your fingertip, works by some sort of exchange of electrostatic energy between the pen (or finger) and the screen. No battery required.
• Wacom: Doesn't say how it works, just that it's made by Wacom.
• Bluetooth: Works by communicating with the computer via Bluetooth.

The Wacom one sounded good, so I bought a Wacom Bamboo Ink. Then I found out two things. First, it operates by using Bluetooth. Second, it's not compatible with my computer, and Asus has their own pen that is. (Neither Asus nor BestBuy told me that when I bought the computer.) Fortunately, purchased through eBay, it was returnable.

So, I got the Asus Pen, and the instructions for it are useless. The instruction sheet that comes with the pen has the same content as the online FAQ page. It says something about the buttons, how to install the battery, and how to replace the nib. Nothing about how to turn the darn thing on or about how to establish communications with the computer. I figured it can't be just capacitive since it has a battery.

So I wrote back to customer support, and they told me that, essentially, once the battery is installed, it's always on (that is, of course, and they of course didn't say this, until the battery runs down, for which there will apparently be no warning). And they said, basically, once the battery is in, just use it.

Huh? So, okay, I put the pen up towards the screen, and lo and behold, before it touches, a tiny circle appears on the screen, which moves around as I move the pen without touching the screen. Bluetooth on my computer is turned off, so it's not using that. There must be some kind of electrostatic or electromagnetic communication between the pen and something inside the screen that only requires proximity, not contact. Basically, that circle is a cursor for the pen. If I touch the pen to the screen, I can draw with it, and if I press one of the buttons, I get a right-click menu. If I press the other (erase) button and touch a line that I drew, the whole contiguous line disappears. None of this is explained in the instructions. But anyway, the pen is working, and my question is, how?

As for the How To Geek article, I see that the Wacom pen I bought operates by Bluetooth, so I wonder if the Asus Pen is operating by the unexplained technology that the article called "Wacom."

So, can someone say how the Asus Pen works, or better yet, post a link to a technical article on how it works?

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Update: The answer from RamHound says the pen is working via Bluetooth. Here is the reason I say Bluetooth is not on. The following screen image is from "Windows > Settings > Devices":

The pen is operating with this setting. That's why I'm pretty sure it's not operating via Bluetooth. Can I be wrong about that?

Answer 1

So, can someone say how the Asus Pen works,

The pen does indeed work through Bluetooth (the buttons on it are programmed to specific actions), if your Bluetooth was enabled, the buttons like the eraser would work.

There are only two pen technologies Wacom and N-Trig (owned by Microsoft). There are no other active pens on the market.

Based on the 1024 levels of sensitivity the ASUS pen supports the technology behind it is N-Trig. Which means the pen is an ASUS branded N-Trig (active) pen.

There must be some kind of electrostatic or electromagnetic communication between the pen and something inside the screen that only requires proximity, not contact.

The communication is done through the touch screen on your device.

or better yet, post a link to a technical article on how it works?

The instruction sheet that comes with the pen has the same content as the online FAQ page.

It’s supposed to be that easy. Since Microsoft acquired N-Trig the only documentation that exists is either through ASUS or Microsoft (I.e for Surface Pen).

But anyway, the pen is working, and my question is, how?

Willing to bet that BT is actually enabled.

As for the HowToGeek article, I see that the Wacom pen I bought operates by bluetooth, so I wonder if the Asus Pen is operating by the unexplained technology that the article called "Wacom."

Wacom does not have a pen with 1024 levels of sensitivity on the market to my knowledge.

ASUS pen also looks identical to that of the Surface pen.

Answer 2

You are totally correct, the pen does indeed NOT use Bluetooth.

Judging by the date, this is probably the model you got: https://www.asus.com/ie/accessories/stylus/all-series/asus-pen/

The "Asus Pen 2.0" (SA203H) does support bluetooth but that's a newer pen with a rechargable battery, the older SA200H and SA201H models and this one do not have Bluetooth and take an AAAA battery instead.

Your pen uses a special extra layer in the display specifically designed for reading the active pen position and data.

The pen emits electromagnetic induction that's sensed by the pen layer in the display assembly. Data is sent to and from the pen following MPP (Microsoft Pen Protocol, formerly N-Trig), the protocol is just a set of rules of how the data should be structured so that all devices that implement it speak the same language and the pens from different manufacturers work on each other's devices.

The pen emits information like the pressure (your pen has 1024 pressure levels that range from 10 to 300 grams), judging by this, your pen is probably MPP 1.5 and does not support tilt information. The later models like SA201H work with MPP 2.0 and do send tilt information. The only thing active pens don't send yet is roll information, which is quite a bit more complex, Apple Pencil Pro does do this and it measures roll using a gyroscope sensor in the pen.

Your pen has 0.2mm linearity, that just indicates how far off the pointer might go from your actual pen position when drawing a line.

Usually the touch screen and pen layer are connected over an I2C bus (kinda like USB but embedded).

If you go into Device Manager > Human Interface Devices (HID), you can find HID-compatible touchscreen and HID-compatible pen. Below that is also I2C HID-device. If you right click and disable the touch screen there, the touch screen will be turned off. The pen will still work though, so it's possible to have no touch screen but have the pen work as the touch screen and the pen layer are a separate thing, though they are a similar technology. You can also disable the pen and have the touch screen enabled.

When you bring the pen near the display (about 1cm) it wakes up a low power microcontroller in the pen and begins transmitting data packets structured according to MPP. I think the polling rate is dynamic usually 120Hz or higher.

The pen can be sensed from a distance without touching because the field can be sensed from a distance, this is the case for regular touch too but they normally tune touch screens to only consider a measured disturbance in the field a touch when it goes beyond a threshold which is normally calibrated to be exactly when the skin makes first contact with the display. Active pens do emit a specific field instead of just disturbing the field of the display like your finger does, this allows the digitizer to way more accurately know the difference between a pen and something else and also sense it from a larger distance.

The pen does not use the data of the pressure sensitive tip to determine when the pen is actually touching the display, it just uses the field strength, the same way it determines wether a finger is touching or not. I suppose that's done so that if your pen maybe fell on the tip and the pressure sensitivity is off, it will still properly determine the difference between hovering and touching and it's probably also just more accurate to do it like this rather than using the pressure data.

I tried an HP and Lenovo pen on an Asus touch screen and they do not work on the Asus, but the HP works on Lenovo and vice versa, the Asus pen does not work on both the HP and Lenovo laptop I tried. Turns out HP, Lenovo and Dell and some others use AES which is Wacom's proprietary protocol and everything separately designed. I can imagine many people bought an AES pen online thinking it'd work on their device with MPP digitizer, and vice versa.

AES sends capacitive bursts while MPP sends capacitive modulation patterns. MPP is an open spec, Microsoft Surface, Asus, Dell and HP seem to support MPP (might vary on specific device models but I'm sure all Microsoft Surface devices use MPP). Asus pens would thus work on Microsoft Surface devices, and Microsoft Surface pens would owork on Asus devices.

There is also Wacom EMR which is used with pens that don't have a battery in them.

The newer pens that support Bluetooth only use Bluetooth for extra features like some extra button, so if your pen has an eraser button it has the entire Bluetooth thing in it just for a single button. Bluetooth does mean the pen doesn't have to be near the digitizer to transmit the data over MPP, so you can use the button on the pen from a pretty decent distance to perform actions from a distance in presentations or so. Bluetooth can also send battery information, ususally the Bluetooth pens come with a rechargable Lithium battery built in too instead of AAAA, which makes sense since Bluetooth draws much more power and the alkaline battery would have to be replaced too quickly instead of last for like a year.

It's definitely very interesting and I also only know the tip of the iceberg on how it actually works, finding info about the real workings of devices isn't easy as nearly everything online is either an oversimplification or it's straight up false information. I'd love to hear in depth explanations of exactly how it works from someone who knows what they're talking about, but I'll have to rely on my own research for now.

It's been a while but I hope this information is still interesting to you as it is to me.

Answer 3

Look for something like “active stylus” in the Windows settings. With that turned on, the Asus will, indeed, work as soon as it has a battery.