Does anyone have any idea how a graphics tablet works? I recently bought one and I love it but I can’t figure out how it works. It has to plug into both the serial port and the keyboard port. It also has both a wireless three button mouse and a wireless pen that can tell how hard I am pushing on the pad. The pen also has an eraser! I hope I’m not resting my hand on some kind or antenna or something like that.
Don’t know the mechanics of the actual tablet, but the keyboard connection is to get power.
Not so sure about that billehunt. It is definitely possible (and even common) to draw power from the serial port. Moreso than from a ps/2, but that’s probably because the ps/2s are usually taken up by keyboards and mice.
TheNerd wrote
Must disagree. Which pins would that be from? Here’s a site that shows DB9 and DB25 RS232 pinouts.
Inever a good idea to draw power from some signal that you assume is high, like DSR. First, there’s no guarantee the signal will be high. More importantly, the amperage you can draw from a signal pin will be minimal, certainly not enough to power an appliance.
“Inever” ==> “It is never”
You can difintaltey draw power from the serial port. What you do is set the CTS or asimilar pin on the PC high and use the power from that pin. You can’t draw much power but serial micro work that way.
Serial mice draw power that way. Sorry for the spelling error in the previous post.
gazpacho wrote
Good point. But for peripherals that require any decent juicage, this won’t work. A typical place to get power is the keyboard. I have two or three gadgets that do that, including a Connectix video camera and a portable graphic tablet.
What graphics board???There are so many & so many possible ways they could work. Might have some info at howthingswork.com
I have an older Wacom tablet. I can’t find the manual, and searching Wacoms site doesn’t have the manual for my model.
While searching, I found this link from PC Graphics and Video Magazine, where someone is reviewing a setup, including a wacom tablet. He says,
Surely, this doesn’t close my case, but it’s one data point. I’ll search around a bit more at Wacom’s site.
Okay, I must clarify what I was attempting to say. I didn’t mean that you couldn’t draw power from the ps2, i just thought that it was more common from the serial. For example serial LCD modules do this, and several GPS receivers have the capability to do this (Unfortunately not mine, I have to use the car’s cigarette lighter).
Most graphics tablets have a grid of extremely fine wires inside them as sensors. The pen probably induces a small current in the wire, or changes the capacitance, or using some other similar effect to allow electronics in the tablet to detect which wires the pen is currently over.
You can draw power from a serial port, but you can’t sink a lot of current (20 mA at most, as I recall). As a designer of a component that uses less than that, you still may not want to use the serial port in case someone plugs your hardware in parallel with some other current-sink, like a copy-protection dongle.
The Keyboard connection has a nice +5v connection with lots of available, stable current.
DHanson covered it pretty well…
You can power a device from the serial port, witness mice. But you can’t get much power, witness the lack of halogen lights that plug into serial ports. 
Serial ports experience the same power spikes when something is plugged in as a PS/2 port does, but they are buffered against it. The Keyboard connectors (especially ps/2 for some reason) are not, because you don’t often unplug/plug in a keyboard while the power is on. (And it’s only a chance of problems, not a guaranteed, or even likely, problem.)
My tablet pen has a battery in it. I like it that way. It has a little more weight & its more independent.
Thanks for your answers. It is a new Wacom 4x5 board and plugs into both the serial AND keyboard ports. The instructions DO warn about plugging and unplugging the unit with the computer turned on. The ‘fine wire’ theory sounds about right. I think the resolution is just over 1000/inch. So it must be real fine wires! There is no place to fit normal electronic chips unless they are the very flat style like we had back in the military. Who knows, maybe this thing is just one big chip! (the little woman still won’t let me take it apart yet.) Now if I could only figure out how it knows left, center and right click on the wireless mouse.
According to http://www.ozemail.com.au/~djand755/Hardware/Intuos.htm
Keep in mind that you don’t need 1000 wires per inch to get 1000 steps/inch resolution. The position can be interpolated from the signal strength of the two or three closest wires. Also, a tablet has plenty of thickness for microchips. They are about the thickness of a digital watch, and they contain electronics, right?
Sorry, don’t know how the buttons work…