TheaterFire

Photo of the Day

Posted by Current_Yellow7722@reddit | vintagecomputing | View on Reddit | 9 comments

Photo of the Day
It's the weekend, so, I got the blurry photos. Any idea of the computer?

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9 Comments

Far_Relationship_742@reddit

I want to go to there.
View on Reddit #85304499

Parking_Constant_960@reddit

It’s a light pen terminal system, the light pen detects an electron beam hitting the screen of the crt and sends coordinates to the computer which now knows where you are. Then you can use the light pen like a stylus! It’s a rare sight to see one today irl.
View on Reddit #85000067

agent_flounder@reddit

That's super cool. This vaguely reminds me of some computer I used briefly long ago. I wish I could remember. How would this thing work, though? The stylus has no way to know its own position... Maybe the pen sends a signal and then something can determine where (which pixel) it was processing when the pen signal arrived and make that available to the CPU. I wonder how the pen detects the electron beam in blank spots? I guess there's some minimum number of electrons shot at the dark spots, ie minimum brightness. Fascinating. I wonder why it didn't catch on.
View on Reddit #85019126

luckless_optimist@reddit

As someone who made a couple of light pens as a kid, I can answer this one. Put simply, you need to have a lit pixel to detect the pen position. The basic idea is to wait for an interrupt signal from the pen. There is a button on the pen, like the fire button on a joystick. When the button is pressed it triggers an IRQ signal that then transfers control to a separate routine. That routine reads the current position of the election beam from the video circuitry. Now, for a terminal it's pretty easy to ensure the pixel is lit up. When the button is pressed the screen can be flashed as a solid colour, or the text can be inverted. For the former, this ensures every pixel is lit up and when the beam passes the photo transistor in the pen then that triggers the video circuitry to record the position in a couple of registers representing the x and y position on screen. That's exactly how the light gun worked on the NES. For the latter, if the first pass didn't yield any results then the screen can be inverted and the pixel detected. Inverting the screen is trivial for a terminal since it has full control of the video attributes of the characters, and taking 1/30th second to get the position rather than 1/60th isn't something a human is going to be able to distinguish.
View on Reddit #85207372

agent_flounder@reddit

Fascinating! I really appreciate you taking the time going into detail on how this works. I didn't realize the pens had a button to trigger the position capture. Cool.
View on Reddit #85207950

Parking_Constant_960@reddit

It could be a Tektronix 4010 / 4014
View on Reddit #85000084

Parking_Constant_960@reddit

But I actually think it could be just a concept photo.
View on Reddit #85000111

phire@reddit

The house is certainly a concept, with the computer terminal that folds away into a coffee table, and... is that a videophone in the wall? So yeah, I'm not sure that's a real computer or terminal,
View on Reddit #85004430

Parking_Constant_960@reddit

It most certainly could have worked if you had a couple million dollars
View on Reddit #85009613