Cheap composite monitor

[Chuck(G)]

even if it has its limitations.

Like a light pen doesn't work with these?

 

[Eudimorphodon]

Like a light pen doesn't work with these?

That ship has sailed, exploded, sank to the bottom of a trench, and had its fragments carried off by giant crabs.

You know what might be a fun project, though? Designing a circuit that maps the input of a capacitive touchscreen overlay to the position of the “raster” on a LCD screen set up to accept composite input. In principle at least this seems like a “not that hard” project; it should be simple enough to map Hsync/vsync timing into an XY grid and fire a trigger when the touchscreen input coincides with the raster position in time…

Actually, this seems like something you could pull off with the RGB2HDMI device; it already line-syncs a raster with a Raspberry Pi’s framebuffer. Just tack a mouse cursor on top and trigger a GPIO line that activates the light pen input on the host computer.

 

[Eudimorphodon]

... FWIW, the only TRS-80 light pens I know of are the "other kind" that should probably work on an LCD as long as it doesn't have more than a frame or two of lag, IE, the "draw a few boxes on the screen and flash them to figure out which one you're pointing at" variety. Like this. This method was actually used for quite a few video game light guns (For instance, the NES Zapper.); the reason they usually break on LCDs is frame lag, not because they can't "catch the raster". Which I guess opens up another interesting can of worms for trying to come up with a definitive workaround.

 

[Phil_G]

My SC/MP running via a chatpad keyboard and a 4.3 inch composite reversing screen, £12 including delivery from ebay,
I bought one then a couple more when I realised how good they were.
Fifty pence for size reference, I couldnt find a banana

 

[Chuck(G)]

... FWIW, the only TRS-80 light pens I know of are the "other kind" that should probably work on an LCD as long as it doesn't have more than a frame or two of lag, IE, the "draw a few boxes on the screen and flash them to figure out which one you're pointing at" variety. Like this. This method was actually used for quite a few video game light guns (For instance, the NES Zapper.); the reason they usually break on LCDs is frame lag, not because they can't "catch the raster". Which I guess opens up another interesting can of worms for trying to come up with a definitive workaround.

Of course, that's not using the light pen feature of the 6845... I wonder how many retro fans look at the data sheet today and say "what's a light pen"?

 

[Eudimorphodon]

Of course, that's not using the light pen feature of the 6845... I wonder how many retro fans look at the data sheet today and say "what's a light pen"?

That said, even back in the day how many people actually got to use a light pen, even if their computer had a port for it? They were never exactly common.

The Commodore VIC-20 and 64 had support for them and it was fairly easy to DIY one, so it's not like cost was prohibitive by the mid-80's, but... did anyone actually use one? I have vague memories of seeing them once or twice at computer shows/user group meetings, and that's it.

 

[Chuck(G)]

Not on the PC, but on a 8-bit system using an 8275 CRTC. For very coarse selection, such as an array of 10 boxes, it could be useful, I suppose. I really don't know if the resolution was much better than a touchscreen using an x-y array of IR LEDs and phototransistors placed in front the screen.

 

[Eudimorphodon]

I really don't know if the resolution was much better than a touchscreen using an x-y array of IR LEDs and phototransistors placed in front the screen.

The 6845’s light pen register latches the current refresh memory location when strobed, so in theory at least it gives you character cell-sized resolution. But there is a gotchya there that depending on what latching/delays might be going on in your pixel generation hardware you might need to apply a correction factor. (So many weird things happen with 6845s in practice, like the 6845 being effectively clocked at half the character rate with the bottom memory address bit being derived from the CRTC’s clock phase. In a case like this your minimum granularity will be two cells horizontally.) So, yeah, it’s not exactly high res digitizer material.

(Looks like the 8275 latches the current character/row counter values, so, effectively the same theoretical resolution.)

 

[Zare]

Can't comment for back in the day since I was born in 80s but lightpen is something we were schooled about and nobody ever saw one. Every other peripheral taught, printers, plotters, mice, joysticks, whatever else, could be seen around.

 

[mdog69]

My SC/MP running via a chatpad keyboard and a 4.3 inch composite reversing screen, £12 including delivery from ebay,
I bought one then a couple more when I realised how good they were.
Fifty pence for size reference, I couldnt find a banana

Toying with getting one of these.

Does that screen have three connections - DC power, "reverse" (output flipped L/R) and "AV" ("straight" input)?
I assume the monitor will normally use the "AV" input and only switches to the "reverse" input when it detects a signal (which is provided by a camera which is activated by reverse)? Or is there another connection to detect when you engage reverse?

(Asking so I don't end up with a turkey)

 

[Phil_G]

It has simple composite and audio in, no mirror image, no reversing guide bar overlay,
its just a simple composite monitor and therefore ideal for our purposes.
For £12 they are an absolute bargain, I dont know how they make them for that price.
I generally use a subset of the Geoff Graham PIC, gives async-serial in composite video out very cheaply and effectively.
The audio is ok for beeps, but you cant expect hifi from a 1" speaker...