OK, 14H30 here, spent all my time since I got up, working on this thing !
Did what I said last night, took the time to probe all 8 pins on that back plane connector, to look more closely at what might be video signals.
It all went well and I thinkj I can confirm it's indeed our much desired video output, and it's well and alive !
So as explained, no appropriate Dupont wires here, so I dug out and old floppy drive ribbon cable and sacrificed it. Works just fine. I was able to secure my scope probe and have both hands free to observe the signal at great length.. and take pics and even a video, going modern here !
So here is what I could figure out :
Pin #1 = GND
Pin #2 = Slow Clock signal, about 21.5ms / 46.5Hz , short 1.04 ms pulses.
Pin #3 = Video data / signal
Pin #4 = +12V
Pin #5 = Fast Clock signal, about 65µs / 15kHz, short 8µs pulses.
Pin #6 = Video data / signal
Pin #7 = Video data / signal
Pin #8 = +5VSo we have 3 video signals. So that means the programmer, well its big brother the 5000D version, was meant to be connected potentially to fancy colour monitors.
Unsurprisingly, since the CRT controller is monochrome only, I witnessed that all 3 video signals showed identical content. I guess this makes it compatible with colour monitors.
So that's good, it means I don't need to search for an antique monochrome only vintage monitor. A colour one hence maybe of a more modern / easier to find, design, might be compatible. With some luck we might find out later that it's compatible with good old VGA, in which case I can just grab any of my old monitors here !
But it's too early to be that optimistic just yet...
Clocks : I guess the slow one could be the vertical sync signal, telling the monitor when to start drawing a screen / frame ? 46.5Hz is about the kind of refresh rates we used to have on old monitors eh ? Sounds plausible.
Then the faster clock I guess is the horizontal sync signal ?
My time/frequency measurements are only approximate of course, just me trying to measure them as best I could on the scope, but maybe that's good enough to try to figure out what kind of old video standard it might correspond to ? VGA or not ? CGA ? Whatever.... anyone knows these things ?
Signal amplitude : I notice that the video signal is not TTL / 5V, despite the CRT chip being of course TTL levels. They get only to 4V.
I am thinking that it may be normal. Maybe the last 1V is used as headroom to make the text brighter than the normal text.
OK so now for the dessert : I scoped, and filmed, what the video signal is doing during the entire bot process.
I used two probes. First signal at the top, in small to preserve screen real estate, is Vertical Sync, for reference, so that we know where a frame starts and ends, so we can make sense of the contents of the video data, know where it starts where it ends.
The boot process / video lasts about 1min30sec so I am sure you will make the effort to watch it, hopefully....
Before you do watch it, let me explain :
- I set the time base to 5ms so that we can see two screens worth of data : one screen on the first half of the graticule, then another screen, and towards the end we see a little bit / beginning of the third screen.
- I gave up using the scope (my Philips Combiscope) in analog mode because it was unwatchable and signals hard to see. Switched to digital mode. The pulses look crap of course, but at least the screen is not blinking, the brightness is even between ground level and the pulses, and you can see easily the pulses. In short it looks horrible but it conveys the important message much better... so I went for that. Lesser of two evils.
OK so here is what is going on :
- First, you see no signal. Programmer is powered up.
- Then I power it up, and soon signals stabilize.
- We can see that the video content appears to start on the falling edge of the Vsync signal.
- We have data/content during the first third of the screen, then dead space for about half of the screen, then at the end we have some content again.
- This content remains unchanged during the entire boot process
- Then when the boot is complete (you can hear the floppy drive stops making noise), the content changes, with only a little bit of data at the very beginning and at the very end, and all dead space in between
- Now during the boot process, do you notice something interesting ? Look at the first packet of data. At the end of this packet, there is an area that literrally "blink", regularly, like clock work, during the entire boot process. It blinks at the same speed that a blinking cursor on a text screen would !
SO !
From all that, I think we can say that :
At power up, the programmer displays some content for the top third of the screen, maybe some splash / welcome screen, and below that a blinking cursor to indicate that booting is in progress. The at the very bottom of the screen, some content again, maybe a status bar or god knows what.
Once booting is complete, it clears the screen, cursor stops blinking, and we are left with only a bit of text at the top, maybe a command prompt, and a little bit of text at the bottom, maybe a status bar, menu bar detailing options, whatever.
How cool ! Just looking at my scope and living the boot process as if I had a monitor actually plugged in, trying to imagine what would be going on on the monitor !!
Oh I love this thing, so fun and exciting working on it !!
OK so we are making good progress here aren't we ? We know we have a video output, that it's actually active even though not available to the user at the rear. That's fantastic news. And we now also have details about the signals being generated, and their timing and amplitude !
So now let's figure out what kind video standard that is, and let's plug some monitor !!!
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