5
\$\begingroup\$

I am hacking around with an old (CRT) VGA monitor. Trying to make an oscilloscope.

What I want is to make the monitor display a blank white screen at all time, when the power is connected of course. I do not care if I cannot use my monitor for its normal job anymore.

Is it possible to short some pins of the VGA connector to mimic a blank white screen?

\$\endgroup\$
  • 3
    \$\begingroup\$ I 'think' you'll need at least HSync and VSync pulses in the range that the monitor accepts. Where are you getting those from? I think you can tie the RGB lines to a voltage on the pin as I remember. \$\endgroup\$ – kenny Mar 26 '12 at 16:16
  • \$\begingroup\$ When I measure HS and VS with my multimeter it measures some current on these pins, so I don't know if I need to supply anything to these pins. \$\endgroup\$ – Qurben Mar 26 '12 at 17:10
  • \$\begingroup\$ @Qurben - You need to read up on VGA signalling, HS should have a short-duration pulse for every vertical line on the monitor every 1/60 s, and VS should have a pulse every 1/60 s. You'd need to check them with a oscilloscope and find their frequencies, on-times, and phase relationship. A multimeter isn't enough. \$\endgroup\$ – Kevin Vermeer Mar 26 '12 at 18:38
  • \$\begingroup\$ I read some specifications on the signal format and I think I am not able to create a device which makes my monitor display a white screen, so I will try to find out what turns the monitor off and fiddle around with the monitor's pcb (and kill my monitor probably) \$\endgroup\$ – Qurben Mar 26 '12 at 21:15
  • \$\begingroup\$ I understand the question, but I am not sure I understand how making a screen completely white is a step towards making an oscilloscope? \$\endgroup\$ – Cybergibbons Jun 15 '12 at 12:13
3
\$\begingroup\$

In VGA, the RED, GREEN and BLUE signals are analogue, in the range 0-0.7V. To have a white screen, you'd have to supply 0.7V to all three RGB inputs.

I'm not sure about HSync and VSync, in the worst case, you're going to need them as well in order for the LCD monitor to display anything.

\$\endgroup\$
  • \$\begingroup\$ The OP specifically said he has a CRT, not LCD display. And yes, you definitely need both Hsync and Vsync within a range that monitor supports to get a display. \$\endgroup\$ – Olin Lathrop Mar 26 '12 at 18:24
  • \$\begingroup\$ My bad, I meant to say monitor. I guess I'm just too used to calling them LCDs. \$\endgroup\$ – Armandas Mar 26 '12 at 18:56
  • 1
    \$\begingroup\$ The other answer is better, as you neglect to mention the need to switch the RGB lines when not in the active area. If you don't do that and simply drive a DC level, you will likely find that the display rapidly fades to black. \$\endgroup\$ – Chris Stratton Jun 15 '12 at 16:50
4
\$\begingroup\$

It's not a simple wiring job unfortunately.

Like armandas said, you'll need to put 0.7V down the R, G, B signals, which seems easy. However, in order for the monitor to come out of suspend and display an image, you must also provide valid HSYNC and VSYNC waveforms.

You'll also need to turn off the display (so, 0V down the R, G and B lines) when either HSYNC or VSYNC are active.

To do all this, you'll need a clock generator (around 25 MHz for 640x480), two counters (one for horizontal position, one for vertical position), eight comparators (four each for horizontal/vertical), and some glue to combine the results.

But then your troubles are just beginning, because you've just made the timing generator of a raster graphics display. There's lots more work ahead of you before finally digitising, rasterising and displaying an oscilloscope signal on the CRT.

It's likely that one of your best options is to use one of the CRT display controllers suitable for microcontrollers. Some solutions are described in this question.

You should look at this question too, because it's pretty much identical to yours.

\$\endgroup\$
  • \$\begingroup\$ It's actually pretty easy to build the necessary state machine with a CPLD or even fast processor. \$\endgroup\$ – Chris Stratton Jun 15 '12 at 16:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.