VGA Port Testing without monitor- Help

[cellsuit]

Hello to you all,

I am new to this forum, hopefully I can help some of you guys in the future with my electronics knowledge.

I am looking for a solution to a testing problem that I hope some of you could help on:

How do you test a VGA port on a computer motherboard or VGA card without a monitor ?

 

[Mikebits]

With the ease of obtaining a monitor, why would you want to test without one? I would suspect the setup required to do what you propose would cost more than a cheap monitor.

 

[cellsuit]

I am looking for a solution for some automation during pc testing, so I don't need someone looking at the color bar test on the screen.

 

[Mikebits]

Ok,that makes sense. To bad you don't know anyone who works for NVIDIA. I have no knowledge in that field, so I have no suggestions. That is not to say, someone else might

 

[Sceadwian]

A simple frequency counter should work. Just feed the horizontal and vertical refresh lines into separate counters and you can tell what resolution and refresh rate the monitor is in. I can't think of any practical away of testing the actual image coming out but if you just want an 'alive/dead doing something' If you need something more sophisticated than that you need to tell us exactly what you're testing the output for.

 

[cellsuit]

Thanks, I will give it a try.

Not sure how it will tell me the screen resolution just from the hsync/vsync frequency, can you tell me more ?

Cheers
Cell

 

[Sceadwian]

If you count the number of hsync pulses that come in between each vsync pulse you have the number of horizontal lines at least, on most PCs this can easily lead to the vertical resolution as well. I think they're both ramp voltages (triangle waves) You could possible test R/G/B lines using a test pattern diplayed on the monitor and setting up addition counters on the R/G/B lines that are reset on each horizontal/vertical pulse.

 

[Someone Electro]

No the Hsync and Vsync are not ramps.Ramps are put in the coils on the tube.

The Hsync will just pulse on every end of the line and Vsync on the end of the last line. I agree if you used a counter it could verify correct operation. Since in most cases evrythyng works or there is no signal at all.

 

[Sceadwian]

Out of curiosity how does the monitor generate the ramps from the sync pulses?

 

[Someone Electro]

It all figures it out from the timings.Because each resolution has its own timing. If your curious how the VGA signal works just try Google it should find some explanations..

 

[Mikebits]

Just a guess, but I would think the rising edge of the pulse starts a ramp generator.

 

[Someone Electro]

Well its a bit more complicated than that, the higher the resolution the "steeper" must the ramp be because the horizontal scan must go faster for higher resolutions.

 

[jordanx]

VGA output hardware test device

Greetings all. Anyone know how to create a simple LED based VGA OUTPUT hardware device for testing ??? What i need to use it for is to prevent my good (or old monitor from damage) when testing suspect VGA adapters.

A simple row of LEDS indicating active signal output on various instances
like, BLUE - light, RED - light, YELLOW - no light. Something like that. I am assuming that defective VGA adapters would not output these signals, eh ??


thanks for any assistance.

 

[Sceadwian]

Your best bet is to test for horizontal and vertical sync signals, but that's still no garuntee. The chances of a faulty video card physically damaging a monitor are so low there's no reason to worry about it.

 

[jordanx]

Thanks for your input sceadwian. I see what you;re saying but, from my experience, bad parts have a 30:70% ratio of "carry-over" of damage. Kinda
like a naughty virus infection. So i have to be 99.99% in isolation when i test
PC components. I heard of a device that does do the output test, its called a
"status indicator box". Just a plastic box with LEDS and a vga cable, and maybe some simple circuitry inside. A company is charging $120 bucks for each unit plus shipping ! From my experience, these custom devices usually
dont carry any major circuits and they make a killing for each unit.

I figure all signals have a "paired" line. So a good VGA must have all signals
outputted, hence a "custom" green LED on the unit would light up. For bad VGA output one, or more, or ALL pairs would NOT be outputting any signals. So my theory is that a simple row of LEDs can be used to diagnose for a simple normal VGA output.

Thanks again !

 

[Sceadwian]

VGA is not paired. There are 5 lines, horizontal pulse, vertical pulse, and then the R G and B lines. How are you testing these cards specifically? Do you insert them into a test machine that attempts to display an image on them, because if you are you can simply test it with a multi meter. Set the screen to display a 100% white signal and check the DC voltage on the R G and B signal lines, the voltage should be around .7V for each line. A little more information about your exact test method is really needed.

 

[Sceadwian]

I just realized we hijacked someone elses thread =O jordanx you should post this as a new thread.