Troubleshooting VGA to RGB monitor connection
This document tries to help you to troubleshoot the connection between VGA and RGB monitor.
General information on troubleshooting monitor problems
Monitor will not sync problems
There are wide variety of causes for a monitor that wil not display a stable or properly configured image. First check that the cable connections are correct and secure. Then check that the sync frequencies and polarities you have configured to your graphics card are correct (nor that in quite man modern monitors the sync polarity does not matter).
If those are fine, we need to look at the specifific symptoms:
- Lack of horizonal sync: Picture drifts smoothly horizontally. This may mean that the horizonal ync signal is missing, you are trying to sync to frequency which is outside monitor specs (above or below operating range) or there is some fault in the sync-processing circuitry inside the monitor. Note that depeneding the difference between the video horizonal rate and the free-run frequency of the horizonal oscillator, the picture may be torn left or right or have multiple images superimposed horizonally.
- If the picture rolls shoothly up or down this can be caused because of missing vertical sync signal or the monitor sync processing circuitry can't sync to that (propably vertical sync rate out of monitor specs).
- If the picture is torn then correct vertical sync signal is reaching the monitor but monitor has problem in locking to it. Check the rate specification and stay within the specs.
- If the picture jumps or vibrates vertically the problem can be caused by unstable vertical lock. This symptom could be due to scan-rate problems or fault inside the monitor. Check also that the monitor and your PC get the power from same electrical outlet, because sometimes ground loop problems can have this kind of symptom when the monitor get the power from different electrical outlets.
- Multiple or repeated images problem (horizonally or vertically): There may be multiple images side-by-sde, on top of each other, or interleaved. The most likely cause is driving the monitor with an incorrect scan rate.
Color problems
If you have problems with monitor colors, then the most propable reason for color problems are the monitor settings and RGB signal connections.
- If you have one color totally missing, then check the connection of that color signal. There might be a bad connection or short circuit in the cable. If you have made a special cable yourself, check that you have wires all the wires corectly, because a wrongly connected cable can cause also this problem.
- If one color is much brigher that other colors there might be somethign wrong in the monitor settings for that color. Check also that the color input is properly terminated (75 ohm termination is the industry standard for analogue RGB signals, but in some monitors this can be disconnected which is not what you want to do). If you have accidentally removed the termination, the voltage in this line will be higher than it shoudl be (typically around two times what is needed).
- If you are sending black image to the monitorm but the picture is not black, then there is somethign definat�ly wrong in the monitor adjustments or you have connected one of RGB lines to some entirely wrong pin.
Double picture on the screen
If you see two equal images on your monitor screen side by side, the reason for this is taht you are sending the picture to the monitor at exactly double the HSYNC rate it can handle. This particular monitor happened to sync to every other of the HSYNC pulses (some monitors sync like that and some don't sync at all in this kind situation.). Two images side by side is caused by the fact that you see two picture scanlines side by side on the monitor screen. This double image problem occurs for example if you try to send a standard VGA (640x480 mode 60 Hz, 31.5 kHz HSYNC) picture to a monitor which can handle 15750 Hz HSYNC rates (some monitors work like that and some do not sync at all).
To silve this problem you have to change the screen monitor setting on your graphics card to halve the HSYNC rate the card sends out so that it maches to the value that your monitor wants.
Measuring tips
Sync signal frequency
- Frequncy counter: Connect the sync signal wire to the input of the frequency counter and read the frequency the counter shows.
- Oscilloscope: Adjust the time scale so that you get two sync signals on the oscilloscope screen. Now you can read here the time between sync signals and calculate the sync signal rate from it (frequency = 1 / time).
Sync signal polarity
- Multimeter: Disconnect the cable form the monitor. Put the multimeter to DC voltage scale. Measure the voltage on the sync signal wire coming from the VGA card. If the voltage is 4..5V then the sync is negative, if it is 0..1V then the sync is positive.
- Oscilloscope: COnnect the oscilloscope to the sync wire. If you see upgoing pulses, then sync is positive. If you se down going pulses, then the sync is negative.
Common problems
PROBLEM: My monitor picture looks like a scrambled cable broadcast
Your computer's resolution or refresh rate is not set to a value which that monitor can handle. Another possibility is missing, wrongly connected or otherwise wrong sync signal. Here are most important thing to check in this situation:
- Double-check the monitor data and the screen refresh settings you are using.
- Double-check the sync wires connection in monitor wiring.
- If you use an adapter circuit with active electronics in it check that it gets proper power and is working properly.
- If you can measure, check the sync polarity, sync signal rates and sync signal lengths really going on those wires.
PROBLEM: The monitor is not syncing to VGA picture at all
Look the question above.
PROBLEM: No picture at all in monitor
Check that the TV is in AV channels and the drivers are correctly activated in computer. Check that the connections are made correctly and the circuit gets enough power. Make sure that there is +5V open circuit voltage visible at adapter SCART connector pin 16. Make sure also that R8 gets +5V power and has correct resistance value. If this is all correct, the do all the things mentioned in problem solving above.
PROBLEM: I connected the circuit to my monitor and saw that the picture on tv is almost two sreens divided with vertical black line.
You are trying to output video signal with higher horizonal sync frequency than the monitor cna handle. The monitor has synced to every other horizonal sync signal. You can correct this problem bower the screen refresh rate to some valu that your monitor can handle.
PROBLEM: Monitor picture scrolls up/down
You are aither using a vertical sync rate which the montor can't properly sync to or verticla sync signal is missing. Chekc the monitor wiring so that the sync signal really get to the monitor in the right way. If there is no problem in wiring, then try to use a little bit lower or higher vertical sync rate to see if this helps to solve the problem.
PROBLEM: You see the picture "waving" or you see horizonal dark band scrolling over the picture
You have ground loop problem. Connect yout monitor and the computer to same power outlet. The problem should go away.
PROBLEM: Picture looks like the picture below and scrolls
---------------------------
/ /
/ /
/ /
/ /
/ /
/ /
/ /
---------------------------
The problem is clearly related to sync signal produced by the sync converter or the sync signals don't get otherwise properly to the monitor. Other possibility is that you are using a screen refresh rate that is a little bitt of the range that your monitor can properly handle.
PROBLEM: The picture in TV scrolls with a very fast speed from the upperright to the lowerleft and DOS propmpt looks like this:
/--/------/|
|C/ C:\> / |
|/C:\> C/C:|
|------/---|
The problem might be caused one of the following reasons:
- If you are using just cable chek that sync wiring are wired correctly and the sync signals are really getting to the monitor.
- If you are using sync conversion circuit then the circuit does not output proper sync signals to monitor because of some fault (you need an oscilloscope to test if the correct signal exists)
- Check the screen refresh rates and sync polarities you are using.
PROBLEM: Wrong colors
Check the wiring of RED, GREEN and BLUE signals. The might be short circuit or bad connection in one of those signal wires.
PROBLEM: Picture does not sync at all
Possible reasons for this problem:
- Sync signal do not get to the monitor correctly (wrongly connected wiring or conversion circuit is not working correctly)
- Wrong sync signal polarities
- Wrong screen refresh rate
PROBLEM: The edges of my screen are missing on my RGB monitor.
Adjust the video signal timings and/or monitor picture size controls to make the monitor picture to have right size.
PROBLEM: The picture does not fill the complete monitor screen
Adjust the video signal timings and/or monitor picture size controls to make the monitor picture to have right size.
PROBLEM: Picture has ghosting or is blurred
There is problem in your wiring. You have used too long cable or cable type not suitable for high frequency video signals.
PROBLEM: Radio interference from circuit
Make sure that all wiring is shielded cable and the shield is connected to signal ground and connector shells. Use shielded connector shell and metal box for the circuit. If this does not keep the interference levels acceptable, you have to install small ferrite core around the cable near connector going to VGA card.
What should the sync signals look like on oscilloscope ?
Horizonal sync
The HSYNC output should look like this (negative sync):
---_------_-----_-----_-----_-----_-----_---or this (positive sync):
_ _ _ _ _ _ _
--- ------ ----- ----- ----- ----- ----- ---
The time between sync pulses is typically in the range of
15-64 microseconds (15.6-65 kHz horizonal sync rate).
Vertical sync
The VSYNC output should look like this (negative sync):
---_------_-----_-----_-----_-----_-----_---or this (positive sync):
_ _ _ _ _ _ _
--- ------ ----- ----- ----- ----- ----- ---
The time between sync pulses is typically in the range of
10-20 milliseconds (50-100 Hz screen refresh rate).
Composite sync
The CSYNC output should look like this:
---_------_-----_-----_-----_-----_-----_---And once for every field like this:
---_------______-_____-______-----_------
Tomi Engdahl <[email protected]>