Microsoft KB Archive/938411

= The screen may flash when you connect an external display or when you use an external display on a Windows Vista-based portable computer =

Article ID: 938411

Article Last Modified on 8/2/2007

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APPLIES TO


 * Windows Vista Home Basic
 * Windows Vista Home Premium
 * Windows Vista Ultimate
 * Windows Vista Business
 * Windows Vista Enterprise
 * Windows Vista Starter
 * Windows Vista Home Basic 64-bit Edition
 * Windows Vista Home Premium 64-bit Edition
 * Windows Vista Ultimate 64-bit Edition
 * Windows Vista Business 64-bit Edition
 * Windows Vista Enterprise 64-bit Edition

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Important This article contains information about how to modify the registry. Make sure that you back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows registry



SYMPTOMS
On a Windows Vista-based portable computer, you use a VGA cable to connect an external display. For example, you connect a monitor, a projector, or a television to the portable computer. The screen may flash when you connect the external display or when you use the external display. This problem occurs if one of the following conditions is true:
 * The VGA cable is long.
 * Multiple VGA cables are connected to one another.
 * The VGA cable does not have effective electromagnetic shielding.



CAUSE
This problem occurs because of electromagnetic noise on the VGA cable.

In Windows Vista, the Transient Multimon Manager (TMM) detects the connection and the disconnection of external displays that use an analog connector. The TMM polls the analog connector one time per second. However, because of electromagnetic noise on the cable, the TMM may incorrectly detect that the connected display is frequently being connected and disconnected.



Correct the VGA cable problem
Ask the system administrator to examine the VGA cable. If there is a problem with the cable, use a cable that is shorter or that is of better quality.

Set the TMM not to periodically detect the analog connectors
Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

Set the TMM not to periodically detect the analog connectors. To do this, follow these steps:  Click Start, click Run, type regedit, and then click OK. Locate and then click the following registry subkey:

 On the Edit menu, point to New, and then click DWORD Value. Type PollingInterval to name the new value, and then press ENTER. Double-click PollingInterval, type ffffffff in the Value data box, and then click OK. Exit Registry Editor, and then restart the computer.</li></ol>

Note Even if you set the TMM not to periodically detect the analog connectors, an independent hardware vendor (IHV) graphics driver may detect the connection. Therefore, this problem may still occur. If the problem still occurs, contact the manufacturer of the portable computer or the vendor of the graphics driver.

Keywords: kbtshoot kbprb kbexpertiseinter KB938411

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