Wednesday, May 20, 2009

KVM switch problems

There are two problems with VGA and KVM switches: the pin-9 problem and the pin-12 problem.

The pin 9 problem

Some monitors wait for a valid +5V level (TTL) on pin 9, using this signal to detect the computer. If the KVM switch doesn't provide +5V (pin 9 is floating) or pin 9 is missing on the connector of the cable, the monitor will not startup properly (sometimes the I/O light will flash slowly). The solution is to hardwire the console-side pin 9 of the KVM switch to +5V and ensure that the cable between the KVM switch and console has a wired pin 9.

The pin 12 problem

As mentioned under 'Disabling DDC', a machine connected to a KVM switch, but during startup not connected throughout the KVM switch to the console, will fall back to the default (lower) resolution. The solution is to remove the pin from one end of the VGA cable and to disable any plug and play for the monitor. Systems which use X11 to control the display (such as GNU/Linux or *BSD) will need to edit /etc/X11/xorg.conf so that: