KVM switches are great, letting you share one keyboard, monitor and mouse with multiple computers. Unfortunately, mouse drivers seem to be very sensitive and sometimes you'll lose the mouse when you switch. This isn't just a SCO problem; I've seen it on NT too.
The mouse gets "lost" - stops working on one or more of the attached systems. Sometimes you can fix it by getting the screen to refresh on the system with the "dead" mouse, but sometimes nothing but a complete shutdown and reboot will restore the mouse. On Linux/SCO systems try switching away with CTRL-ALT-F1 and then back (CTRL-ALT-F2 on SCO, usually CTRL-ALT-F7 or F8 on Linux).
Some motherboards just won't work with some KVM switches at all. That's probably a matter of power levels rather than anything else, but it can be very annoying. I've often just used separate mice for certain systems rather than fight with it. A higher quality (more expensive) KVM may solve the problem for you, and even just a better grade of KVM cables can sometimes help.
The KVM switch may fail in this way if power is lost: your systems reboot, but the mouse (and maybe other things) don't work. Again, better KVM's have features to prevent this, and a UPS is a better idea anyway..
Here's an old newsgroup post suggesting some debugging on SCO Unix (it didn't help, though).
From: Bela Lubkin <email@example.com> Subject: Re: Mouse issues on IBM @server xSeries 335 using OSR5.0.6 Date: Tue, 22 Jul 2003 00:13:38 GMT References: <firstname.lastname@example.org> Try flipping away, flipping back, not touching the mouse, flipping away, flipping back, _then_ try the mouse. Try this with increasing numbers of back-and-forth flips before you touch the mouse; up to a total of 4. I am not suggesting these as workarounds, but probes to try to understand the problem. What I'm trying to probe is: the keyboard mouse driver expects to see data from the keyboard mouse in a certain sequence. It expects a packet of 3 or 4 bytes (depending on whether it's a non-wheel or wheel mouse). I'm imagining what would happen if, during the KVM flip, the driver saw a single byte of garbage. It might think it was the first byte of a packet, after which it would be off by one in interpreting packets. If each flip produces one garbage byte, flipping 3 or 4 times might get you back in sync. There's a problem with this theory: the driver attempts to detect this condition by rejecting additional bytes of a mouse packet if too much time has elapsed (defined as 1/4 second). This defensive check should prevent the above scenario. But maybe it doesn't quite work right. To enhance your testing, you can turn on a keyboard mouse driver debug flag. The flag is `kbm_noisy' and the easiest thing is to turn it on in your live kernel. Do this: # /etc/scodb -w scodb> kbm_noisy=1 scodb> q The change will persist until you reboot (or change it back to 0 in the same manner). I would like to know whether you get any "kbmintr" warnings with it turned on, when the mouse is in the bad state. You can also set a second variable, `kbm_dbg', to values of 1 or 2. Setting it to 1 causes it to print information on what it's sending up to the mouse reader; 2 causes it to additionally print the actual mouse bytes as they are received. 0 turns it off. This output is extremely verbose for practical purposes, but might be helpful in understanding your problem. All of the output produced by these two debug variables appears on the console. Under X, it will appear in an "Error" window. You will probably find it easier to decipher behavior on a text multiscreen. In particular, set `kbm_dbg=2' and flip back and forth, see if the act of flipping is producing any mouse bytes. >Bela<
Got something to add? Send me email.
More Articles by Tony Lawrence © 2013-07-21 Tony Lawrence
There is no reason anyone would want a computer in their home. (Ken Olson, president, chairman and founder of DEC)