On some controllers the Keyboard and Mouse operation becomes Flakey with SERCOS enabled, specifically when the Cyclical interrupt gets installed in SERCOS Phase 4.

In Phase 4 the SERCOS chip takes over running the operating system timer interrupt, which is a lengthy interrupt. SERCOS adds additional Timer interrupt responsibilities. There are additional interrupts that are added in this state. All of these interrupts were higher priority than the PIC Keyboard Interrupt which processes Mouse and Keyboard data. The Serial port hardware has no recieve FIFO so it is important not to delay processing the KB interrupt for too long or it will lose characters, resulting in Flakey operation.

This has been resolved by raising the priority of the KB interrupt, higher than the OS Timer interrupt and the extra SERCOS interruts. The KB interrupt is pretty small so it should not have much impact on these other interrupts.