An “always on” human machine interface (HMI) is a key requirement for many industrial control systems. Nonetheless, the HMI can become unresponsive or unusable when system events impose a heavy load on the central processing unit (CPU). This paper explores how system designers and developers can use CPU time partitioning to prevent this problem and guarantee both local and remote operator access in any scenario. With time partitioning, developers and system designers can allocate a guaranteed portion of CPU time to each software subsystem, regardless of system load. As a result, each subsystem, including the HMI, can remain continuously available.