“I've always thought that PC operating systems like Windows and DOS are not a stable enough platform for control applications. How does SoftPLC protect against this?”
SoftPLC is not a Windows application, nor a DOS application -- it is a real-time, multi-tasking operating system, or 'kernel'.
SoftPLC systems include DOS as a boot mechanism and file directory structure only. Once SoftPLC is loaded into memory, it is in control of the computer CPU at all times. DOS is never called again until SoftPLC is shut down. This structure gives SoftPLC reliability and characteristics of proprietary dedicated hardware PLC's, such as deterministic scan time, protection from bugs in the operating system or other software applications, etc.
Unlike some computer control software products which try to run multiple applications simultaneously under a standard multi-tasking operating system, such as Windows NT/2000, and must deal with operating system issues, task priorities, etc., SoftPLC is a dedicated system -- optimized for speed and reliability.
You don't need to worry!