"RogerN" fired this volley in
m:

his will allow the
Arduino to control several things at once instead of doing one thing
at a time. I think the correct name for it is non-preemptive
multitasking.

Most PLCs that have an event-triggered extension language would have to be
qualified as doing "preemptive multi-tasking". Once off in a module, you
can't service the ladder until the routine returns. Some don't even permit
servicing I/Os _during_ the routines, only setting internal registers that
will be transferred to the hardware once the ladder loop resumes.

Lloyd