if anybody knows how round robion scheduling is performed if TIMESHARING is set 0 in RTX51 Tiny.

Think outside the PC box!

Why on earth use a URTOS to do a round robin? All it will do is impose some naming conventions on something you, yourself, can write in 10 minutes or less and use meaningful names for (e.g. ProcessSerialInput instead of task3).

how long does it take to code

While (1)
{
ProcessAcme();
ProcessMagic();
ProcessTrick();
ProcessGumbo();
ProcessFnox();
)

Erik