OK. Multi-core MCU I think is expensive. Using more than one chip also.

So why not use a bigger PIC with more memory to do the job of all others smaller PIC's?

How? Well I have never done it but for a long time I was thinking to do it like that:

1. All tasks even loops should execute only for a fraction of time, a slot.
2. An interrupt handler would share time slots to all loops or subs.
3. The interrupt handler might also grab the serial characters and buffer them as long as the serial transmission is slower than the handler execution time.
4. If a loop has to execute 10 times, then a counter and a flag could be used so that every time that loop is given time, it knows where it was and what to do.

In general I think these are the main rules to make an RTOS on a PIC. May be if we work together we can built an RTOS of our own for control applications. One man is difficult to accomplish this task.

Ioannis