>    I would like you to tell me wich are the the most
> important reasons why a PIC microcntroller would (or would
> not) be able to replace a PLC.
>    For example, supose the need for replacing the semaphore
> PLC control in a city.


I'd be very aware of accepted standards when making this judgement.  I
have worked on lots of automated sawmill equipment applications where a
PLC would be about the last logical choice due to the calculations
involved, but the sawmill electricians program in PLC ladder logic only.
There are times when efficiency, cost, speed, flexibility and
development time are not as important as the comfort zone of those
involved in influencing the purchasing decision, and more importantly,
the availablity of fairly universal maintenance and support 24 hours per
day.  In addition, PLC's are packaged in a format already suitable for
application in industrial settings.

There certainly are opportunities for micro controllers replacing PLC's,
possibly for traffic light controls (I know nothing about traffic lights
beyond green=go/red=stop), but beware of how and why the controls are
selected, and how they are supported.

Good luck!

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.