i will be highly obligd if someone can just throw some light on geral plc develoment

1) why do you enter a new question in this old thread?

2) developing a PLC is plain vanilla, nothing complicated, the issue is the ladder logic entry program running on the PC, that is where the real work is. The inertia in the industry is such that trying to sell something with another means of entry is, at best, a seep upohill battle.

Today you can buy a PLC for less than $200, there is no justified reason to make "a PLC". There are many control applications that can be handled more efficiently by specialized uC designs than a general purpose PLC, go for those.

Erik