AVR-Chat

hi
this is my first mail to group
i am working on my final year project can i want to replace the analog 
module of plc in my project therefore i have to design a 
miicrocontroller interface card(the graphic display is to be viewed on 
pc)for this i am using RS232 but i have to send the status signal from 
computer to interface card and then to logo for this task i dont know 
what should i use please let me know thanks