Weight bridge machine serial port problem

Question

Hi guys i have problem .I have weight bridge using for weight raw materials of aggregate or sand or 

cement.when car come it come empty meaning weight bridge take only weight of car without any weight of cement or sand or any thing

but when it back it come loaded with raw material(cement or sand) that mean when car it come back it include car weight + raw materials

I using c# 2005 visual studio

I need source code for windows form c# if possible for

1- recieve value of weight bridege to textbox1 directly without press button or any thing when go inside company include empty car(weight of car only)

2-recieve value of weight bridge to textbox2 directly when car back from company loaded with raw materials(car weight + raw material weight)

Can any one help me in this