Yau,

I'd recommend that you on do all the control logic in the PLC. The scale should be able to interface directly with the PLC. Some scales actually have alarm points that can trigger failsafe closing of the valve if the weight exceeds a limit. The PLC itself is more reliable than a PC, and therefore the PLC should be in control of reading the weight and controlling the valve.

Can you explain better about what the weighing software is and how you are using it?

>Trying to link up a weighing software to control the closing and opening of a valve for a hopper. What it does is that when a weight target is keyed in to the system, the filling of a tanker is initiated until the target weight is achieved where the valve will then be shut. The process is done via a programmable logic controller.
>Any suggestions on how this should be done using VFP on WinXP? The program needs to be fail safe as the valve control failure will result in dangerous chemical spillage.