Yau,

>I'm using MSCOMM32 for the reading of the RS232 data but not sure how to trigger the valve. More unsure what would happen if the PC somehow hangs!

Which is exactly why the scale and PLC should be in direct communication with each other and not dependent on the PC. You should consider using an industrialized, hardened PC too.

A lot of PLCs come with a builtin network where you can add a card to the PC to put it on the network then the PC is just another device as far as the PLC is concerned. The PC can read the weight from the PLC memory which has read the wieght from the scale. The PC can write to the PLC to establish the weight limit that the rest of the control logic running on the PLC uses to control the valve.

>The weighing software receives a string of text data from the weighbridge via a RS232 port. It captures and then displays the current weight constantly. The software also generates weighbridge tickets confirming a vehicle's weight on its round trip in and out of the compound. (e.g when it comes in, if it is delivering goods, it will weigh more than when its on its way out. The system then prints out a ticket to confirm the entry and exit weight and the net weight to determine whether its a purchase or sale transaction)
>Now, the customer wants to use the software to fill oil tankers. A picture of the tanker is to be displayed and a set point for the filling is keyed into the system. Once the filling starts, the software will show the percentage filled on the picture of the tanker. Once the target weight is achieved, the software is to send out a signal via an I/O card to trigger the closing of the valve.