Hi tguclu ,
Sorry if my response is too late as I did not notice your reply until today !
** And about your question :
As Tom says , I think my friend that you – at first - have to be familiar with PLC before learning about writing SCADA using visual basic .
Then you must know about PLC's communications protocol (e.g Modbus )
www.modbus.org
you must know what is a protocol at first .. Search about it .
Then you must be familiar with visual basic , I believe that it is the best programming language for automation .
You may read this book "SAMS,Teach your self visual basic in 21 days"
It is a very good book , you may also read the company's book about data base ( for me I did not read it )
Note : you do not have to be an expert in visual basic .
After you collect these knowledge you may go for a company that provides special activex controls for automation and scada stuff ( you can download demo in most cases )
Search by google and you 'll find a lot of them ( I use Adlink's active x controls "DAQbench" )
** About OPC :
You can find information about it here :
www.opcfoundation
And we can say in brief that OPC technology made the life easier as you do not have to learn the PLC's/device communications protocol in details to achieve the communications successfully .
I believe that : (( Visual basic 6 + OPC >= powerful & low cost SCADA package ))
I hope I gave you information that you need
Ayman