Alan, Thanks for your response... This is where I'm stuck. I've assigned some Global Variables in Ultraware, and accessed them fine in my C program. No problem there. The problem is getting the PV to be able to access those variables. I'm getting error 683 - File access error on the PV600. Is a start, I've just got one button assigned on my PV screen, with a write tag address of B3:0/0 and a Indicator tag of B3:0/1. As simple as it gets... I'm sure the solution is simple, but hard to find with no documentation.