I had a customer with a very simular application, but was using InduSoft WebStudio as the SCADA instead of CiTect. It ended up being a bad NOE module in the end, but had alarms and bad data from time to time. CiTect has really good drivers I've been told, so I don't know if it would be a driver issue. The only other problem I've seen with Quantium hotstandby and HMIs is the fact that Modicon does an IP address swap for the hotstandby processors. I've seen panels (not PCs) talking to hotstandby processors actually not reconnect to the new "master" processor after an IP swap, it turned out that the driver used on the panels was tied to the MAC id of the processor and not the IP address. Hope you might find this info useful. The bad NOE module was hard to find, the first reaction was to point fingers at the software guys.