Introducing significant changes (or even a new system) to production systems (i.e. manufacturing execution system) on a production floor has always been a challenge. A key issue is the need to conduct high confidence testing validating the correctness and performance of the new system. Currently, production requirements (i.e. lack of equipment time) usually limit the quantity and quality of the tests we can conduct. We have devised an approach to provide a highly realistic testing environment by simulating FAB wide equipments interacting with the MES system via the equipment integration (EI) components just as it would in actual production. This way, we are able to validate the target production system with high confidence without the need of actual equipment time