A system for testing control units via simulation includes: a simulator; a host computer; and at least one connection for a communication system. At least one communication tool is stored on the system. Real control units are connectable to the system via the communication system. At least one controller is provided on the system for the connection to the communication system. Driver software for the at least one controller is stored on the system. The at least one communication tool is configured to generate communication code for communication between simulated control units and/or the real control units, wherein the communication code is configured to interact with the driver software and to relay signals and/or messages from the real and simulated control units to the driver software and to receive the signals and/or messages from the driver software. A loop mode is provided for the driver software.