An on-vehicle device for a driver monitoring system can be configured to be isolated from the vehicle bus while connected to the vehicle's OBD port. In a fully-isolated mode, the device only receives power and ground from the vehicle's OBD port and there is no other communication sent or received from the vehicle bus. In a passive mode, the device can obtain some information from the vehicle bus but does not request any information on the vehicle bus. The device may be undetectable on the vehicle bus. Optionally, the device receives commands via a communication interface to be switched among the three modes: fully active mode, fully isolated mode and passive mode. The commands may come from a server and may be communicated over the internet or over a cell network to a communication module on the device.