This part of ISO 14230 specifies the physical layer, based on ISO 9141, on which the diagnostic services will be implemented. It is based on the physical layer described in ISO 9141-2, but expanded to allow for road vehicles with either 12 V d.c. or 24 V d.c. voltage supply. NOTE In this part of ISO 14230, values given in parentheses apply to 24 V d.c. systems.