The CiA 424 series of documents specifies virtual device interfaces for door controllers and door units. The door controller sends two different commands to the outside door units, another command is used to control the in-vehicle doors. The door units confirm the commands by corresponding door state parameters and provide door diagnostic information. The door units provide several specific process data (e.g. door hold open time), too. All devices support the heartbeat protocol. The default bit-rate is 125 kbit/s. It is recommended to use RJ45 or DE-9 connectors.