A communication system and method are disclosed for serially transmitting data in automation systems. In at least one embodiment, the system includes a base module as the system master, a plurality of extension modules as the slaves and a bidirectional communication connection that serially connects the base module to the extension modules. A status as group master for the serially subsequent extension module is allocated to at least two of the extension modules, the base module communicating with the extension modules depending on said allocation.