RDM or Remote Device Management is an extension to DMX that allows for bi-directional communication with DMX devices. RDM will enable features such as device discovery, status monitoring and remote configuration.
The same data link used for basic DMX is used bidirectional for RDM (i.e. link 2 in cables/connectors is still unused).
The DMX controller decides which device/fixture is allowed to send RDM data back to the controller. The Controller sends RDM commands by making a packet with start code 0xCC (instead of 0, which is used for the basic lighting control).
Each RDM enabled device is assigned a 6 byte UID comprising of a 2 byte manufacturers ID and a 4 byte serial number. It is used to identify all devices on a universe (enumeration process) and sending RDM commands to them, e.g. to set their DMX address from the DMX controller.
Official name: ANSI/ESTA E1.20, Entertainment Technology - Remote Device Management over USITT DMX512
An effort is underway to develop an RDM over ACN protocol, see E1.33