WHAT IS RDM Signal?
RDM signal describes a method of bi-directional communications over a DMX512 data link between an RDM signal capable controller and one or more remotely controlled RDM signal capable DMX512 devices. It allows discovery of devices on a DMX512 network and the remote setting of DMX512 starting addresses, as well as status and fault reporting back to the control console. While basic DMX512 is a unidirectional protocol, RDM signal is a bi-directional protocol. RDM signal is most commonly used to configure and monitor the health of theatrical lighting equipment. RDM signal is a half-duplex bi-directional serial protocol. RDM signal equipment is discovered by the controller searching for RDM devices. There is no inherent reliability, though some operations can be verified through the use of time-outs.
Remote Device Management (RDM signal), is a protocol based on DMX512 with bidirectional communication capability between a lighting controller and RDM signal-capable lighting fixtures or devices. In this way, information from fixtures can be given back to the controller, a feature that standard DMX512 does not support. RDM signal packets are inserted between the existing DMX512 data packets being used to control the lighting data. The DMX-512 specification always requires that DMX packets begin with the start code. The default Start Code is 0x00 (also known as the Null StartCode). By using the start code 0xCC, RDM signal packets can be safely inserted between DMX data packets without older non-RDM aware devices attempting to read them.
The RDM signal standard addresses this problem by ensuring that in all cases (except discovery) only one device is authorized to be transmitting at any given time (somewhat similar to thetoken passing approach). Only the controller (of which there can be only one) can start an RDM signal exchange. Responders can speak only if spoken to. The controller will always initiate all RDM signal communication. All RDM signal devices have a unique identifier (UID) that consists of a manufacturer IDand serial number. General communication with a specific fixture occurs in a request-response pattern. The controller sends the request to the device, addressing it by the device’s UID. When the request has been sent, the controller relinquishes control of the DMX512 line for a given period of time, so the device can transmit its response.
Unicast communication is the only way in which data can be retrieved from a fixture, other than its UID, which can be obtained using the discovery mechanism mentioned above. If the device does not respond within a given period of time, the controller can assume communication has failed, and may retry. RDM signal equipment is configured by messages defined in the standard that manipulate a set of parameters.
A single RDM signal controller searches for devices. After finding a device, the controller may ask for any status the device has or set its configuration. RDM signal devices may not initiate communication, they may only respond when spoken to.
Plugs for DMX512
A DMX512 controller is connected to a TIA/EIA-485 rated cable (typically 100-120ohm) in a daisy chain. Category 5e unshielded twisted pair (UTP) cable is also permitted. At the end of the chain of devices the cable is terminated with a resistor that matches the impedance of the cable. In many cases, this termination is built into the receiving device. Plugged connections use 5-Pin XLR connectors.
Limits of RDM Signal
The maximum number of RDM signal devices on a single daisy chain is limited to 32 unit loads (which may be more than 32 devices) by the DMX512-A standard.
Interoperation with other protocols
Interoperation between RDM signal and other protocols is achieved via gateways that translate between the protocols. Gateways are known to exist for ACN and some proprietary Ethernet protocols.