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A full length DMX512 packet includes 513 data bytes. For conventional DMX
communication, the first byte should consist of all 0s, while the remaining 512 bytes
contain the data intended to control the 512 DMX devices connected to the link. That
first byte has been referred to variously as the Start Code, Null Byte, 0 Byte, etc. RDM
data looks very much the same as DMX data, except that the packet length can vary
with each communication, the longest packet is 257 bytes, and the first byte contains
different data than all 0s. The data in that first byte identifies the packet as an RDM
rater than DMX packet.
An RDM system consists of a controller and one or more responders. These units will
normally be operating as a conventional DMX network. When RDM control activity is
needed, the controller will issue a special RDM data command to alert any connected
RDM devices, and then may issue any of a variety of other RDM commands to trigger
actions or
responses
in the connected responders (dimmers, fog machines, whatever).
As mentioned above, the RDM data is mixed in with conventional DMX data and in
theory should be invisible to conventional DMX devices on the link.
An RDM session must begin with
RDM Discovery
, during which the controller identifies
and builds a list of all the connected RDM responders connected to its data link. Once
the controller has discovered all of the connected responders, they can be monitored,
adjusted, configured and generally managed individually over the RDM data link. During
normal RDM operations, the Discovery process is periodically repeated so that added,
removed, or failed devices can be identified and the controller’s list can be updated.
There are a few caveats that should be understood when working with RDM.
•
RDM data replaces DMX data, and so reduces the resolution of the DMX data
stream, particularly when lengthy, complex operations like Discovery are
occurring. This loss of resolution can affect the performance of sensitive DMX
and RDM controlled devices (like LED fixtures) and can even make the whole
system pause or appear sluggish while the RDM operation is occurring.
•
Some conventional DMX devices ignore RDM start codes. Although for many
years, the DMX512 Standard has required DMX devices to not respond
incorrectly to data packets with alternate (non-0) start codes, there are units out
there that completely disregard the start code, which means they will do some
strange things when presented with RDM data.
For these reasons, we recommended that RDM users test their system well in advance
of show time to identify any non-compliant DMX devices that might flip out during RDM
operations, and so that operators can experience the loss of resolution that occurs
during RDM operations and decide if they should limit RDM sessions to non-critical
periods, such as during the pre-show checkout.
