RDM

The ArtRdm packet is used by all Art-Net devices for the transfer of RDM (Remote Device Management) data.

The following screen shot shows a DMX-Workshop capture of an RDM transaction using two ArtRdm packets.

The first packet contains an RDM Get command requesting queued messages from the RDM responder. In the second packet the RDM responder replies with sensor information.

The following screen shot shows a Wireshark capture of an ArtRdm packet.

The Wireshark dissector is very powerful for ArtRdm as it dissects both the ArtRdm packet and also the RDM payload. In this example the RDM payload contains a Get command for sensor data.

ArtRdm – Packet Definition

ID
The ID field is 8 bytes in length and contains the null terminated string of ASCII characters:
A r t – N e t
The purpose of the field is to identify the packet as an Art-Net protocol packet. All Art-Net packets start with this field. Receivers must check this field is correct and discard the packet if it is not.

OpCode
The OpCode field is a 16-bit word transmitted low byte first. The OpCode identifies the packet type. In the ArtRdm packet this field is set to ‘OpRdm’.

ProtVer
The ProtVer field is a 16-bit word transmitted high byte first. It identifies the version of the Art-Net protocol. The protocol version is not related to Art-Net 1, II, 3 or 4. Only one protocol version has ever existed in production equipment which is 14₁₀. Receivers should use the following good packet test:
ProtVer >= 14
The ProtVer may be increased in a future release to identify significant functionality changes.

RdmVer
Art-Net provided RDM support prior to its formal release. The original draft format and the standard differ in structure. This field defines the version of RDM that the node supports.

All modern products will set this field to ‘1’. However controllers must honour the RdmVer field when parsing ArtRdm packets.

Filler / Spare
These fields are not currently in use. The transmitter must send as zero. The receiver must ignore the contents of these fields.

Net
The low 7-bits of Net defines bits 14-8 of the 15-bit Port-Address that must process this packet.

Command
The Command defines how the RDM data should be processed. Currently there is only one option:

Address
Address defines bits 7-0 of the 15-bit Port-Address that must process this packet.

RDM
This is a variable size structure of 26₁₀ to 257₁₀ bytes in length. It holds the RDM packet as defined in the ANSI E1.20 – 2010 standard.
The first byte in the RDM field is the RDM Sub-Start-Code, which is slot number 1 of the RDM packet and will always contain a value of 1.