HON’s Wiki # DMX512
Home / Lighting
Contents
General
- Used for controlling stage equipment like RGB(WA…) lights, moving heads, fog/haze/smoke machines and stuff.
- “DMX512” means digital multiplex with 512 addresses/elements. The ANSI standard, based on the older USITT standard “DMX512”, is known as “DMX512-A”.
- RDM (remote device management) is a separate protocol used to remotely configure “menu settings” (like the DMX address) on devices.
- Art-Net is a separate protocol used to transfer DMX512 and RDM over an IP network. sACN is a related protocol, often used together with Art-Net.
Protocol
- A unidirectional, master-slave, bus protocol, with a single controller as the master and one or more units/devices/fixtures as slaves (master-slave terminology still used here).
- A closed group of a controller and some units is called a “universe”. Each universe consists of 512 channels (aka addresses or slots) (where 1-511 are usable). Some mixers support multiple universes, but all must be electrically disjunct and running on separate cables.
- Each unit uses one or more consecutive channels, starting at the pre-configured channel/address.
- Each channel takes a value between 0 and 255 (8-bit), representing e.g. an color component value.
- Electrically, it uses the RS-485 serial protocol.
- RS-485 is designed to be a balanced (or differential) system using twisted pairs, to reduce both emitted EMI and received EMI (after merging the signals to get rid of common-mode noise). In DMX this is supported by XLR5 but not XLR3 connectors.
- RS-485 does not use any kind of error checking and correction.
Cabling and Programming
- Typically using XLR5 (compliant) or XLR3 (commonly used but non-compliant) connectors.
- Units are addressed using a pre-configured address. Multiple units may use the same address if they should be controlled using the same address and are roughly channel-compatible.
- When connecting multiple units to the same controller, the units must be daisy chained together (unless using some fancy splitter). A maximum of 32 units may be chained (according to the specification). Units typically have one input and one output for proper daisy chaining (even if they have multiple outputs, you should only use one).
- Long lines (sum of all cables in the daisy chain, about 150-300m) should generally always be terminated using a DMX terminator for best possible signal integrity. For short runs it will often work without a terminator (until it won’t). This is in order to reduce reflected signals (inter-symbol interference from signals bouncing back from the end) and noise. The terminator is a connector containing a 120ohm resistor (depending on the characteristic impedance of the cable, but typically 120ohm).
- Although both audio and DMX (may) use cables with XLR3 connectors (DMX should use XLR5, though), DMX should use cables specific to DMX to meet the DMX/RS-485 cable requirements (i.e. not mic/speaker cables).
- For less expensive cabling for permanent setups, Cat5e or Cat6 may be used instead, with 1-4 universes per cable. Not that it should be terminated according to ESTA’s DMX512-over-Cat5 specification, see this.
- Using proper cabling, it may sometimes achieve data rates over 10Mb/s and distances over 1000m.
- To run multiple DMX universes over a large area, Art-Net (or sACN) may be used to run DMX over IP. Special entertainment-class switches are recommended for this, but normal unmanaged and managed switches are typically good enough.
Art-Net 4
- Used for sending DMX512 and RDM over Ethernet/IP/UDP (port 6454).
- A node is a device that translates to/from DMX512. A controller is a device that controls/monitors fixtures/devices.
- Supports 32 768 universes (as of v4). The 15-bit port-address attribute consists of te net (bits 14–8), the sub-net (bits 7–4) and the universe (bits 3–0). Each node should generally use only a single net and sub-net value, with 16 universes available.
- The IP address is either derived from the MAC address or assigned using DHCP (if capable). If using DHCP, the subnet may be routed. If static/deterministic, subnet
2.0.0.0/8 or 10.0.0.0/8 will be used (controllers should use both to discover all devices).
- Controllers use the
ArtPoll packet as a directed broadcasts to discover other controllers and nodes, every 2.5–3 seconds. It may optionally target a range of port-addresses. Devices should wait a random delay in 0–1 seconds before answering with an ArtPollReply packet to avoid congestion.
- Controllers may reconfigure the IP address and port-addresses of nodes (if supported).
- The
ArtDmx packet is used to send DMX512 data to/from devices/controllers. DMX outputs should periodically retransmit the last DMX frame it received. DMX inputs with active, non-changing DMX input should periodically retransmit the last DMX frame into Art-Net. ArtDmx should always use unicast, with universe subscribers discovered from ArtPoll replies.
- The
ArtSync packet may optionally be used to synchronize multiple universes, e.g for use with video. It uses broadcast, unlike ArtDmx which uses unicast to each node. Nodes boot into non-synchronous mode, but switch to synchronous mode when the first ArtSync packet is received. In sync mode, nodes must buffer ArtDmx packets until receiving the next ArtSync packet. After 4 seconds of not receiving ArtSync packets, sync nodes should switch back to non-sync mode.
- Node DMX inputs are all enabled by default, but can be disabled by the controller to avoid pointless Art-Net traffic.
- Node firmware may be upgraded over Art-Net.
- Supports Remote Device Management (RDM). Nodes individually handles RDM discovery (full and incremental) and maintain their own RDM device lists (
ArtTodData packets are automatically broadcast on any table of devices (TOD) changes). Input gateways are used for DMX controllers to query node device lists out of Art-Net and back into DMX/RDM, by querying Art-Net nodes (with a ArtTodRequest broadcast packet) and maintaining its own device list built from other nodes’ device lists. Art-Net controllers acts the same way as input gateways. RDM get and set commands may be used from the controller over Art-Net (using a ArtRdm and ArtRdmSub packets).
Remote Device Management (RDM)
- Used to remotely configure “menu settings” (like the DMX address) on devices.
- This is extremely useful both for fixing settings on inaccessible devices, as well as making it generally more practical to configure devices.
- Electrically, RDM is run on the same cable as DMX512 and transmits data between DMX signals/commands on the same data wires.
- All of the controller, devices and eventual splitters/repeaters need to support RDM in order for it to work.
- Devices can be easily discovered by the controller by running an RDM scan.
- Devices not RDM-compliant may exhibit issues like flickering when RDM traffic exists on the line.
Resources
hon.one
| HON95/wiki
| Edit page