Difference between revisions of "ArtNet"

Revision as of 18:44, 10 July 2009

ArtNet is a protocol for sending DMX data over IP networks. In order to understand ArtNet it helps to know a bit about DMX and IP, so here it goes....

What is DMX?

DMX is a standard for controlling theater equipment (mainly lighting). In a theater, there are normally large numbers of lights, that have to be turned on and off independently. One solution to this is to wire the theater like a houseand have a desk of on/off switches with a mains cable running to each light.

There are a couple of issues with this however. First, mains cables are relatively thick, even with 20 lights and 20 cables, a lot of wire is involved. Secondly, if we ever wanted to move the desk, we'd have to run a new wire for every light which is most theaters would be a horrendous job. Finally, doing the same thing many times isn't the way we engineers like to work :)

So, instead of a cable from the desk to every light. why not put all the mains switching gear near the lights and run a low voltage control signal from the desk? Sounds good? Well thats exactly what DMX does.

DMX is a serial protocol, so it sends data one bit at a time over the wire. Each DMX cable can control up to 512 channels (for now, each channel is a light) and each channel takes a value between 0 and 255. If conventional lights are being used, each light occupies a single channel and the value of the channel is the intensity of the light (0 = off, 255 = full).

In a DMX cable, only 3 of the 5 pins are used (signal +, signal -, and ground). Although this cable is blue, it's not CAT5!

A DMX network is formed in a daisy chain fashion, so one device is the source (most often the lighting control console) and the other devices are the receivers. Each receiver has a preset address, which corrosponds to the channels of DMX data (from 1 to 512) that it is interested in.

What are IP Networks?

Like DMX, IP is a protocol. Where as DMX connects lighting equipment, IP was designed to connect computers. For the scope of this discussion, IP operates on top of another protocol called Ethernet. Ethernet cables are typically blue and have a plugs similar to phone connectors on the end.

Ethernet is wired in a star fashion (extended star to be precise), so an Ethernet network looks a bit like this:

ArtNet and DMX

Back to ArtNet. ArtNet then is a way of sending DMX information over IP networks. So what are the advantages ?

• An ethernet cable can carry a lot more information than a DMX cable (Part of my work is to identify exactly how much is "a lot").
• Hardware for ethernet networks is much cheaper than DMX networks
• IP is not tied to a single phyiscal medium, this means it's as easy to send the DMX data via wireless links as it is to send it down wireless. Coupled with point two, this means we can have very low cost wireless DMX links.
• Continuing from point three, this means that it's also easy to send DMX data over the Internet. Ever wanted to control your lights from a suburb away?
• ArtNet supports bi-directional communication, this means that nodes can send information back to the controllers (DMX also supports this via RDM, but you may have to upgrade your equipment).