How To Wire DMX

About DMX512

The communications standard "DMX512" covers digital multiplexed signals. It is the most common communications standard used by lighting and related stage equipment.

DMX512 provides up to 512 control "channels" per data link. Each of these channels was originally intended to control lamp dimmer levels. You can think of it as 512 sliders on a lighting console, connected to 512 light bulbs. Each slider's position is sent over the data link as an 8-bit number having a value between 0 and 255. The value 0 corresponds to the light bulb being completely off while 255 corresponds to the light bulb being fully on.
Note: If you need more then 512 channels, we are talking about Universe for each DMX cable chain.

DMX512 specifics

DMX512 data is transmitted at 250,000 bits per second using the RS-485 transmission standard over two wires. As with microphone cables, a grounded cable shield is used to prevent interference with other signals.

There are five pins on a DMX connector: a wire for ground (cable shield), two wires for "Primary" communication which goes from a DMX source to a DMX receiver, and two wires for a "Secondary" communication which goes from a DMX receiver back to a DMX source. Generally, the "Secondary" channel is not used so data flows only from sources to receivers.

DMX512 is connected using a daisy-chain methodology where the source connects to the input of the first device, the output of the first device connects to the input of the next device, and so on. The standard allows for up to 32 devices on a single DMX link. Although each device has an input and output connector, these are merely wired together -- no re-transmission or amplification is performed by each device.

Each receiving device can be set to the "starting channel number" that it will respond to. For example, if two 6-channel dimmer packs are used, the first dimmer pack might be set to start at channel 1 so it would respond to DMX channels 1 through 6, and the next dimmer pack would be set to start at channel 7 so it would respond to channels 7 through 12.

The DMX512 communications protocol is very simple and robust. It involves transmitting a reset condition (indicating the start of a new "packet"), a start code, and up to 512 bytes of data. Data packets are transmitted continuously. As soon as one packet is finished, another can begin with no delay if desired (usually another follows within 1 ms). If nothing is changing (i.e. no lamp levels change) the same data will be sent out over and over again. This is one of the best features of DMX512 -- if for some reason the data is not interpreted the first time around, it will be re-sent shortly.

DMX512 pinout and cabling details

Standard DMX connectors use XLR connectors with five pins:

Pin 1: Shield (ground) Pin 2: Primary Data Complement (-) Pin 3: Primary Data True (+) Pin 4: Optional Secondary Data Complement (-) Pin 5: Optional Secondary Data 2 True (+)

A DMX output connector is always female, and a DMX input connector is always male. Some manufacturers use 3-pin XLR connectors, eliminating Pins 4 and 5.

The DMX512 signal is transmitted via the industry standard interface EIA485, more familiarly known as RS485. RS485 is a balanced connection. The standard wiring is a twisted-pair, shielded, low-capacitance data cable designed for RS-485 -- never use standard microphone cable. Recommended cables are Belden 8227, Belden 9156, Belden 43906 (European DMX Cable Version).

Data is transmitted in serial format asynchronously with the transmission speed of 250 Kbps. Voltage on both pins ("+" and "-") should be between +12 volts and -7 volts (measured to ground). EIA485 defines that the signal voltage between the two wires should be at least 200 millivolts.
Higher voltage on the "+" pin and lower voltage on the "-" pin results in a digital "1".
Higher voltage on the "-" pin and lower voltage on the "+" pin results in a digital "0".
The ground wire is only a reference point and often used for shielding.

DMX devices such as lights are connected in a daisy-chain fashion: from the controller to light #1, to light #2, to light #3 and so forth.
The final device in the daisy-chain must be terminated. Terminating plugs contain a 120 ohm resistor soldered across pins 2 and 3. The terminator functions by absorbing signal power which would otherwise be reflected back into the cable and degrade the data.

-- PeterK - 15 Aug 2007