DMX Receiver Base Address

This is a short guide to configuration of base addresses. Each DMX fixture requires a start address or base address in the range 1- 511. The address determines the first slot that is used by the receiving device.

All receivers on a DMX bus are allocated a base address that may be represented by a 9 bit binary number. In genrality a single fixture may use one or more consecutive slots.

Here are some examples:

The base address may be expressed in binary. For example a DMX address of 40, sets the binary bits at positions 4 & 6 to a '1' value.

Base address bit pattern, note the representation is typically shown lsb first.

There are three common ways to set the base address:

Setting the DMX Base Address using DIP Switches

This method usually uses a 10-way DIP switch, which consists of 10 microswitches. Each switch is wired so that when it is “on”, it connects an input to ground (inputs to microcontollers normally float to a logical “1” when they are not connected, although for some inputs it is preferable to add a high-value resistor to the positive supply to ensure predictable operation). Since a DMX address requires 9 bits (each digit corresponding to 1 2 4 8 16 32 64 128 256), this leaves one switch spare - this is often used to configure an option (e.g., when set to one indicates a self-test mode).

AVR-based DMX receiver showing the DIP switches (on left in blue) used for configuration.

DIP switch setting for a base address of 10 (2+8=10). The least significant bit is
usually on the left side, hence switch 1 has a value 1, switch 2 has a value 2, switch
3 has a value 4, switch 4 a value 8, etc. Note: 10th bit is used for other configuration.

DIP switch setting for 24 (16+8=24). Note: 10th bit is used for other configuration.

Multiple Devices

A fixture requiring one or more channels for control begins to read the channel indicated by the start address. For example, a fixture requiring 7 channels that was addressed to have a base address of 100, would read data from 100,101,102,103,104,105 and 106. The next consecutive device could use a start address of 107.

Example showing slot 7 being read as output 3 of device 2, configured with a base address of 5.

Here are some examples:

Two identical devices with the same base address will be configured to operate exactly the same.

Using fewer switches for configuration

Although DMX allows any base address in the range 1-512, some devices place constraints on the base address, for instance to only allow addresses to be configured on 8-slot boundaries, or to not support the full range of values.

For instance, a manufacturer may choose to allocate some of the switches for configuration settings. These switch positions could allow a general-purpose board to configure the outputs in various ways: some as digital outputs, some as dimmer (PWM synchronised to mains zero crossing), some as direct PWM for servo control or LED lighting control).

In the examples below, a weighting of 23 (8) is used for the first switch. The slot value is derived from the DIP switch setting on switches 1 to 5 by multiplying the binary value by 8 and then adding one to result. This gives a maximum base address of 249. A similar configuration is used for the Milford Instruments DMX receiver (Parte number 1-479).

DIP switch setting for 1 = 8*0+1 (for an 8 channel device configured in 8 channel groups). Note: bits 6,7,8,9,10 are in this case used for other configuration.

DIP switch setting for 9 = 8*1+1.

DIP switch setting for 41 = 8*5+1.

Two things to remember to check is which switch represents the least significant bit of the address, and to note the weighting for this bit (i.e. does switch 0 represent 20 (1) or it could represent some other value, e.g. 22 (4) or 23 (8) ).


Resources

See also:

A lot of people find binary manipulation hard to visualise, and some helpful people have built applets for phones, iPads and computers to help people get to grips with this. Some example calculators are available on-line at:


Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2016)