DMX512 is an agreement over the connection between lighting controllers, dimmers, scrollers, scanners, etc.. This protocol is developed from the need for standardization and flexibility. DMX512 is 1986 compiled by the " United States of institutes for Theatre Technology " (USITT).
DMX512 contains practically that:
1. cable type is standardized
2. connector type is standardized: 5-Pol XLR
3. one cable transmit the control information for maximal 512 channels
4. the cable is easily through plugged from device to device  

What is the DMX512 protocol ?

DMX stands for Digital MultipleXed: The information of the control channel is transmitted in digital form, channel for channel, so send successively (multiplex) over the cable. The speed, in which this takes place, is so high that it occurs as simultaneous. However, this speed causes that a DMX network must be structured in specific way (see under DMX wiring).
The information of the control channel has the form of an alternating voltage model. Hereby the voltage can only take two values.

Digital, bits and bytes
To DMX512 applies: The voltage between the conductors of the cable is either approximately +2,5[v ] or approximately -2,5[v ]. If the voltage is for 4[us] long positive, then a ' 1 ' is transferred. If the voltage is for 4[us] long negative, then ' 0 ' is transferred. So the content of the information is bivalent: either ' 0 ' or a ' 1 '. This smallest quantity of the information is called "bit". The transmission of one bit information lasts 4[us], according to DMX512. By combining several bits, the information can obtain several values. The combination of 8 bits results in 256 different values: 00000000=0, 00000001=1, 00000010=2, 00000011=3, etc.. An expressing of the information in ' 0 ' and ' 1 ' is called digitalis.
A combination of 8 bits is called a byte. With the DMX512 the information for a control channel is expressed in a byte. If  the control information is directed toward a dimmer, then the illuminating intensity can be set up to max. 256 different values. (resolution performance of 256).
This step size is mostly sufficient in practice, because for lights with a performance > 100W an intensity step of 1/256 is not perceptible. For moving lights a smaller step size can be necessary. The DMX protocol is then modified to16 bits. (The Toddler can be set up to16 bits to control of moving lights.)

The structure of the DMX512 signals
The DMX512 signal consists of a repetitive bit stream, which is structured as follows (see also following figure):
The beginning of the bit stream is marked by the fact that during 88[us ] a ' 0 ' is transmitted. This mark in the DMX signal is called "break".
After the break signal a so called "start byte" follows. This start byte determines the target from the following bytes. For the dimmers the start byte consists of eight '0-s'. The start byte extends the possibilities of the DMX512 protocol.
After the start byte maximal 512 bytes will follow with information for the control channels. The sequence number of the bytes determines the channel number, for which the information is intended.

Start bit, stop bit and date frame
In the data flow of the '0-s' and '1-s' the separate bytes are to be differentiated by the following appointments:

The transmission of a date frame lasts 44[us]. The time between the transmission of the successive bytes and break is arbitrary, within certain boundaries. DMX512 is called therefore an asynchronous data protocol.

DMX wiring

The DMX signal consists of a fast changing voltage level. Think of changes in nanoseconds. This causes the following appearances, not perceptibly with analogue control: Because of these deviations, you have to take notice of the following installation rules.

Which cable?
DMX512 contains the electrical specifications the industrial RS-485 of data protocol. RS-485 is a symmetrical voltage control: both the signal and the inversion of it, regarding " common " (= reference), are transferred. In addition a receiver gets the information from the difference of these two signals. Disturbance, in both signals available, is omitted in such a way. This increases the working reliability.
A shielded cable with a twisted pair of conductors is necessary. The characteristic impedance must be approximately 120[Ohm ]. The twisted pair of conductors transmit the signal (marks with ' + ') and its inversion (marked with '-'). The shield used as low impedance connection for the signal reference. It is very important for the control of the common mode voltage and for the EMC behavior of the cable. You find a suitable cable under: DMX cable

Which connectors and how to connect them?
If connectors are used, then it must be 5-Pol XLR types.
Pin 1 = signal reference = cable shield
Pin 2 = signal inversion = ' - '
Pin 3 = signal = ' + '
Pin 4 = optional (e.g. acknowledgment)
Pin 5 = optional (e.g. acknowledgment)

How long may DMX the cable be?
The length of the control cable limited through:

If all rules are considered the cable can be 500[m], with two receivers and a conductor diameter of 0,2[mm2 ]. In case a larger distance must be bridged, a repeater must be used. With several receivers the quality of the receivers determines the maximal length of the cable, for which a repeater must be used.

How to wire?
The structure of a DMX network earns special attention. To avoid signal distortion by reflections the following must be considered:

A trouble free process

Disturbances with digital control in general, so also with the DMX, can appear as short unforeseeable disturbance of the normal operation: The installation can work well for hours, and stammers then suddenly. A trouble free process is only achieved  if one takes consideration on the high frequency character of the control signal. Therefore: This information is offered to you by Theater Technisch Lab BV, manufacturer of DMX devices, digital dimmers and digital lighting control desk.