DMX512 is an agreement over the connection
controllers, dimmers, scrollers, scanners, etc.. This protocol is
from the need for standardization and flexibility. DMX512 is 1986
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
3. one cable transmit the control
information for maximal
4. the cable is easily through plugged
from device to
What is the DMX512
DMX stands for Digital
The information of the control channel is transmitted in digital form,
channel for channel, so send successively (multiplex) over the cable.
speed, in which this takes place, is so high that it occurs as
However, this speed causes that a DMX network must be structured in
way (see under
The information of the control channel has
the form of
an alternating voltage model. Hereby the voltage can only take two
Digital, bits and bytes
To DMX512 applies: The voltage between the
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.
the content of the information is bivalent: either ' 0 ' or a ' 1 '.
smallest quantity of the information is called "bit". The transmission
of one bit information lasts 4[us], according to DMX512. By combining
bits, the information can obtain several values. The combination of 8
results in 256 different values: 00000000=0, 00000001=1, 00000010=2,
etc.. An expressing of the information in ' 0 ' and ' 1 ' is called
A combination of 8 bits is called a byte.
With the DMX512
the information for a control channel is expressed in a byte.
the control information is directed toward a dimmer, then the
intensity can be set up to max. 256 different values. (resolution
This step size is mostly sufficient in
for lights with a performance > 100W an intensity step of 1/256
perceptible. For moving lights a smaller step size can be necessary.
DMX protocol is then modified to16 bits. (The Toddler
can be set up to16 bits to control of moving lights.)
The structure of the
The DMX512 signal consists of a repetitive
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
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'
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
- A byte is preceded by a '0':
this is called "start bit "
- A byte is terminated by two
'1-s': these are called " stop
- With the receipt of the
start bit always eleven bits " are
read ": these eleven bits form the date frame.
The DMX signal consists of a fast changing
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.
- Waves exists: A level change
does not manifest itself directly
everywhere on the cable.
- There is signal distortion
by wave reflection at cable transitions.
Transitions are: attached receivers, connectors, and the end of the
- There is signal distortion
through frequency dependency of
the signal run time. This limits the cable length.
- The induction
characteristics and the capacity of the cable
step into the foreground. A measure for these characteristics is the so
called characteristic resistance (characteristic impedance) of the
This determines the resistor, necessary to terminate the cable end.
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
of these two signals. Disturbance, in both signals available, is
in such a way. This increases the working reliability.
A shielded cable with a twisted pair of
necessary. The characteristic impedance must be approximately 120[Ohm
The twisted pair of conductors transmit the signal (marks with ' + ')
its inversion (marked with '-'). The shield used as low impedance
for the signal reference. It is very important for the control of the
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
The length of the control cable limited
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
must be bridged, a repeater must be used. With several receivers the
of the receivers determines the maximal length of the cable, for which
a repeater must be used.
- Voltage drop in the
- Signal distortion by run
time difference for the harmonious,
of which the signal is composed
- The wiring in the receiver
and the quality of the used plug
- The number of the receivers,
that are attached over the cable
- The specification of the
transmitter and the receiver (EIA
How to wire?
The structure of a DMX network earns
To avoid signal distortion by reflections the following must be
- The DMX cable must go from
receiver to receiver (= "daisy
chain"). A device controllable with DMX is prepared for this, by means
of the XLR plug and socket connectors for respectively DMX in and DMX
- At the last device the cable
termination must take place.
This means that between ' + ' and ' - ' a resistor must be set, which
is equal to the characteristic resistance of the cable. This is
by putting into the DMX out a plug connector, in which a resistor of
] is soldered.
- In an attached device the
connection of the XLR connector
and the receiver IC may not be longer than 30[cm ].
- If the cable is splitted, an
active "splitter" must be used (see
A trouble free
Disturbances with digital control in
general, so also with
the DMX, can appear as short unforeseeable disturbance of the normal
The installation can work well for hours, and stammers then suddenly. A
trouble free process is only achieved if one takes
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
- Consider above mentioned
- Make sure that the
transmitting and receiving part is galvanic
ally separate in the devices. So never connect the shield of the cable
with the housing or the earth. Only then, the differences momentary in
the potential of the earth do not have an effect.
- Do not put the control
cables into the proximity of the mains
cables. Mains cables can produce magnetic fields. The effectiveness of
the cable shield for low frequency magnetic fields is nil. For fields
high frequency the shield has only effect if the manufacturer of the
has executed the DMX in and DMX output according to the EMC standard
- Pay attention to the
presence of the CE marking.