DMX512
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:
- A byte is preceded by a '0':
this is called "start bit "
- A byte is terminated by two
'1-s': these are called " stop
bits "
- With the receipt of the
start bit always eleven bits " are
read ": these eleven bits form the date frame.
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:
- 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
cable.
- 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
cable.
This determines the resistor, necessary to terminate the cable end.
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:
- Voltage drop in the
conductors
- 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
connectors
- The number of the receivers,
that are attached over the cable
- The specification of the
transmitter and the receiver (EIA
RS-485 specification)
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:
- 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
out.
- At the last device the cable
termination must take place.
This means that between ' + ' and ' - ' a resistor must be set, which
value
is equal to the characteristic resistance of the cable. This is
achieved
by putting into the DMX out a plug connector, in which a resistor of
120[Ohm
] 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
PASS1-3
en ISO2-4)
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:
- Consider above mentioned
wiring rules
- 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
with
high frequency the shield has only effect if the manufacturer of the
devices,
has executed the DMX in and DMX output according to the EMC standard
- Pay attention to the
presence of the CE marking.
This information is offered to you by
Theater Technisch Lab
BV, manufacturer of DMX devices, digital dimmers and digital lighting
control
desk.