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GENERAL
INTEREST
8-Channel DMX
Demultiplexer
eight DMX channels = 8 x 0-10 V
(with relays to come soon)
Design by B. Bouchez
bbouchez@netcourrier.com
‘DMX512’ stands for ‘Digital Multiplex’, which is a technique that allows
up to 512 channels to be driven sequentially (multiplexed) using a single
cable. The circuit presented here is intended to be used to control
DMX512-compatible devices.
the DMX512 standard, we recom-
mend that you read this
article [1], as well as
the one on the
MICI–DMX interface [2].
Put briefly, the DMX512
protocol allows an
RS485 link to be used to
connect up to 32 devices
over a distance of up to
1000 m. The data are
transferred at a rate of
250 kbits/s, with the
data format being 8 data
bits, no parity, 1 start bit
and 2 stop bits. The infor-
mation needed for the
various channels is sent
sequentially in the form
of 8-bit values ranging
from ‘0’ (‘off’) to ‘255’
(‘on’). In order to mark the
start of the series of 512 val-
ues, communication is inter-
rupted by simply generating a
‘Break’ (logical ‘0’ level) with a
duration of at least two frames.
Finally, a ‘high’ level lasting at least
8
The DMX512 system was defined in
1986 by the USITT (the US organisation
responsible for the development and distrib-
ution of standards for the theatre world), but
it did not really take off until the mid 1990s,
after the use of automatic spotlights became
popular.
Several articles devoted to the DMX512 sys-
tem have
already appeared in earlier
issues of
Elektor Electronics
. The
first one appeared in the first quar-
ter of this year. If you want to know
more about the technical details of
s is generated to mark the start of
the first byte following the break.
µ
56
Elektor Electronics
11/2001
GENERAL
INTEREST
+5V
+5V
C4
+5V
IC2
47n
74F573
+5V
C5
C1
11
40
C1
1
47n
EN
1
µ
10V
20
9
30
C7
ALE
RESET
1
28
IC2
R1
VPP
IC1
47n
10
D0
D1
D2
D3
D4
D5
D6
D7
39
38
37
36
35
34
33
32
D0
D1
D2
D3
D4
D5
D6
D7
2
3
4
19
10
AD0
AD1
AD2
AD3
AD4
AD5
AD6
AD7
1D
A0
A1
A2
A3
A4
A5
A6
A7
18
9
8
7
6
5
4
3
17
5
6
16
IC3
15
11
D0
D1
D2
D3
D4
D5
D6
D7
D0
7
8
9
14
12
D1
13
13
EPROM
D2
12
15
D3
S1
16
80C32
27C256
D4
1
2
3
4
16
1
2
3
4
5
6
7
8
21
22
23
24
25
26
27
28
25
17
P1.0/T2
A8
A9
A8
D5
15
24
18
P1.1/T2EX
A9
D6
14
21
23
19
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
A10
A11
A12
A13
A14
A15
A10
A11
A12
D7
13
5
6
7
8
12
2
11
26
27
A13
A14
10
9
OE
CE
22
14
20
DMX CHANNEL
29
PSEN
S1:
1 = 8
2 = 16
10
P3.0/RXD
3 = 32
4 = 64
11
P3.1/TXD
5 = 128
12
13
14
15
5
P3.2/INT0
P3.3/INT1
+15V
6 = 256
7
IC5.B
6
P3.4/T0
P3.5/T1
R2
IC5, IC6 = TL084
16
+15V
P3.6/WR
31
17
EA
P3.7/RD
10
K1
X1
X2
8
1
2
3
4
5
6
7
8
IC5.C
4
4
20
19
18
D2
9
IC5
11
IC6
11
C6
C9
L1
1
H5
µ
47n
47n
X1
10V
3
-5V
1
C3
C2
IC5.A
7
2
33p
33p
24MHz
8
2
SDI
VRHA
VO1A
VO2A
VO3A
VO4A
VRLA
3
4
5
6
1
+5V
ISO
IC4
+5V
9
CLK
12
14
IC5.D
C11
13
C10
R3
47n
DAC8800
ISO
47n
13
19
15
16
17
18
20
LD
VRHB
VO1B
VO2B
VO3B
VO4B
VRLB
8
7
IC8
2
12
IC7
1
CLR
R6
5
7
IC6.B
6
10
RD+
8
CLK
R5
390
6
2
3
RD–
R
Ω
7
5
6N137
TD+
5
14
11
3
10
K2
TD–
D
6
8
1
2
3
4
5
6
7
8
IC6.C
+15V
9
AC A
+5V
R7
IC9
IC10
4
LTC490
7815
7805
SHLD
B1
R4
ISO
3
1
IC6.A
C12
2
C13
C14
C15
AC B
D
1
1000
µ
25V
100n
100n
100n
B80C250
12
+5V
+5V
ISO
POWER
14
IC6.D
AC C
13
IC12
1
6
VI+
VO+
5
B2
CTL
2
4
VI-
VO-
C16
C17
C8
AC D
NMF0505S
ISO
1000
25V
100n
100n
µ
IC11
B80C250
010002 - 11
79L05
-5V
Figure 1. The schematic diagram of the demultiplexer shows that it is a microcontroller system reduced to its simplest form.
11/2001
Elektor Electronics
57
GENERAL
INTEREST
Why do we need
a demultiplexer?
As already mentioned, ‘DMX512’ stands for
‘Digital Multiplex’, since data for the 512
channels are sent sequentially (multiplexed)
over a single cable.
Devices that come standard with a DMX512
interface are generally more expensive than
equivalent devices that can be remotely con-
trolled by an analogue signal. Besides, for cer-
tain types of equipment it is not necessary to
have a built-in interface, whether DMX or oth-
erwise. This is particularly true of spotlights
used in theatres or television studios that
only allow the brightness to be adjusted. This
category also includes special-effects
devices, such as stroboscopes and smoke
generators.
For such simple applications, either simple 0-
10 V remote control is used or the control is
entrusted to a dimmer.
The task of the demultiplexer is to provide a
set of control voltages that allow all of theses
types of devices, which previously could only
be controlled by a voltage between 0 and
10 V, to be controlled using DMX512. If we
add relays to the repertoire of our demulti-
plexer, it can also control simple ‘all-or-noth-
ing’ devices (those not having an analogue
control input, in other words). With a bit of
practical skill, it’s even possible to fit our
demultiplexer to devices that are not
designed for remote control and thus turn
them into semi-professional devices, but let’s
take things one at a time….
K1
L1
C3
R2
D2
C2
H4
C
C9
X1
C6
B2
~
IC4
D
IC11
K2
C1
A
~
B1
B
C12
IC12
C10
C5
C7
S1
R3
1
8
D1
C11
C14
010002-1
H2
-
+
C15
Figure 2. The printed circuit board and component layouts for this project.
board layout easier.
Readers who are familiar with the
80C32 (such as those of you who
have followed the microcontroller
course in
Elektor Electronics
) may be
surprised to see this microcontroller
being used in this manner, since its
UART cannot detect the Break sig-
nals that form the basis for synchro-
nising DMX512 frames. There’s no
need for concern; the detection of
Break signals is definitely within the
scope of the 80C32’s capabilities.
This is because our program simply
detects the status of the ninth bit of
every byte. Since the DMX512
frames consist of bytes with no par-
ity, the ninth bit is actually the image
The real work…
A quick study of the electronics, whose
schematic diagram is shown in
Figure 1
,
shows that once again a microcontroller has
the leading role. Here we have used an 80C32
in the well-known classical manner, with
external program memory in the form of an
EPROM. (By the way, we could have just as
well used a microcontroller with an on-board
EPROM, which would make the schematic
diagram even simpler.) Using an EPROM
makes it easier to implement a new version
of the program in the future. As is usual in
such cases, the address bus is demultiplexed
by IC2, which provides the eight least signif-
icant bits of the address to the EPROM (IC3).
As you can see, not all of the address lines of
the EPROM are actually used. This is because
the program is so compact that it would eas-
ily fit into a 27C64. However, these devices
are becoming increasingly rare and thus more
expensive, which is good reason to use a
27C256 and leave the unnecessary address
lines unused. This also makes the circuit
Figure 3. A fully assembled example of the DMX demultiplexer.
58
Elektor Electronics
11/2001
GENERAL
INTEREST
COMPONENTS LIST
Resistors:
R1 = 220k
Ω
R2, R4 = 220
Ω
R3 = 1k
Ω
R5 = 390
Ω
R6, R7 = 4k
Ω
7
Capacitors:
C1 = 1
F 10V radial
C2,C3 = 33pF
C4-C7, C9,C10,C11 = 47nF
C8,C13,C14,C15,C17 = 100nF
C12,C16 = 1000
µ
F 25V radial
µ
Semiconductors:
B1, B2 = B80C250 bridge rectifier (80V piv,
250mA cont.)
D1 = LED
D2 = zener diode 10V 500mW
IC1 = P80C32SFPN (40-pin DIL case, tem-
perature range = 0-70°)
IC2 = 74F573 or 74HCT573 *
IC3 = 27C256 (programmed, order code
010002-21
)
IC4 = DAC8800FP(Analog Devices)
IC5, IC6 = TL084
IC7 = LTC490 CN8 (Linear Technology)
IC8 = 6N137
IC9 = 7815
IC10 = 7805
IC11 = 79L05
IC12 = NMF0505S or TMA0505S
010002-1
Miscellaneous:
K1, K2 = 8-way SIL pinheader
L1 = choke, 1
H5*
S1 = 8-way DIP switch
X1 = 24MHz quartz crystal
Mains transformer, secondary 2x15V at 3VA
IEC mains appliance socket with earth con-
nection and internal fuse, 25mA
Enclosure, size 200x80x132mm, e.g., Telet
type LC270
PCB, order code
010002-1
Disk, project software, order code
010002-11
* see text
µ
010002-1
(C) ELEKTOR
of the stop bit and thus must be a ‘1’.
If the microcontroller detects a byte
in which the ninth bit is a ‘0’, this
means that the DMX interface is in
the synchronisation phase. Although
this is a somewhat unorthodox
approach, it works perfectly (we
have used this method for several
years in professional designs with-
out any problems).
It’s not necessary to say much about
the peripheral components around
the 80C32, thanks to the use of a
standard circuit. However, note that
a 24-MHz crystal must be used in
order to provide the serial port with
a data rate of 250 kbits/s. Normally,
it is not easy to find a 24-MHz crys-
tal. Consequently, we have included
a small coil (L1) to force the crystal
to oscillate at a harmonic overtone.
In the section of this article that
deals with the practical aspects of
the design, we describe how to
determine whether this coil is actu-
ally necessary.
The address selected using the dip
switches is read via the P1 bus.
Since our multiplexer has eight voltage out-
puts, it occupies eight DMX channels. The
DMX address of the demultiplexer is config-
ured ‘modulo 8’, which means that it can be
located only every eight channels (1, 9, 17, 25
etc.). In order to determine the channel num-
ber of the first DMA channel, all you have to
do is to add the values of all of the closed
switches (as marked) and then add 1 to the
sum. For example, if none of the switches is
closed, DMX channels 1–8 are occupied. If the
first and third switches on the interface are
closed, the demultiplexer occupies eight
channels starting with channel number 41 (=
11/2001
Elektor Electronics
59
GENERAL
INTEREST
8 + 32 + 1), or in other words, channels
41–49. You will probably notice that only the
first six lines are used to select the DMX
address. The other two lines are used to acti-
vate certain demultiplexer options. Presently,
only P1.6 is used, in order to activate the
‘relay output’ option.
The connection with the DMX bus is pro-
vided by IC7, a standard RS-485 transceiver,
whose output is not used here. Resistors R6
and R7 pull the line to a defined level in the
quiescent state, which prevents incorrect
behaviour if the interface is disconnected
from the DMX bus. We have chosen to have
the DMX output run via solder pads in order
to leave ourselves a free choice of XLR con-
nectors, since both the 3-pin and the 5-pin
versions are used. At the end of this article,
we provide more information about these
connectors.
In order to avoid problems with ground loops,
the LTC490 is electrically isolated from the
rest of the circuit by means of a fast optocou-
pler (IC8). This part of the circuit is powered
by a static DC/DC converter (NMF0505S)
with 5-V input and output.
In order to generate the eight control volt-
ages, we use an 8-channel DAC from Burr
Brown (type DAC8800). Besides the fact that
it contains eight R-2R converters, the major
advantage of the DAC8800 is that it can be
driven via a synchronous serial link, which
allows the circuit board to remain relatively
simple. Without this component, we would
have had to connect eight DACs (such as the
DAC08) in parallel and use an address
decoder, which would make the circuit board
layout considerably more complex.
Since the manufacturer of the DAC8800 rec-
ommends keeping the supply voltage on
pin 7 at least 4 V higher than the maximum
output voltage, in order to ensure maximum
linearity, the supply voltage for IC4 is 15 V. It
should be noted that compatibility with 5-V
TTL outputs is guaranteed by an internal
voltage regulator.
The reference voltage inputs VHRA and
VHRB can be used to set the maximum out-
put voltage of the DAC. Here they are con-
nected to a simple 10-V Zener diode that pro-
vides the reference voltage. In spite of the
fact that this solution is far from being as pre-
cise as a ‘real’ reference voltage, it has proven
to be perfectly satisfactory for our purposes,
and besides that it is inexpensive.
Since the output impedance of the DAC8800
is rather high, problems could arise if the out-
puts were directly connected to devices
whose input impedance is only a few thou-
sand ohms. For this reason, each outputs of
the DAC are buffered by opamps (TL084).
Since the saturation voltage of the opamp
standard version
(5 pins)
simplified version
(3 pins)
3
3
4
2
input,
mal
e
Pin
Function
5
1
2
1
1
2
3
4
5
0V/Ground
Data -
Data +
free (optional data -)
free (optional data + )
3
3
2
4
input,
female
1
5
1
2
010002 - 12
Figure 4. Just for reference: 3-way and 5-way XLR connectors.
output stage prevents the opamp
from working down to 0 V if the neg-
ative supply pin is connected to
ground, it is instead connected to
–5 V.
The final item in the schematic dia-
gram is the power supply, which
does not need any particular expla-
nation except with regard to the
transformer to be used. The ‘AC A’
and ‘AC B’ inputs must be connected
to a transformer with a secondary
voltage of 18 V, in order to ensure
that the voltage on the input of IC9
is sufficiently high.
As regards the negative supply,
everything depends on the manu-
facturer of IC11. If you use a
MC79L05 (Motorola is now ‘ON
Semiconductor’) or an equivalent
device, there is no problem. This is
because the MC79Lxx ICs from this
manufacturer are specified for 30 V
(18 V
ac
, rectified and filtered, gives a
peak voltage of 27 V). With this reg-
ulator, you can thus use the other
secondary winding of the trans-
former connected to ‘AC A’ and
‘AC B’. The problem is that some
manufacturers of this type of voltage
regulator do not specify the input for
anything higher than 15 V. In case of
doubt, a second transformer with a
9-V output must be used for this part
of the supply.
components, the practical imple-
mentation of the demultiplexer,
using the printed circuit board
shown in
Figure 2
, should not pre-
sent any problems.
Even if you decide not to use IC
sockets for economic reasons, it’s
hard to overstress the fact that you
should always use sockets for IC7
and IC8. Should there be any serious
problems on the DMX bus (excessive
voltages, electrostatic discharges
etc.), it is a lot easier to replace these
IC’s if they are mounted in sockets.
Pay attention when fitting IC3, the
EPROM, since its orientation is
opposite to that of the rest of the ICs.
We would also like to point out that
a programmed EPROM can be
obtained from the usual reliable
source (Readers Services) under
order number
010002-21
.
In light of the low power consump-
tion of the circuit, a small piece of
aluminium is adequate as a heat
sink for IC9 and IC10.
As far as K1 and K2 are concerned,
you can choose whatever best meets
your needs (screw terminals, DIN
connector, XLR connector etc.). In
our case, as can be seen from the
pictures at the head of the article
and in
Figure 3
, we chose to use DIN
connectors with the outputs divided
into two groups of four each, but you
are naturally free to make your own
choice.
Once all components have been fit-
ted and checked, you can apply volt-
age to the circuit. Now you can
check the various supply voltages, in
Get out
your soldering iron!
Due to the small number of compo-
nents and the absence of any critical
60
Elektor Electronics
11/2001
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