Doug Fleenor Design DMXOPTO-5 User Manual

DMX512 OPTICAL ISOLATOR

OWNERS MANUAL

Doug Fleenor Design
396 Corbett Canyon Road
Arroyo Grande, CA 93420

[phone]

revision 2.0
December, 1996

PRODUCT DESCRIPTION

The DMX512 Optical Isolator (DMX Opto Isolator) provides complete
electrical isolation between lighting consoles and associated
dimmers when installed in the control line. The electrical
connections is broken by using an optocoupler which passes the
control signal using a beam of light. Designed to protect the
console from damage in the event of dimmer failure, the Opto
Isolator may also be used to eliminate ground loops, protect
against lighting damage, and buffer the DMX signal at control
line intersections (wyes).

The input stage of the DMX Opto Isolator is passive. The United
States Institute for Theater Technology (USITT) standard for
DMX512 calls for the input of a receiving device to meet EIA-485
specifications. The DMX Opto Isolator's passive input does not
adhere to EIA-485, and so it's input does not strictly meet the
DMX512 specification. See this manual's section on installation
for more information on the Opto Isolator's input, why it doesn't
meet DMX512, and why it doesn't need to.

The DMX Opto Isolator uses a 10,000 Kilo Baud optocoupler. The
DMX512 data rate is 250 Kilo Baud. The reason for using such a
high speed optocoupler is to insure that the DMX Opto Isolator
does not introduce any signal degradation, even when several are
daisy-chained together.

The DMX Opto Isolator's output section is fully DMX512 and EIA­485 compatible. The output driver is also capable of 10,000 Kilo
Baud operation.

SETUP AND CONNECTIONS

The DMX Opto Isolator is designed to be placed as close to the
console as possible. In fact, it is possible to plug the
isolator directly into the output jack of the console, and the
control cable onto the end of the isolator. This method of
connection eliminates the need for any additional cables.

In some installations, the console is too close to the wall to
allow the isolator to "hang off the back". In other cases the
likelihood of damage to the "extended connector" makes the direct
installation unwise. In these cases, a short (1 to 2 meter)
DMX512 cable may be used between the console and the isolator, so
that the isolator may be placed on the floor or table near the
console. (The isolator does not need to be close to the console
for proper operation. However the longer the cable between the
console and the isolator, the less protection the isolator will
provide from electro-magnetic induction).

The DMX512 input to the isolator is a male 5 pin "XLR" style
connector, the output is a female 5 pin connector. These are the
connectors specified in the DMX512 standard. The mating
connector for the input to the isolator is a female 5 pin
connector such as the Switchcraft model A5F. The mating
connector for the output from the isolator is a male 5 pin
connector such as the Switchcraft model A5M.

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The input to the DMX Opto Isolator is passive, and, since the
DMX512 standard specifies an active input, the DMX Opto
Isolator's input
not a serious drawback, however, since the Opto Isolator is
designed to be the only load on a particular console port
(whereas there are usually several dimmers connected to one
port). The isolator's output meets the full DMX512
specification, however, and restores the full drive capability of
the port. (The fact that the isolator's input is passive is
another reason it is recommended that it be placed close to the
console).

does not meet the DMX512 specification. This is

DMX Opto Isolator Input Pinouts

5 pin male "XLR" style

(accepts DMX512)

1 - No Connection
2 - Data Complement (-Data)
3 - Data True (+Data)
4 - No Connection
5 - No Connection

DMX Opto Isolator Output Pinouts

5 pin female "XLR" style
(meets DMX-512 standard)

1 - Common
2 - Data Complement (-Data)
3 - Data True (+Data)
4 - No Connection
5 - No Connection

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BREAKING GROUND LOOPS

The term Ground Loop refers to an electrical connection that
results in two paths for current from one point to another, with
one of the paths usually occurring through the building's
electrical ground system. Ground loops result from two pieces of
electrical equipment being connected to electrical power in
different places, and then being connected together. In audio
systems ground loops frequently occur, and can result in audible
hum or buzz. The art of eliminating ground loops in audio
systems is simplified by being able to hear the result; when the
buzz goes away, the job is done.

Serious ground loops in dimming systems are less common but can
be difficult to trouble shoot if they exist. Unlike audio loops
that result in an audible hum, a ground loop in a multiplexed
dimmer control application may not have continuous visible
symptoms. A ground loop in an analog multiplex system (AMX-192
or CD-80) can result in steady flicker, but even a bad ground
loop in a digital multiplex system may not impair performance for
long periods of time (hours or days). From one standpoint this
is an advantage to digital multiplex... it is much more immune to
noise and ground loops. On the other hand, if a problem exists,
it is harder to pinpoint.

When hum exists in an audio system, one useful tool to isolate
ground is an audio transformer. The transformer passes the audio
signal through a magnetic field while breaking the electrical
connection. The DMX Optical Isolator provides a similar function
for a DMX512 control signal. It passes the lighting control
signal (using a beam of light) while breaking the electrical
connection.

If you suspect a ground loop is causing problems in your lighting
system, a DMX Optical Isolator, installed at the console, will
insure that no ground loop can be made through the isolator.
Make sure, however, that there is no connection to the metal
shell of your control wiring connectors. The isolator's shell is
metal and will carry current around the isolator's internal
isolation circuitry if a connection is made to the shell of the
connector (only pins 1, 2, and 3 should have any connections to
them).

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