Doug Fleenor Design DMXOPTO-5 - Manual (Page 5)

4
PROTECTING AGAINST ELECTRICAL STORM DAMAGE
Voltages induced on long control cables during electrical storms
may be isolated from both the console and the dimmer input
circuitry by placing an Opto Isolator at each end of the control
cable. Shielded cable should then be used to connect the output
of the isolator at the console to the input of the isolator at
the dimmer with the shield connected to pin 1 at the console end
and to a good earth ground at the dimmer end.
Extremely high voltages, caused by direct strikes to lighting
towers or power feeds may still cause damage. With Optical
Isolators installed, this damage will hopefully be limited to the
optocoupler or EIA-485 driver inside the isolator. In the event
of such failure, the Optical Isolator may be easily replaced with
a backup. The isolator may then be returned for repair without
removing the console or dimmers from service. The repair to the
isolator should be less expensive than repair to an unprotected
dimmer or console, plus it's easier to ship.
BUFFERING CONTROL LINE INTERSECTIONS (WYES)
Editors Note: When this product was introduced in 1990 it was our
only isolation product. Doug Fleenor Design now makes a complete
line of isolation and distribution products, many of which are
more suitable for the problems listed below. The following text
is left in this manual as a guide on how to use the Optical
Isolator when more suitable products are not on hand.
The data rate of DMX512 is 250,000 bits per second. This is a
relatively high data rate, and is susceptible to errors caused by
signal distortion. One cause of signal distortion is
"reflections". A reflection is caused when an electrical signal
reaches the end of the wire; a portion of the electrical energy
is reflected back up the wire.
In a simple system, where the console is at one end of the wire
and the dimmer(s) are at the other end, reflections may not be a
significant problem. Since the dimmers are at the end of the
wire, the reflection is not 'seen' by them, and the resulting
signal distortion often does not cause errors.
Consider what happens, however, if a dimmer is placed in the
middle of the wire, such as if a run is made from the console to
the stage right dimmers, and on to the stage left dimmers. The
stage left dimmers are at the end of the cable and do not see the
reflections going back up the wire. The stage right dimmers
(located in the middle of the control run) see the initial
control signal from the console, but it is distorted by the
reflected signal coming back up the line. Two possible solutions
exist to this problem... eliminate the reflections (see the
section on line termination) or isolate the reflections from the
dimmers in the middle of the control run.