DMX512 OPTICAL ISOLATOR OWNERS MANUAL Doug Fleenor Design 396 Corbett Canyon Road Arroyo Grande, CA 93420 (805) 481-9599 revision 2.
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.
The input to the DMX Opto Isolator is passive, and, since the DMX512 standard specifies an active input, the DMX Opto Isolator's input does not meet the DMX512 specification. This is 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.
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.
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.
Isolating the middle dimmers from reflections may be accomplished by placing a DMX Optical Isolator at the "loop through" connection of the dimmer in the middle of the control line.
Sometimes it is necessary for a control run to be split before reaching the first dimmer. The resulting "wye" connection is a reflection nightmare. Now you have reflections coming from two directions, and you can't even terminate both ends of the wye without overloading the console! +------+ +----------|DIMMER| | +------+ +-------+ | |CONSOLE|-----------------+ +-------+ | | +------+ +----------|DIMMER| +------+ Editors Note: The proper item to use in this situation is an isolated splitter.
LINE TERMINATION Line Termination refers to the electrical properties at the end of a control cable. In analog, and analog multiplex systems, line termination is less important than it is in high speed digital applications. Termination always improves DMX512's signal quality. In some simple cases it can be ignored with little loss of reliability. Line Termination becomes more important as cable length increases, and as more dimmers (receivers) are connected to the line.
Typical Characteristic Impedances of Control Wire BELDEN SERIES NUMBER * NOMINAL IMPEDANCE 9804 100 Ohms 8132 120 Ohms 9829 100 Ohms 8332 Not Rated 9501 Not Rated 8102 100 Ohms 9729 100 Ohms 8162 100 Ohms 9990 50 Ohms* 9841 120 Ohms 9880 100 Ohms 8777 50 Ohms* 9873 50 Ohms* 9773 50 Ohms* EIA-485 drivers (on which DMX512 is based) are rated for a minimum load impedance of 60 Ohms. Use of cables with a characteristic impedance of less than 100 Ohms is not recommended.
OPERATION The DMX Opto Isolator has no operator controls; it is fully automatic. It should be left plugged in at all times for automatic operation. When a DMX signal is applied to the input, it is isolated by the optocoupler and instantly (within 1/10,000,000 second) retransmitted by the output driver. When the signal is removed from the input, the optocoupler is idle, however the output driver continues to drive the DMX line in an idle (high) state as long as the isolator is powered.
When the -data line is lower than the +data line, the LED indicator is extinguished and the output driver drives the +data line low, the -data line high. When there is no drive to the isolator (such as when the console is turned off) the LED indicator is extinguished and the output driver drives the data lines to their idle state (+data line high, -data line low). ABOUT THE COMPANY Doug Fleenor Design is a semi-custom design and manufacturing company.
