Doug Fleenor Design 1211 - Manual (Page 4)

TECHNICAL DETAILS
DMX512 uses a "differential" digital signal. A differential signal is a pair of signals which are
inverse from one another. That is, when one is high, the other is low. Differential signals are
common in both analog processing (balanced microphone cables) and digital processing (digital
communications such as EIA-485). Differential signals are used to reduce the effect of electrical
noise on long cable runs. When a differential signal is processed by the receiving circuit
(microphone pre-amp or EIA-485 receiver for example) the noise can be eliminated but the signal
remains.
Differential signals always travel in pairs; the wires they travel on are usually twisted to insure any
noise picked up is of equal magnitude on both wires. The noise must be of equal magnitude on each
wire so that the receiving circuit can effectively subtract the noise and leave the signal. The pairs
of signals are usually denoted by a plus (+) sign on the normal, or "hot", or "true" line and by a minus
(-) sign on the inverted, or "return", or "complement" line. The differential signals on the splitter are
Data+ and Data-.
DMX512 is a standard for lighting control published by the United States Institute For Theater
Technology. The DMX512 standard specifies that the electrical properties of the signal comply with
a standard published by the Electronic Industries Association, EIA-485 (or RS-485). Many
manufacturers make integrated circuits that meet the input and output specification of EIA-485. The
DMX512 Isolated Splitter/Amplifier uses a "75179" as the receiver and "75176's" as transmitters;
both of which were designed specifically to meet EIA-485. Between these circuits is a "6N137"
optical coupler which isolates the two circuits using a beam of light passing through a layer of glass.
The "6N137" is rated to a maximum speed of 10 million bits-per-second and provides isolation to
2500 Volts.
Since the receiver and transmitters all require power, and they must be isolated, multiple 5 volt
power supplies are included. Transformers, each with two secondaries, steps the 120 volt line down
to 10 volts. A 1N4004 diode converts the 10 volts AC to 14 volts DC. A 330 uF capacitor filters
the DC and a 7805 regulates it down to 5 volts.
Resistors are included on the input of the splitter. The one, three, and five output models have 1K
pull up/down resistors on the input to insure the output goes to the idle state when no source is
connected. A 120 ohm termination resistor is across the data lines to prevent reflections. On the
eleven output model the 120 ohm termination resistor is absent since a pass through connector is
provided to loop on to additional splitters. Also, the eleven output model uses 22K pull up/down
resistors so as to not load the line.