Doug Fleenor Design RR (Rotor-Router) - Manual (Page 6)

5
TECHNICAL DETAILS
For those technical types out there, here is a brief description of how the DMX512 combine unit
works.
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, and the wires on which they travel 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 combine unit 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 Combine Unit uses the "MAX483" as receivers and transmitters. The "MAX483" was
designed specifically to meet EIA-485.
The DMX512 signals from the consoles are applied to the input connector of the combine unit.
These are received by "MAX483"s and fed through optical isolators (6N137) to individual
microprocessors (PIC18C242). Each input has its own microprocessor, as does the output. The
microprocessors all exchange data over an 8 bit bus. The output microprocessor sends the combined
DMX512 signal to a "MAX483" which generates the differential output.
The main 5 volt power supply consists of two step-down transformers connected in parallel, a bridge
rectifier, a 2200 uF filter capacitor and a "7805" 5 volt regulator. Up to six DC to DC converters
provide isolated 5 volt power for the isolated DMX512 inputs.