The stage lighting DMX512 control signal transmission line
Since stage lighting control into the digital era, DMX512 protocol long-term holding stage lighting control of dominance, become each stage lighting workers knows a technology. For DMX512 but in practice, the selection and use of a transmission line is very chaotic, use do not conform to the requirements of the signal transmission line phenomenon is widespread. Not only in temporary flow performance, also exist in the construction of theater. Transmission line is the nervous system of the DMX512 control, its quality directly affect the transmission of control signal quality. Inferior transmission line will threaten the reliability of the lighting control and stability, and can bring fatal harm to performance. For theatre, wiring system is the basis of the "one hundred plan", people should give more attention. In recent years, domestic presented a theatre construction, the author took part in a lot of theatrical lighting design review and evaluation, lighting engineering, equipment generally found in the review this problem didn't get the attention they deserve. Even in some national key projects, problem also exists.
DMX512 control system should use what kind of communication cable to make the system work reliably? Why must use special cable? Why use non-standard transmission line problem? To answer these questions must first to understand DMX512 protocol. DMX512 technology academy of drama in the United States (the USITT) formulated specially used in stage lighting console and dimmers between data communication protocol standards, then expanded to many kinds of stage equipment such as light, color changer control field. DMX512 main contents is made to the data communication protocol, which directly related to the signal transmission cable main content includes:
1. The communication interface adopts computer communication in general the EIA - 485 standard, its electrical properties fully refer to the EIA - 485.
2. The DMX512 protocol signal transfer rate of 250 KBPS.
In the face of DMX512 signal reliable transport and should be used under the wire rod are analyzed.
In radio theory has a long term theory, the so-called "long term" refers to the length of the transmission line can be compared with the current wavelength spread along the transmission line to line. Is generally thought that the length of the transmission line is greater than one over ten of a wavelength (l > - l), can be considered as long term (some think l > - l is long term). So long is not necessarily a long line. Such as the average frequency of the PC's CPU has reached thousands of MHZ (G, 109) an order of magnitude, the mainboard of frequency is also corresponding to a, two hundred million, so a small printed circuit board also want to consider in the design of long term problems. In long term situation, some of the common circuit principle is no longer applicable, need to use special transmission line theory to analysis.
DMX512 control signals of the transfer rate of 250 KBPS, namely 250000 binary code sent per second, in other words, send 250000 rectangular pulses per second. According to the frequency spectrum of the signal analysis theory, the rectangular pulse contains a lot of higher harmonic, even if only one of the lowest frequency of fundamental wave and l > - l conditions to calculate, when the frequency is 250 KHZ, wavelength is 1200 m, then transmission line is more than 120 m shall be deemed to be the long term. In fact its harmonic is not allow to ignore, so the distance is shorter. So the DMX signal transmission must be treated as long term.
In ordinary circuit, circuit of capacitance is focused on the capacitor, inductor is concentrated in the coil, the resistance concentration in the resistor, while the resistance of the conductor is distributed parameter, but also its equivalent often seen as a simple resistor, this is known as a lumped parameter circuit. Each specified in the lumped parameter circuit, instantaneous, the voltage and current at every point along the transmission line is the same, can be from any point on a transmission line of process to determine the phenomenon occurs on a transmission line, the voltage and current is a function of time, has nothing to do with the location in the circuit. But in the long term situation, must consider the distribution parameters, the line has a small piece of each of the resistor, capacitor, inductance and conductance. So long term should be considered by many the four-terminal network line unit connected, every success in the terminal network have the line unit resistance R, inductor L, capacitance C and conductance G (as shown in figure 1). The characteristics of long term by its distribution parameters R, L, C and G.