DMX512 Theory

■ DMX512

DMX512 is mainly used for stage lighting and outdoor lighting.

The specific control forms are: WiFi, infrared induction, remote control, human body induction, etc.

1. Contrary to DALI used to express static effects, DMX512 is mainly used to express the dynamic effect of the illuminated object, that is, the lights controlled by DMX give people the feeling that they are always changing, mainly color changes, which are suitable for rendering the atmosphere, so Generally used in landscape lighting and stage lighting.

▲ Shown as above, a number of RGB lamps, each of which contains a DMX module, the module will drive the LED lamp beads to change the brightness and darkness, so as to adjust the color change when it receives the power supply and the controller signal at the same time. .

2. Features of DMX512

■ Each output loop supports up to 512 channels

Each output loop supports a maximum of 512 channels. According to the number of channels occupied by a single device, the maximum number of devices that can be carried by each port is determined.

▲RGBW single pixel lamp connection diagram

■ 256-level dimming grayscale (0-255)

Each DMX control byte is called a command frame, called a control channel, which can control one or several functions of the lighting equipment. A DMX instruction frame is composed of 1 start bit, 8 data bits and 2 end bits in total, and adopts one-way asynchronous serial transmission.

The S in the control command in the dotted line in the above figure is the start bit, with a width of one bit, which is the start flag that the controlled lamps are ready to receive and decode the control data; E is the end bit, with a width of two bits, indicating the end of a command frame .

D0 and D7 are 8-bit control data, and their level combinations range from 0000—1111 to 1111, with a total of 256 states (corresponding to 0 to 255 in decimal). When controlling the brightness of the light, 256 brightness levels can be generated, 0000~ (0) The corresponding light is the darkest, and 1111 1111(255) corresponds to the brightest light. The bit width (width per bit) of the DMX512 instruction is 4 s, the width of each frame is 44 s, and the transmission rate is 250 kbps.

■ Standard EIA485 interface

The wiring method is a bus topology and does not support a ring or star network. Up to 32 nodes can be attached on the same bus.

Structures of various shapes, DMX512 should follow the first bus structure, with a head and a tail.

The picture above is an underground lamp project in Guangzhou. The wiring of the signal line is relatively random (as shown in the picture on the right), which will increase the workload of the follow-up work.

■ Transmission rate 250Kbps

Bit rate refers to the number of bits transmitted per second, bps (Bit Per Second). The baud rate is closely related to the construction specification, and the high baud rate signal has poor anti-interference ability.

DMX512 Notes

In actual operation, sharing a few experiences can reduce some control problems:

① The transmission distance of the super five network cable does not exceed 100 meters;

② When the distance between controllers exceeds 100 meters, use optical fiber;

③ The EIA485 protocol stipulates that the number of nodes carried by each repeater does not exceed 32;

④ The wiring of each control loop must follow the bus structure layout;

⑤ Signal lines and power lines should be laid separately.

No matter how low the probability, bad things will always happen. Even if you use the DMX512 control system according to the steps and standards, you can always encounter various problems.