2024-06-26
Introduction to the protocol DMX512
Introduction to the protocol DMX512
DMX512 Introduction:
DMX512 is a digital dimming protocol, it can be applied to the stage, theater, studio and other places of dimmer and other control equipment for digital control, suitable for a multi-point master-slave control system, its interconnection form adopts a multi-point bus structure, there is no information path blocking problem, its connection is simple, high reliability.
Although the lighting control system has been developed into a networked control system, the DMX512 signal system is still the most widely used system, even if it is a large networked lighting control system, the end of the control is still a large number of DMX512 signal control.
DMX512 protocol is suitable for the point-to-multipoint master-slave control network system. According to the requirements of DMX512 data transmission rate and the characteristics of decentralized control network, the design of the physical layer uses RS-485 bus transceiver, and the bus uses a pair of twisted pairs to realize the connection between the dimmer and the dimmer. RS-485 bus adopts balanced transmission and differential reception, receiving sensitivity is high, and anti-interference ability is strong, the signal transmission distance can reach 1000m. If the dimmer is too far away from the dimmer station, an amplifier or a signal separator can be used to enhance the signal. DMX512 protocol adopts bus structure, but the data is one-way transmission from the dimmer to the dimmer, so there is no competition between each dimmer to use the bus and lead to information blocking phenomenon.
DMX512 protocol is suitable for the point-to-multipoint master-slave control network system. According to the requirements of DMX512 data transmission rate and the characteristics of decentralized control network, the design of the physical layer uses RS-485 bus transceiver, and the bus uses a pair of twisted pairs to realize the connection between the dimmer and the dimmer. RS-485 bus adopts balanced transmission and differential reception, receiving sensitivity is high, and anti-interference ability is strong, the signal transmission distance can reach 1000m. If the dimmer is too far away from the dimmer station, an amplifier or a signal separator can be used to enhance the signal. DMX512 protocol adopts bus structure, but the data is one-way transmission from the dimmer to the dimmer, so there is no competition between each dimmer to use the bus and lead to information blocking phenomenon.
DMX512 packet format:
The DMX512 protocol has certain requirements for the data transmission rate, the typical transmission rate is 250kbps, the corresponding duration of each bit is 4us, the duration of each data frame is 44us, and the protocol also stipulates that each DMX512 data packet supports a maximum of 512 frames of data transmission. The format of the DMX512 packet is shown below:
The DMX512 protocol stipulates that data packets must be transmitted in accordance with the specified format and transmission baud rate. The format of a complete DMX512 data packet consists of an MTBP signal, a BREAK signal, a MAB signal and an SC signal, and the following data frames. Their respective meanings are as follows:
(1) MTBPMTBP (Mark Time Between Packages) marks the completion of the sending of a complete DMX512 data package, and is also the next data package is about to start. The high level is effective, indicating that the current transmission line is idle and there is no data transmission.
(2) BREAKBREAK is the revelation control signal of a DMX512 packet, corresponding to the reset stage after the end of a packet, and the next packet of data should be sent after the reset is completed. The protocol specifies that the BREAK signal is valid at a low level, and the duration is not less than the length of two DMX512 data frames, that is, 88us.
(3) MABMAB (Mark After Break) marks the beginning of a packet transmission. Since the first bit of each data frame is low, MAB signal is added to distinguish the low level of the BREAK from the low level of the start bit of the data frame. The protocol specifies a typical duration of MAB of 8us, a time of two bits, active at a high level.
(4) SC (Start Code) SC is the start code, which is the same as an ordinary data frame, but its 8 data bits are zero, marking the start of the data frame in the data packet.
(2) BREAKBREAK is the revelation control signal of a DMX512 packet, corresponding to the reset stage after the end of a packet, and the next packet of data should be sent after the reset is completed. The protocol specifies that the BREAK signal is valid at a low level, and the duration is not less than the length of two DMX512 data frames, that is, 88us.
(3) MABMAB (Mark After Break) marks the beginning of a packet transmission. Since the first bit of each data frame is low, MAB signal is added to distinguish the low level of the BREAK from the low level of the start bit of the data frame. The protocol specifies a typical duration of MAB of 8us, a time of two bits, active at a high level.
(4) SC (Start Code) SC is the start code, which is the same as an ordinary data frame, but its 8 data bits are zero, marking the start of the data frame in the data packet.
Features:
Compared with other protocols, DMX512 protocol has the following characteristics:
(1) the signal is transmitted based on differential voltage, and the anti-interference ability is strong;
(2) Using RS-485 bus transceiver, the signal can be transmitted long distance;
(3) The data refresh is fast, regardless of whether the output of the optical device needs to be changed, the host must send a control signal, the time between the data frame and the data frame is less than 1s, if the dimmer does not receive a new data frame within 1s, it can be known that the data has been lost;
(4) The implementation is simple and does not require special hardware equipment support.
DMX512 instruction frame introduction:
Each DMX control byte is called an instruction frame, called a control channel, which can control one or more functions of the lighting device. A DMX instruction frame consists of 1 start bit (S), 8 data bits (D0-D8) and 2 end bits (E), a total of 11 bits, using one-way asynchronous serial transmission, as shown in the figure:
The S in the control instruction in the dotted line in the figure is the start bit and the width is one bit, which is the start sign that the controlled lamp is ready to receive and decode the control data;
E is the end bit, the width is two bits, indicating the end of an instruction frame;
D0 ~ D7 is 8-bit control data, its level combination from 00000000 to 11111111 a total of 256 states (corresponding to the decimal number of 0~ 255), control the brightness of the light, can produce 256 brightness levels, 0000~ (0) corresponding to the darkest light, 11111111(255) corresponding to the brightest light.
The bit width (per bit width) of the DMX512 instruction is 4us, and each instruction frame is 11 bits, so the instruction frame width is 44us, and the transmission rate is 1/44us = 250 kbps.
E is the end bit, the width is two bits, indicating the end of an instruction frame;
D0 ~ D7 is 8-bit control data, its level combination from 00000000 to 11111111 a total of 256 states (corresponding to the decimal number of 0~ 255), control the brightness of the light, can produce 256 brightness levels, 0000~ (0) corresponding to the darkest light, 11111111(255) corresponding to the brightest light.
The bit width (per bit width) of the DMX512 instruction is 4us, and each instruction frame is 11 bits, so the instruction frame width is 44us, and the transmission rate is 1/44us = 250 kbps.
DMX512 packet:
A complete DMX512 Packet consists of one MTBP bit, one Break bit, one MAB bit, one SC and 512 data frames.
MTBP(Mark TimeBetween Packets) marks the completion of a complete information packet and is the "idle bit" that the next information packet is about to start. The high level is valid.
Break is an interrupt bit, corresponding to the program reset phase after the end of a packet, and the width is not less than two frames (22 bits). Control data should be sent after program reset.
MAB bit, because the first (that is, the start bit) of each data frame is low, it must be separated by a high level pulse before and After two low level pulses, the high level pulse that plays the interval and separation role is MAB(Mark After Break), the pulse arrives, meaning that the "new round" of control starts again.
SC(Start Code) means the start code frame (frame 0 in Figure 1), which, like subsequent data frames, is also composed of 11 bits, except for the last two high end bits, the other 9 bits are all low levels, usually called frame 0 or channel 0 (Ch~nel No 0). It can be understood as a non-existent channel (non-1 ~istent Channe1).
Break is an interrupt bit, corresponding to the program reset phase after the end of a packet, and the width is not less than two frames (22 bits). Control data should be sent after program reset.
MAB bit, because the first (that is, the start bit) of each data frame is low, it must be separated by a high level pulse before and After two low level pulses, the high level pulse that plays the interval and separation role is MAB(Mark After Break), the pulse arrives, meaning that the "new round" of control starts again.
SC(Start Code) means the start code frame (frame 0 in Figure 1), which, like subsequent data frames, is also composed of 11 bits, except for the last two high end bits, the other 9 bits are all low levels, usually called frame 0 or channel 0 (Ch~nel No 0). It can be understood as a non-existent channel (non-1 ~istent Channe1).
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