DMX 512 Physicals

In fact so many stage lights support DMX512. Most of stage lights must have this function.

The DMX bits are also represented by a digital high (HI) or a digital low (LO).The actual DMX output transmits these HI and LO signals in an electrical form that is explained in the page -

DMX512 PHYSICALS

The DMX data stream clocks out at the rate of 250Khz which means each bit is measured at 4 micro seconds widths.

1) IDLE or NO DMX situation:
In the absence of a valid DMX packet the output of a DMX line will be a continously HI signal.

2) BREAK
The start of a DMX packet is heralded by the output going LO for a MINIMUM period of 88 microsecs. This means 22 LO bits are measured out one after the other. This is known as the BREAK. The BREAK could be longer but not less than 88 microsecs. Experience shows that slightly longer breaks (above 88 microsecs) sent from a console are better since the receiving devices are generally given the algorithm = “Is the BREAK>88 microsecs or 22 pulses”. I keep it generally at 100 -120 microsecs in equipment designed by me

3) MARK AFTER BREAK (MAB)
The MAB immediatly follows the BREAK by making the output go HI for a MINIMUM period of 8 microsecs or 2 pulses. This MAB is a bit of a problem since the difference between the original DMX512 and the current DMX512(1990) standards relate to this period of the packet. The original was set at 4 microsecs or 1 pulse. This created hassles for some receivers for being too short a MAB for detection and was upgraded to 8 microsecs or 2 pulses in 1990. The problem comes when a older console is used with newer receivers or vice versa.Wrong detection will lead to packet rejection or the wrong data going to the wrong channel. This will travel down the line leading to utter confusion. Some receivers have a dip switch to set this parameter for both timings. Again the maximum MAB length could be 1 sec.
My ideal timing would be 12 microsecs.

4) START CODE (SC)
The SC is next in the line. It is easier to remember that the SC is the start of the actual data stream where all individual channel data have the same format. The BREAK & the MAB were of different timings but the SC onwards all frames will have the same structure and timing of 11 pulses or 44 microsecs width. The first one can be termed as data for channel No 0 which is a non-existent channel and represents the SC.

I will first describe the general structure of these channel data frames:

-Of the 11 pulses the 1st one is always LO signifying the Start bit .

-This is followed by the actual data byte of 8 bits (which could be any of 256 values from 0 to 255).

-The frame ends with 2 bits which are HI signifying the two stop bits and end of the channel information.

Channel No ’0′ is the SC, which as things stand, ALWAYS has the databyte = 0 signifying that the following data is for dimmers. As per the current standard, no other value can be used. The option is left open and as and when ESTA specifies, the SC value may be used to tell the receiver that the data following it is meant for a specific type of receiver. That is the end purpose of having the SC….. to be able to segregate a packet of data, receiverwise. But, for the moment, it’s zero which has been specified for dimmers by ESTA. Do remember that this also includes just about any receiving device like dimmers, moving lights or whatever !

5) MARK TIME BETWEEN FRAMES (MTBF)
The mark time between frames can be from a little more than 0 sec to upto 1 sec, but the lesser the better. Each channel frame can have the MTBF before the start bit. The MTBF is obviously HI .

6) CHANNEL DATA (CD)
The CD frames follows the SC frame in a logical manner from 1 to 512 (or less) in the form described above.

7) MARK TIME BETWEEN PACKETS (MTBP)
After the last valid CD stopbits are sent, one full packet is completed and the next packet can start with a fresh BREAK & MAB. However an idle (HI) can be inserted between packets (MTBP), the lenth of which may be a little more than 0 sec to upto 1 sec. It is upto the console designer to design the architecture of the console and the software powering it in such a manner that the data thruput time is at a minimum.

 

When a stage laser light have DMX function, it can be controlled using DMX console.