THE DMX512 "PACKET"

The DMX512 Packet is the heart of the Standard, it consists of a collection of frames "wrapped up" with further synchronization information known as a 'Break' and a 'Mark-after-break'. It is this information that enables a receiver to detect the start of a frame and hence make some sense of the data it is receiving.

Examination of the frame timing reveals that the maximum time that the line can be 'low' is 4µs (Start bit) + (8 x 4µs (Data bits)) = 36µs, however a "Break" consists of a 'low' of at least 88µs, the difference between these two is easily detectable and is used for dimmer synchronization.

The "Mark-after-break" is a 'high' state on the line and is at least 8µs long, the "Mark-after-break" is required so the end of the "Break" can be detected otherwise the "Break" would run into the Start bit of a frame and everything would get very confused! A diagram of the "packet" should help explain things...
Fig 4

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The picture wouldn't be complete without "Start codes" and "inter frame time".The "Start Code" is a frame (like the others) except that it is the first frame after the break and is used as a flag to indicate the type of data that follows.

A value of '0' indicates that the following frames contain dimmer level information.The other 255 codes are not defined in the Specification but some manufacturers use other codes to send product specific information. A dimmer receiving a non-zero start code 'should' ignore the rest of the packet, but be careful - it isn't always checked!

The "Inter-frame-time" is used to slow down the data rate - some dimmers cannot cope with the data running flat out, or, more usually to 'pad' the transmission while the console gets on with some other task. It can have any value from 0 to 1s.

The Specification sets some limits on the timings as follows:

It can be seen from these figures that there is a great deal of flexibility an the data rates, however, it should be noted that a line state continuously 'high' or 'low' for more than 1s is not legal and should be considered an error condition.

IN CONCLUSION (is your brain fried yet?)

Almost all glitches happen with the DMX512 systems are due to lack of knowledge of the system. One area that causes problems is in splitting the signal. Remember the system works by terminating segments. Simply splitting the line (which seems logical) will not work due to the vagaries of Ohms. Doing so will result in the signal being corrupted.

The solution is to use 'splitters' or 'repeaters', powered devices that 'listen' to the data on a segment, and then broadcast, or 'repeat' it onto further segments as required.

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These boxes come in all sorts of configurations, offering more and more freedom, starting at simple 1 to 1 and advancing to multi-ported opto-isolated, with new features being added as fast as designers can integrate them.