Understanding Specifics of the DMX512 Protocol [Newbie Nate's Question of the Day]
- Thread starterNateTheRiddler
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Simple. The cable length limits are, as the name implies, limits on the length of the cable, and not on the length of individual conductors within the cable. Bear in mind that an electrical cable, by definition, is some number of wires joined together in a single unit (and usually with a single outer sheath). If making a DMX512 cable, you just get pull your handy 1700' tape measure out of your bag and make sure you don't have more than 1640' of cable between the ends.
Technically speaking, the length limits are based on any of various electrical characteristics of the cable, and those are in turn determined by the physical construction of the cable. For ethernet, at least as originally conceived, it's the propagation time from one end to the other, so as to ensure proper collision detection: if the cable is too long, it becomes possible for two notes at opposite ends to transmit a short packet simultaneously and finish before the start of the opposite packet arrives, making it seem that there is no collision when there actually was one (and nodes in the middle cannot make sense of either packet). The propagation time for signals in the wire is a shade under the speed of light. For DMX512, I think it might more be determined by the distributed capacitance and impedance of the cable causing enough signal loss and waveform distortion to start to make data transfer unreliable; propagation time is of little concern, basically none if RDM is not being used (and even if it is the timings should allow for much longer cables based on signal propagation times). For AC mains wiring, the limiting factor for length is often based on voltage drop due to the resistance of the wire--and can be overcome by using heavier wire.
Technically speaking, the length limits are based on any of various electrical characteristics of the cable, and those are in turn determined by the physical construction of the cable. For ethernet, at least as originally conceived, it's the propagation time from one end to the other, so as to ensure proper collision detection: if the cable is too long, it becomes possible for two notes at opposite ends to transmit a short packet simultaneously and finish before the start of the opposite packet arrives, making it seem that there is no collision when there actually was one (and nodes in the middle cannot make sense of either packet). The propagation time for signals in the wire is a shade under the speed of light. For DMX512, I think it might more be determined by the distributed capacitance and impedance of the cable causing enough signal loss and waveform distortion to start to make data transfer unreliable; propagation time is of little concern, basically none if RDM is not being used (and even if it is the timings should allow for much longer cables based on signal propagation times). For AC mains wiring, the limiting factor for length is often based on voltage drop due to the resistance of the wire--and can be overcome by using heavier wire.
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