Morning, CB!
Time for another NNQotD! Thanks to recent joblessness, I've been recently taking a signals course from ETC that goes into detail re: the DMX-512a protocol, and I have a few rather practical questions. No need to answer them all at once, since I have quite a few, and have all the time in the world to wait for answers:
Thanks for those of you who take time to answer one or more of these questions. Figured I should use my time "off" to really amp up my educational pursuits in entertainment, and a better understanding of DMX seemed a great place to start.
Also, I'll go ahead and put my ego aside and ask: does anyone have any theatrical studies textbooks they no longer need? I want to keep studying, but I've hit the limit of what I can afford to purchase, and $60+ for each textbook while being out of work really doesn't seem wise right now. I'll happily accept digital copies if they exist, as well. Please DM me if you are willing to help, I really do appreciate it. Thanks!
NOTE: I've gotten some preliminary/rough answers on these questions from Wikipedia (I RTFM'd), but I'd prefer expert opinion/fact over Wikipedia's version of fact.
Time for another NNQotD! Thanks to recent joblessness, I've been recently taking a signals course from ETC that goes into detail re: the DMX-512a protocol, and I have a few rather practical questions. No need to answer them all at once, since I have quite a few, and have all the time in the world to wait for answers:
- As I understand, DMX-512 inherits the properties of its parent protocol, RS-485. Two of these properties are a limiting length of 1,640' of home run wire, and a limit of 31 receiving devices. Is this a hard and fast rule, or a recommendation? I've worked on systems (that I did not design, however) that place much more than 32 devices on a single universe contained within a single cable run. What are the potential consequences of exceeding this limit? Also: I've been taught that 300' is the practical limit for a DMX home run, particularly if non-terminated. Am I remembering that wrong?
- Piggybacking off of question #1, I've noticed that the RS-485 "stub rule" (no more than 12" of wire between data backbone and receiver) is violated by DMX-512, but is considered an acceptable violation. Why is this? Are there no consequences to violating that inherited property? Why does the stub rule exist for RS-485 in the first place? I can't find an explanation anywhere in the course.
- The course has not explained what pins 4 and 5 are for on a 5-pin XLR connector. I assume 4/Data 2- and 5/Data 2+?
- What does terminating really do for a system? I assume that by increasing resistance at the end of a data run, if data were to flash back/reflect, it would be far too weak (not enough voltage) to meet the requisite threshold to trigger the comparator inside the receiving device's decoder? So why not terminate with a much higher resistance than 120ohm? Or is 120ohm more than enough to do the job already?
- Within the structure of a DMX packet, I've noticed that there are specific timings associated with the MAB and interbytes, to allow for counter reset and counter increment, respectively. These times have been set to a minimum of 8μs, but can be slowed if necessary. I understand that if a [less capable] device's counter is unable to reset in time, it will either lose its place in the packet, or perhaps never clear its framing error. Is that what devices such as the Doug Fleenor DMX Decelerator are for? To lengthen timings of the MAB and interbytes to allow inferior/slower counters to "keep up"?
Thanks for those of you who take time to answer one or more of these questions. Figured I should use my time "off" to really amp up my educational pursuits in entertainment, and a better understanding of DMX seemed a great place to start.
Also, I'll go ahead and put my ego aside and ask: does anyone have any theatrical studies textbooks they no longer need? I want to keep studying, but I've hit the limit of what I can afford to purchase, and $60+ for each textbook while being out of work really doesn't seem wise right now. I'll happily accept digital copies if they exist, as well. Please DM me if you are willing to help, I really do appreciate it. Thanks!
NOTE: I've gotten some preliminary/rough answers on these questions from Wikipedia (I RTFM'd), but I'd prefer expert opinion/fact over Wikipedia's version of fact.
Last edited: