dbuckley Posted July 11, 2006 Share Posted July 11, 2006 The DMX512/1990 spec (section 9.0.2) defines pins 4+5 as "optional second data link". So (a) the connection of pin 4+5 is not required by the standard, this optionality is further supported by section 10.0 which lists acceptable cables, the first two listed are Beldon 9841 and "alpha 5271 (one pair...", both of which are single pair cables. DMX512-A lists two types of connection, C1 and C2, for one pair and two pair respectively. (b) The use of the word "second" implies that the other data pair is the first data pair, so one may reasonably conclude it was for similar purpose. Against this is that the first pair are labelled for "dimmer drive". Avo were quite clever in what they did. The second data link is known to be RS485, so the additional cost of implementing the second data link is actually quite small, just a socket, a driver chip, and a few passives. What you do with it is then down to software. All RS485 interface chips are bi-directional supporting both reception and transmission, so any half duplex or one way protocol could be used, and another universe of DMX512 is just one of many things one could do with said extra link. Overall, however, these are the sorts of problems you get when standards are made up by amateurs. Lord knows, DMX512 has served us well, and will do so for some years yet, but as a standard (as is usually understood in the world of standards) its a bit hit and miss. Link to comment Share on other sites More sharing options...
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