Thanks to all for these suggestions. I will look into them and respond, might be a few weeks now due to other jobs. As you feel confident in having eliminated all external and 'user-solvable' issues, it seems likely that the problem is in 'service' level territory which requires information that you probably need to ask your local Martin distributor for e.g schematics and service information including software updates where possible. If sending channel 1 to a Lamp On state level doesn't do anything (yet the fixture is obviously capable of lamping on) then it seems reasonable that either the fixture is not getting that information or doesn't know what to do with it. Perhaps an earlier 812 software version didn't have a DMX lamp on function. The added bonus with this age of kit is the potential foul-up of the Martin 485 vs DMX 'auto' detection along with the dreaded hot-cold swap. With or without further information, then you are looking at board swap diagnosis although I would imagine that in this case there aren't that many different options to swap. If you are going to the trouble of de-rigging and opening fixtures, then you might check for similarities/differences between the problem unit and the a working one including if there is some kind of sticker or other indication of software on the main board. It doesn't seem like the problem would be caused by the mode jumper. As mentioned, something physical shouldn't be ruled out yet. Thanks for your help mate, the thing thats baffling me is that it worked fine before. This leads me to think that I has to be some kind of hardware problem?
