DMX auto configuration issues

[DJ Jules]

Following on from this post (http://www.forum.mobilediscodirectory.co.uk/showthread.php?41528-Wireless-DMX-retro-fit-of-my-fixtures!) yesterday about retro fitting fixtures with Wireless DMX, I got thinking about an issue I've been having for a while with a couple of my fixtures. I've got a pair of Eurolite Scanners and when I put them on the same DMX chain as my Acme Cougar's, they go nuts. Basically they switch from DMX control to master/slave and try and either slave the Cougar's or switch the Cougar's to master!

I normally get round this by putting them on their own DMX chain as I'm lucky enough to be using a DMX controller with four electrically isolated outputs. I'm assuming though that the problems are being caused by whatever custom protocol the Scanners and Cougar's are using to try and identify if there are any other fixtures on the DMX chain that they can talk to and I've been trying to work out how to block this from happening for a while. Yesterdays conversations about Wireless DMX and issues with the DMX Transceiver switching to "Transmit" mode because of the auto configuration stuff going on made this issue more relevant again as I've ordered 6 Wireless DMX PCB's :D I'm fairly sure that quite a few of my fixtures are going to cause issues with the Transceiver mode which will rule out hard wiring the PCB's to the DMX input - which means doing things like wiring the transceivers on separate power and enforcing a strict start up order (great fun if the power fails for any reason!!)

Does anyone know anything about the configuration protocol used by Acme and others? Has anyone else come across this problem? Has anyone found a solution? I've had a Google and have come up with nothing about this (or RDMX, which probably operates on similar principles).

Julian

[bmdj]

The cougars are very bizarre when it comes to the auto master slave function. I have had them both working via dmx wireless but its a matter of turning them on last to avoid the auto mode kicking in. I was considering trying some diodes on the data lines to prevent the wiresless dmx units switching from receive to transmit, but Ive not tried that yet. I think it will be a matter of experimentation. On the plus side you could use two wireless dmx modules on another channel to allow the units master/ slave to each other.

[DJ Jules]

The cougars are very bizarre when it comes to the auto master slave function. I have had them both working via dmx wireless but its a matter of turning them on last to avoid the auto mode kicking in. I was considering trying some diodes on the data lines to prevent the wiresless dmx units switching from receive to transmit, but Ive not tried that yet. I think it will be a matter of experimentation. On the plus side you could use two wireless dmx modules on another channel to allow the units master/ slave to each other.

I was wondering if adding diodes to one of the channels would work, but I've just read RS485 spec and it says that a binary 0 is set when the + line is lower voltage than the - and a binary 1 is when the + line is higher voltage than the - (both compared to the ground reference) - or in other words, both + and - lines can carry voltage both ways so adding a diode will break communication.

I might have to break out the oscilloscope from the loft to see what the DMX signal actually looks like. I don't think I've got the tools to work out what the Cougars are actually doing though.

Reading the RS485 spec is interesting as the protocol is actually intended to be able to run bi-directional communications, but the DMX spec states that it's unidirectional. Found the spec (http://en.wikipedia.org/wiki/RDM_(lighting)) for RDMX (RDM) though, and there's an interesting bit in there about compatibility with older devices which probably tells us what we need to know:


Any devices that provide galvanic isolation or buffering on the DMX line (such as DMX splitters) have traditionally been designed to allow transmission in one direction: from the controller to the devices. As RDM requires bi-directional communication these devices will typically fail. Only devices that have been designed with RDM compatibility as a feature will typically work. Older DMX splitters that are not RDM compatible should still reliably send the DMX data, and block RDM communication.

So, I need a unidirectional DMX buffer or a DMX repeater. Probably something like this (http://www.ebay.co.uk/itm/DMX-Inline-Buffer-Isolator-PCB-DMX-512-Opto-Iso-DMX-Voltage-Protection-/271234264646?pt=US_Stage_Lighting_Controllers_Dimm ers&hash=item3f26d26e46). Shame they've blurred out the IC details in the photo, $35 seems a bit steep for what it is. I'm guessing it's probably this one (http://uk.farnell.com/exar/sp485cs-l/trx-rs485-lp-half-duplex-8pdip/dp/1831934) though which would do the job by itself as the Optical isolation isn't really needed.

Hmmm, going to have to do some more digging :D

Julian