DMX Isolation - Page 1
Forum section: electronics / projects

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 05, 2017, 07:41:08 am
content: I'm thinking about building a DMX interface (I could just buy one but...) to control a few lights during some stop frame animation.DMX seems an interesting protocol and a slight variant of normal serial so it seems to be doable using a PIC UART...I have looked at a few designs and there seems to be a variation regarding the amount of isolation provided.The simplest designs directly expose the differential pair and have a common ground... e.g MAX481CPA, SN75176B types.The really expensive designs use something like a ISO35T which is a very expensive ($10) and rare chip.I imagine there could be some middle ground for opto isolation?If I were to extend my lighing outside (i.e. hostile environment with conductive water) I can see that isolation may become important.Any recommendations on the level of required isolation for DMX?Thanks in advance

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 05, 2017, 08:15:45 am
content: Depends on the power supply - if your local PSU isn't connected to mains ground, isolation isn't generally necessary, as teh main issue is ground loops and differences in ground potential. As long as the supply grounds are isolated ( which will usually be the case unless you're using PC power supplies).DMX receive with a normal UART is very simple - framing error = break condition. The only time it can get fiddly is with FIFO'd UARTs, as their behaviour is often not clear from the datasheet & you have to experiment to see how they handle breaks and how to clear the condition without losing data in the fifo.Although DMX is 8N2, for receive-only, setting the UART to 8N1 gives better tolerance to poor signal conditions.

author: mrpackethead
author_rank: Super Contributor
timestamp: July 05, 2017, 08:53:28 am
content: I use the ISO35, and its not $10. i think it mmight be $3. The alternative is that you can use a discrete optoisolator, and a small isolated DC/DC converter. If you want to do it properly.Theres LOTS of gear as you say that does not have any isolation

author: Kjelt
author_rank: Super Contributor
timestamp: July 05, 2017, 09:32:28 am
content: Do you need the optional data2 ?If not an ISO15 would suffice saving almost half the cost.

author: mrpackethead
author_rank: Super Contributor
timestamp: July 05, 2017, 09:55:26 am
content: In my case yes I do. My interfaces are bidirectional so I can do dmx in and out (rDm). Do this discreetly and it ends up being three OPTO

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 05, 2017, 10:53:46 am
content: Quote from: mikeselectricstuff on July 05, 2017, 08:15:45 amThis is early days but I can imagine that ultimately I will be trying to use the same interface in three incarnations:Driven by a PC, using PC power over USB so referenced to PC powerIf I do my own light controller ( at the lamp end ) referenced to a supply at that end driving a lamp via a Triac or relay? (Not sure of that detail yet / I might actually just buy that part)Possibly as a remote controller attached to Ethernet or WiFi. Very likely referenced to a ground in common with a cheapo 12V chinese PSU powering to 5x string(s) of 300 WS2812A lightsI'm conflating a few different projects together but it would be nice to have a common approach.Quote from: mikeselectricstuff on July 05, 2017, 08:15:45 amThanks. I think I can see where you are headed there.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 05, 2017, 11:01:36 am
content: Quote from: mrpackethead on July 05, 2017, 08:53:28 amTo be honest cost isn't a huge factor since this is for local consumption only (and compared to time, component cost is trivial) I will look around for a better price if I go that route.I also found this article...http://www.analog.com/en/analog-dialogue/articles/isolated-rs-485-in-dmx512-lighting.htmlQuote from: mrpackethead on July 05, 2017, 08:53:28 amI'm not familiar with anything like that. Would you buy such a thing as a module or try and hand craft it?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 05, 2017, 11:10:20 am
content: Quote from: NivagSwerdna on July 05, 2017, 11:01:36 amBecause it's a cheap & easy solution. Magnetics for isolated converters are much less available as standard parts than simple inductors for nonisolated converters. Off-the-shelf isolated converters are cheap nowadays.For examplehttp://uk.farnell.com/recom-power/ree-0505s/converter-dc-dc-1x-o-p-0-2a-5v/dp/2218599http://uk.farnell.com/recom-power/r1sx-3-33-3-r/dc-dc-converter-3-3v-303a/dp/2747534A very easy but expensive solution is the ADM2582, which integrates the isolation and dc-dc converter for a single chip isolated RS485 solution

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 05, 2017, 11:17:49 am
content: And if you want receive-only without too many nodes, the NVE passive-input isolators, e.g. IL610 are an option.

author: ajb
author_rank: Super Contributor
timestamp: July 05, 2017, 12:21:07 pm
content: Quote from: mrpackethead on July 05, 2017, 09:55:26 amUsing an ISO35 because you want RDM doesn't make any sense. RDM is half-duplex on the primary data pair (because the utility of the second pair was never firmly established so it wasn't implemented reliably--hence so much gear has 3-pin XLRs) so you would be tying A/B and Z/Y together to basically turn it into an ISO15. Implementing the second data pair is going to require a second transceiver.There's not a tremendous amount of value in implementing the second data pair unless you're trying to squeeze two universes into one physical connector or have an option for a non-DMX full-duplex interface of some sort.Personally, I usually go with separate isolated DC-DC, digital isolator (much smaller than three optocouplers, and not much more expensive), and standard transceiver. The PCB space isn't much more than some of the all-in-one solutions, and it permits some supplier diversity.Regarding level of isolation, it depends on your environment. Generally, the purpose is just to break a potential ground loop of a couple of volts, but in a system of mains-powered devices, think about what could happen if something goes VERY wrong. Risks increase in setups where signals cross multiple mains power domains (think big festivals with multiple generators), so take the level of care appropriate to the level of risk. In systems of more than a handful of devices, an isolated splitter becomes mandatory. In a typical lighting setup, an isolated splitter with a sensible topology is nearly as good as having isolated interfaces on every unit.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 05, 2017, 12:29:37 pm
content: Quote from: ajb on July 05, 2017, 12:21:07 pmCan you suggest a part? In Opto land 6N137 seems very popular.

author: ajb
author_rank: Super Contributor
timestamp: July 05, 2017, 12:42:42 pm
content: Quote from: NivagSwerdna on July 05, 2017, 12:29:37 pmJust looking at the board next to me, Si8641. DMX is only 250kbaud, so just about any digital isolator will be fine as long as it has the number of channels you need in each direction. If you only ever want to receive, then a single opto is fine, otherwise you need at least two output and one input channel for TX,DE, and RO.

author: mrpackethead
author_rank: Super Contributor
timestamp: July 05, 2017, 01:49:34 pm
content: Quote from: ajb on July 05, 2017, 12:21:07 pmYes, RDM is interleaved between the DMX frames. The two pairs in the ISO 35T are complemtry, one TX pair and one RX pair. to get a bidirectional interface you need to use both of them. The TX pair and the RX pair are joined in parrallel on the isolated side. The reason for doing this, is because I wanted the ability to loopback in some circumstances.. ( this was a dmx diagnostics tool and we were doing some trickery ). If i did'nt want to do this, i agree, the 15 would suffice...