StuartS Posted February 6, 2018 Share Posted February 6, 2018 Has anyone used signal diodes to block interference on DMX lines?We have two low end moving heads hopefully receiving control via an inexpensive wireless link. One of the heads has the irritating trait of going into a demo mode in the absence of DMX. Not only that but the unit defaults to a ‘master’ state and squirts some sort of signal out of its input in the hope of being heard by another of its kind in the ‘slave’ mode. This has the effect of putting the wireless unit into transmit, so, unless the components are powered up in the right order and DMX trasmission is solid, the heads ignore control.Short of putting in an active isolator, I wondered I could use low voltage drop diodes in the D+ and D- lines to stop the receiver seeing spurious signals from the head and thereby remain in the receive mode. Then when DMX is established it should take control.Anybody tried it? Link to comment Share on other sites More sharing options...
gyro_gearloose Posted February 6, 2018 Share Posted February 6, 2018 Do you have to use wireless DMX? Your current setup requires that everything is powered up in the right order, and that you maintain a solid wireless link. Even if you do manage to stop the receiver from seeing the data sent out from your heads, they will still go into master/slave mode when you lose DMX. If it were me, I'd stick our Linebacker between the receiver and the heads in order to maintain a steady DMX signal. Link to comment Share on other sites More sharing options...
timsabre Posted February 6, 2018 Share Posted February 6, 2018 Short of putting in an active isolator, I wondered I could use low voltage drop diodes in the D+ and D- lines to stop the receiver seeing spurious signals from the head and thereby remain in the receive mode. Then when DMX is established it should take control. This will not work, the DMX line is a differential signal so needs to both source and sink current. (When data "high" state the A line sources current and the B line sinks it, for data "low" state it is the other way round). What is the model/manufacturer of the problem fixture, the "master" mode can often be disabled but it is usually not obvious how... Link to comment Share on other sites More sharing options...
electronicsuk Posted February 6, 2018 Share Posted February 6, 2018 A dumb DMX splitter or isolator should do the job, as most are designed for one-way communication only, input to output(s). Link to comment Share on other sites More sharing options...
BenEdwards Posted February 6, 2018 Share Posted February 6, 2018 A dumb DMX splitter or isolator should do the job, as most are designed for one-way communication only, input to output(s). We have aTranscendent S4 DMX Booster/Splitter, indexpensive and opticaly isolated (on eatch of the 4 chanels and input). Link to comment Share on other sites More sharing options...
StuartS Posted February 7, 2018 Author Share Posted February 7, 2018 1517951473[/url]' post='559466']1517946425[/url]' post='559460']Short of putting in an active isolator, I wondered I could use low voltage drop diodes in the D+ and D- lines to stop the receiver seeing spurious signals from the head and thereby remain in the receive mode. Then when DMX is established it should take control. This will not work, the DMX line is a differential signal so needs to both source and sink current. (When data "high" state the A line sources current and the B line sinks it, for data "low" state it is the other way round). What is the model/manufacturer of the problem fixture, the "master" mode can often be disabled but it is usually not obvious how... Just to update you, I have wired two diodes into a DMX lead and in a quick test it does seem to pass signal in the forward direction. This makes sense in that, relative to ground, both data lines go positive, albeit never at the same time. I have yet to try it in anger. The problematic fixture in question is a just cheap little moving wash under the brand name Tomshine which boasts RGBWA and UV. Obviously if the application demanded professional gear I wouldn't be averse to using linebackers or opto isolators, and wired connections, but in this case a short rig time and simplicity rules. Link to comment Share on other sites More sharing options...
timsabre Posted February 7, 2018 Share Posted February 7, 2018 Just to update you, I have wired two diodes into a DMX lead and in a quick test it does seem to pass signal in the forward direction. This makes sense in that, relative to ground, both data lines go positive, albeit never at the same time. I have yet to try it in anger. The problematic fixture in question is a just cheap little moving wash under the brand name Tomshine which boasts RGBWA and UV. Obviously if the application demanded professional gear I wouldn't be averse to using linebackers or opto isolators, and wired connections, but in this case a short rig time and simplicity rules. Um, there's nothing to pull the lines low though except the tiny load of the DMX receiver chip in the fixture - it's basically floating or pulled high. I think you are setting yourself up for further unreliability here. Link to comment Share on other sites More sharing options...
Brian Posted February 7, 2018 Share Posted February 7, 2018 Why not disable the transmit side of the faulty device? It's likely to use '176 transceiver so just cut the tx enable pin and tie it to the inactive state. Link to comment Share on other sites More sharing options...
sunray Posted February 7, 2018 Share Posted February 7, 2018 I'm trying to remember back a long way but I'm pretty sure the lines switch back and forth in polarity and fairly big between something like -9V to +12V the same as RS232 possibly? If that is the case diodes can not work. I must stress that it is many years since I have got deeply involved in this sort of detail. Link to comment Share on other sites More sharing options...
timsabre Posted February 8, 2018 Share Posted February 8, 2018 DMX (RS485) goes between 5v and 0v. Link to comment Share on other sites More sharing options...
Brian Posted February 8, 2018 Share Posted February 8, 2018 DMX (RS485) goes between 5v and 0v. Unless your transceivers are running on 3.3V ;) What is important is not the voltages put out by the transmitter but the VOLTAGE DIFFERENCE between the A and B lines seen by the receiver. The difference needs to be greater than 200mV in one direction to be seen as a logic '1' and 200mV in the other direction to be seen as a logic '0'. These differences can be between two signal that are anywhere between +12V and -7V wrt the receiver's 0V pin. But with an isolated receiver you could, in theory, transmit DMX on top of one phase of a three-phase mains feed. Not that it's recommended. Or safe. Link to comment Share on other sites More sharing options...
sunray Posted February 9, 2018 Share Posted February 9, 2018 This all makes sense, The only times I've looked at these voltages have been around 1985 and 1995. 200mv seems a small voltage for this sort of application but I have no reason to disagree. Link to comment Share on other sites More sharing options...
sunray Posted February 9, 2018 Share Posted February 9, 2018 Out of curiosity I looked at the DMX out of a Transcension SDC816. with a 'scope.Measuring between E and D- readss -2V to +6V and E to D+, +7V to -1V when connected to an earthed dimmer pack & terminator.Without the earthed pack in circuit the signal is too noisy to take meaningful readings in the same way but measuring directly across D- & D+ there appears to be a voltage swing of 17V with no terminator or 15V with.All voltages are in round numbers. If these voltages are typical then diode isolation would be very unreliable. Link to comment Share on other sites More sharing options...
richardash1981 Posted February 10, 2018 Share Posted February 10, 2018 But with an isolated receiver you could, in theory, transmit DMX on top of one phase of a three-phase mains feed. Not that it's recommended. Or safe. I have encountered RS-485 run referenced to mains neutral. Provided that the trailing single-core neutral lead was plugged into the banana jack. If it wasn't, the "0V" potential drifted up to line voltage via the slave device's built-in PSU ... Link to comment Share on other sites More sharing options...
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