I often get questions asking about DMX and how it works so I thought it might be useful to recycle my presentation about DMX from the 2012 Academy that talks about the basics of the DMX protocol.
What can DMX do?
• DMX is simple – it is designed to control
devices, usually lights
• This may also mean just turning on and off a device
• Work with desperate devices and software applications
Where did “DMX” Come From?
• DMX was designed as a public standard to allow hardware and software vendors to all be able to
design inter-operation devices
• Designed in the early 90’s
• Developed by USITT – United States Institute for Theatre Technology
• Designed to be very reliable (but not guaranteed – no error checking)
What is DMX?
• DMX is a protocol
• DMX is a public standard
- E1.11 (ANSI)
- Just a set of rules
• DMX runs “over” other protocols or wiring systems
What DMX isn’t
• DMX isn’t a wiring standard
• DMX isn’t a physical “thing”
• DMX isn’t complicated
• DMX isn’t the perfect protocol
RS485 and DMX
• It is important to understand that RS485 ISN’T DMX and vice-versa
• RS485 is the most common method of DMX transmission
Here is a table that shows how DMX (E1.11) compares to other layered systems:
RS485
• RS485 is the “road” on which DMX runs
• Very robust – designed for industrial environments back in the 1970’s
• Differential signaling system (positive and negative voltages)
Here is an example of how a differential signalling system works:
RS485
• Able to handle speeds up to 35 Mbit/s depending on cabling
• Cable lengths up to several thousand feet
• Two wires + ground (optional)
• Allows for a variety of wiring topologies
• RS485 is the basis for DMX (E1.11) , LOR, Pixelnet and Renard protocols
RS485 Termination – Yes or no?
• PRO: Termination “dampens” the reflections of the signal in the cable
• CON: Termination “sucks up” power on the line, lowering the voltage and thus the distance
• The RS485 (DMX) specs call for termination (100-120 ohms) with standard DMX cable
LOR Controllers don’t use termination
• No one best answer – sometimes it is necessary…sometimes not
• A scope is the best tool for looking at the quality of the signal
• For video showing the effects of termination:
Splitting RS485
• Some controllers passively split the connection (LOR/LE Express) and some actively split and then repeat the signal (LE Express)
• Ground wire often not connected in holiday lighting controllers
Controller Count per line
• Technical limit to the number of devices on a single DMX line is 32 but many more are
possible depending on the line load per controller (actually the RS-485 chip). I've personally used 90, 3 channel DMX controllers on a single RS-485 line.
• Controller counts can be increased with the use of repeaters (not common)
DMX over Ethernet (E1.31)
• No RS485 - DMX is instead sent over standard Ethernet/Wireless using TCP/IP
• Allows many universes over a single network connection
• Used when distances are far or channel counts are high
• EtherCongateway (J1SYS) / SanDevices controllers or other high-channel count pixel controllers
DMX Channels and universes
• There are 512 channels (9 bits) in a DMX universe
• One transmitter (RS-485) per universe
• One transmitter (E1.31) for many universes
• Universes are effectively unlimited
• Universes are not “connected” to another universe
DMX Protocol Internals
• DMX runs at 250Khz or 4 micro seconds widths/”slices” of time – 1 Microsecond (µs) = .001 milliseconds (ms) / 1000 ms = 1 second
• MTBP – Mark Time Between Packets (idle)
• Break – Starts with 88 µs low/ 22 pulses (get ready…I’m about to send data)
• MAB – Mark After Break ~12 µs high / 2 pulses
• Channel Data - 44 µs / 11 pulses for each channel (shown in red below)
– Start bit– 1 bit low
– Data bits – 8 bits (0-255 that define the level of light intensity)
– End bits – 2 bits high
• First channel zero, has the start code of binary 00000000 (zero)
• MTBF - Mark Time Between Frames 0-1 seconds high (the next channel is coming up)
• All 512 channels are sent one after another until the next MTBP and the process restarts
• 1,000,000µs (1 second) / 22,722µs = 44.01 Hz or 44(times per
second)
• This means that as long as your sequences contain timing no smaller than 22ms or .022 seconds, the timing of the display will be as expected
Output Adapters / Dongles
• DMX must be generated by a device
• Devices can be “Smart” or “Dumb”
– Smart – Command is sent to device from the sequencing software (say…Channel 1 at 128 bits) once and the devices keeps repeating it 44 times/sec. This way the PC doesn't need to keep repeating it.
– Dumb – Commands from the sequencing software have to be re-sent over and over 44 times/sec. This puts a larger load on the PC
When I started researching RGB and pixel solutions in early 2010, there was just about no information on the internet as to what all the teminolgy, protocols, wiring, controllers and other items were. After extensive research and work with others, I decided to document all the information I knew at the time in the form of a video series. The focus behind the videos was to simply show each type of lighting, controller, wiring, etc in a non-vendor specific manner so that someone just getting started out could get up to speed on the terminolgy and functions. This information, while produced in late 2010, early 2011 is older now, every bit of it is still completely relevant. Since this information is squirrled away on our website, I though I would repost it here for those new to RGB and pixels.
The videos below cover the general topics of RGB as they relate to Holiday Lighting.
Video #1
What is RGB?
What are the different type of RGB lighting devices?
What is Basic RGB and RGB Pixels?
How to Pixels get their addresses?
How do you assign channels to pixels?
What are controller macros?
Video #2
How do I fix dead pixels?
What is the difference between three and four wire pixels? Does it matter?
How do I know which end of my pixels to hook to the controller? Does it matter?
How do I power my pixels?
Why are there different voltages for pixels?
What is the difference between centralized and de-centralized power distribution of pixels?
How many pixels can be on a single controller output?
What is color depth in pixels? Why does it matter?
Video #3
What are the different type of pixel chips?
What quality issues should I be aware of when purchasing pixels?
What are the drawbacks of RGB over just regular Christmas lights?
What advantage do Basic RGB and RGB Pixels give me?
Video #4
What are the different types of Basic RGB controllers?
Why would I want to use basic RGB controllers and LEDs instead of pixel based controllers and LEDs?
What are the different types of RGB Pixel controllers?
Updated 25-Aug-2015 with Brilliant Bubs and Updated Spredsheet In our prior blog article – Outlining Your House in RGB Lights – A Primerwe covered the high level decisions that needed to be made when designing system for RGB lights on the fascia of a house.This article, unlike the first, provides in-depth information on how to layout and spec such a design.Not included in this article are the specific step-by-step instructions on how to wire controllers, wire power supplies, how to solder or any other physical setup – but don’t worry – that is actually the easy part, it is the design that most people find the more complicated part.I should also mention, just as our blog article No Free Rides – Cheap Isn’t Free When It Comes to RGB – there isn't an “off the shelf” solution for doing this – each house, each display, each budget and each level of skill is different and the only person that knows all these requirements is yourself.So, expect to spend some time doing research for a complete solution. One last thing - if you have not already done so, be sure to start with our RGB Projects Primer post. This post covers all the major steps you need to consider when designing a pixel based project.
Ok, so let’s get started.We have selected the house of Nathanial R. to build this real-world sample project from.This house (shown below) has many areas that need to be covered – 19 in all, so this house will serve as a good example of how a house with many complex roof lines can be handled.
Design – Always the first step
Often people try to determine which hardware will fit their house without first starting with a design.The absolute best method is to always start with a design prior to purchasing anything.Design can comprise many aspects but in essence it is – What do you what your display to look like?
Normally you would look at the entire house/display and determine all the elements that you wanted to add – mini-trees, megatrees, bush lights, candy canes, cutouts, garland, icicles, borders, etc. and then create separate “projects” to focus on each one.In this case we are focusing on a sub-element of Nathanial’s display – to outline the fascia of his house with RGB pixel lights.He has already determined that he likes this look but just isn't sure what is required and the issues involved in the project.
So, start this project by going outside and taking a photo of your house from the approximate location(s) that your viewing audience will view your display.These photos will serve to allow you to sketch out your design and to also layout measurements, cable routing locations, controller locations and other relevant information.
Smart or Dumb?
This is the first major intersection on your project – you need to determine if you want to go the dumb RGB or Smart RGB route.Here are some of the pros and cons of each for this project:
RGB Smart/Intelligent/Pixel
RGB Dumb/Basic
Lighting Costs
Smart RGB modules and strip are about 10-30% more expensive than dumb of the same physical items (modules, nodes, strip).
Lights are about 10-30% cheaper than Smart.
Lighting Functionality
Smart RGB lights can be controlled either by individual lights or in small sections (usually less than 3” each).
Dumb RGB lights cannot be controlled individually – all lights attached to a controller will be the same color and intensity.
Controller Costs
Smart RGB controllers are about 50% to 100% more but generally you need fewer since pixels can be used with power injection.
Controllers are less expensive but you are likely to need more of them along with more power wiring. Long runs are extremely complicated to perform with dumb RGB.
Controller Functionality
Smart pixel controllers are able to control each light or small section of lights individually.
Dumb controllers can only control a string of lights, not individual lights within the string.
Today, in 2015, it is hard to make a good case from going with dumb RGB for a house outline in all but few cases. Pixel solutions are easier setup than dumb and the cost differences are minimal when all other factors are included. If you are going dumb RGB outline, you should know why it was selected over smart. A few caveats on this – some people would design this using pixels and using either functions in their software or controller, would limit the control to sections or controller outputs.Some pixel controllers have an option to control an entire length of pixels with just three DMX channels instead of each pixel with three individual DMX channels.This allows simplicity in setup and sequencing at the trade-off control but still allows moving to a full pixel display later on down the road when you are ready to do so.
For simplicity and brevity, we will be focusing only on pixels in this case since this is the direction that Nathanial has decided to go - though you will find dumb or basic RGB have many similar design requirements (power, signal) but just slightly different hardware (controllers and lights) than pixels.
House Measurements
An absolute requirement for this project is to determine where you want your RGB lights to be located.To do this you will first need to determine where you will be installing them and how you will be installing them.We won’t delve into specific mounting methods as there are just too many options depending on your house constructions and overall look you are going for – such as under soffit “wall washing” instead of fascia mounted or directly viewable RGB lights. (See our budget PVC pipe mounting method here and our high-end aluminium mounting strip here.)
So, get out your ladder, tape measure, thinking cap and determine the exact length of each section where you wish to place the RGB lights and record it on the photo of your house.When you are done you should end up with a photo that looks something like this:
You can see that he has measured each and every section of the fascia that he intends to mount the RGB lights to.
Why does it matter that you are accurate in the measurements you take?The reason is that, depending on the RGB lights you intend to use, even just several inches of RGB pixel lights could result in 3 to 6 DMX channels and if you miss a few inches here, a few feet there, your final design could be off several dozen or hundreds of channels depending on the size of your display.This is not to even mention the issues in properly calculating the power consumption.
Selecting RGB LED Types
The next step is to determine the actual type of RGB Pixel LEDs you will be using.Which pixel type you select depends on a number of factors, such as the density of lights and control you want, the amount of cost you want to expend, the type of “look” you are going for and a number of other minor factors.Let’s go through some of the areas you should be looking at when you select the type of RGB pixels you intend to use:
·The “look” you want to achieve – There are a variety of different types of pixels on the market – nodes (with and without diffusers), strip and modules.You need to determine the type of look you want – is it a more retro look like with C7/C9 diffused RGB nodes or do you like the close spacing in strip that offers a more “neon” appearance?
·Costs – Each different type of RGB light has different costs.Some of this cost can be a result of a better quality pixel, level of control it offers or density the RGB lights in the pixel. Typically, strip lighting is always going to be the lowest cost method of getting RGB lighting onto a house outline.
·Mounting – Each pixel type has a different method of mounting.Some may be able to use existing mounts such as those based on C7/C9 devices (rare) or they may require a mounting substrate (PVC pipe, 1"x2", etc).Think about how you will be mounting the type of RGB pixel you select and what effects it will have on the mounting method and the cost of that method.Also keep in mind how complex it will be to mount and dis-mount the pixels – saving 50 cents per pixel won’t seem like a great deal if it takes you two times as long to mount them.
The Number Crunching
A this point you have now selected the types of lights you want to use and you have collected the lengths of each section on your house that the lights will cover.The next step is to enter this data into our spreadsheet.This spreadsheet (requires MS Excel 2007 or later) only requires you to enter each of the lengths (sections) within your project and select the type of light (strip, module, node) you wish to use and it will then output costs, number of RGB and DMX channels and power requirements.This gets you part of the way toward your final design and it also allows you to play around with different lighting to determine channel counts and costs but it can’t completely design your display because there are just too many factors to take into account.There are many small but important decisions that need to be made based on the specifics of your house and design.Here are some of the issues:
·Location (and type) of controller(s) – There are three basic options for pixel controllers – a centralized controller solution, a de-centralized controller solution and a mixed centralized/de-centralized controller solution.
oCentralized – These controllers, such as the AlphaPix 4 and AlphaPix 16, are units that take an input of DMX data (over Ethernet using the E1.31 protocol) and output over 4 or 16 outputs (separate strings or DMX universes).This means that you can hookup a single string of RGB pixels, ranging from 320 (AlphaPix 16) to 680 (AlphaPix 4) pixels per controller output.There are some issues here though – there is usually a limit to the length of distance between the controller output and the start of your string of lights and these limits vary but are often (with most pixels HolidayCoro sells) around 15 to 20 ft.
oDe-Centralized – These controllers, such as the EasyPix allow you to take an input of DMX data (from a DMX dongle, such as the ActiDongle or from the RS485 output of our AlphaPix controllers (3 on the AlphaPix 16 or 1 on the AlphaPix 4) and output the data to a pixel string, usually up to 170 pixels per controller.
oMixed Centralized & De-Centralized – There maybe cases, most often with large houses or wide spaces, where it makes sense to use a mix of these two controller types.
oPower Management – Just as important as getting a data signal to pixels is getting the proper amount of power to the pixels and there are a myriad of issues involved, such as:
oCable/wire gauge – This is a complex issue but it really boils down to ensuring that for the amount of power you will be using, that you have sufficient method to get it to where it is needed and this means using a wire gauge that meets that need.We've talked about this in our “Technical Guide to RGB Wire Selection” article, so we suggest starting there.Be aware that the wire gauge involved isn't just that leader cable from the power supply to the pixel but the actual gauge of the wire inside the pixels themselves.
oPower consumption of each pixel (or group of lights that make-up a pixel) – The spreadsheet will help with this calculation but also be aware that there is a limit to the number of pixels that can be run in a continuous length due to power consumption by the pixels themselves.Some pixels may only be able to be run continuously in lengths of 40, 50 (common in 5v pixel nodes), 100 or they may be able to take a single injection of power at the start and run for a full universe of 170 pixels (510 DMX channels).For most of HolidayCoro's pixels, you'll find a information on power consumption and length limits before power injection is required.
This is why you see 12v pixels vs. 5v pixels.A 5v pixel is closer to the actual voltage of the LED and IC chip using the power and thus is more efficient as where 12v has to be “dropped” down by use of a voltage regulator circuit (not as common) or dropping resistors which waste the power.The trade off is that you generally can run 12v strings of pixels, given the same gauge wire, longer distances than you can 5v pixels without the need to re-inject power as often. This is an entirely separate and complex discussion. In general, you'll find it "easier" to work with 12v RGB lighting which is more forgiving than 5v lighting which requires more accurate planning and power management.
The Real World
Now that we've covered many of the basics, we are ready to turn our attention back to Nathanial’s house and start working through the design.First, we input the data into the spreadsheet (version 2.1 / 25-Aug-2015) and select RGB pixel strip as the type of lighting we want to use (RGB Pixel Strip in this case):
This provides us with some important starting points:
·The total number of DMX channels is 1,308 and since each DMX universe is 512 (actually 510 for pixels or 3 channels * 170 pixels), we end up with needing 3 universes.
·We will need a total of 436 RGB sections and/or RGB channels
·We will need about 324 watts of power
So, now let’s go back to the photo of the house and the location of each section of RGB strip he intends to use:
The first issue we need to address is that with some controllers (does not apply to HolidayCoro AlphaPix controllers) no one continuous length of strip can be more than 170 RGB pixels/channels (510 DMX channels - Red, Green, Blue channels * 170 pixels) in a typical controller.As a result, we need to look at logical sections that might be a continuous length and determine if they are under 170 RGB channels or less.If we look at the front section of the garage facing the street, we have a total of 26 linear feet of strip we need to place.We can see that the spreadsheet has calculated that this is a total of 80 RGB channels – OK, good there, what about if we include the side of the garage over the garage doors at 34 feet.We get 182 RGB channels – nope, can’t do that, its 12 RGB channels over the output of any single output (170) so our options are to either adjust the lengths and remove a section, or break it down into two sections.We are going to go with two sections for the garage section – with 80 RGB channels for the front of the garage facing the street and 102 RGB channels for the section of the garage over the garage doors.
We’ll do the same for the top of the house with the sections that are 8.5’ + 8.5’ + 1’ + 9’ + 15’ + 11’ for a total of 53’ linear feet and a total of 160 RGB channels.Then for the bottom section of the front of the house at 6.5’ + 6.5’ + 9’ + 9’ for a total of 31’ or 94 RGB channels.So, to summarize, we have the following segments:
·Garage Front:80 RGB Channels & 240 DMX channels
·Garage Entry:102 RGB Channels & 306 DMX Channels
·Front House Top:160 RGB Channels & 480 DMX Channels
·Front House Bottom:94 RGB Channels & 282 DMX Channels
You can see these numbers match perfectly to the spreadsheet calculations of 436 RGB channels and 1,308 DMX channels.Of course this assumes that your controller can handle a total of 170 RGB channels per “output” – adjust as necessary to meet the specific requirements of your pixel controller.
Power Distribution
This is where things can get a little tricky – while we will be selecting a controller that can output a total of 170 RGB channels, that doesn't mean that if we apply power to the start of the strip (in this case), that a sufficient amount of power will make it to the very end.As power runs down the strip, most of the power is lost to the LEDs using the power and the remainder is lost to the resistance in the strip itself or the wire between modules.So, we go back to the spreadsheet again and we look at the power consumption for a given grouping of sections.We get:
·Garage Front:60 Watts
·Garage Entry:76 Watts
·Front House Top:119 Watts
·Front House Bottom:70 Watts
Here is where it gets a little grey – you can check with your lighting vendor and they may list voltage drop over a given length of RGB pixel – say, input of 12v, 11.5v at 50 pixels, 8.5v at 100 pixels and 6v at 150 pixels but that isn’t common, so more than likely you’ll need to measure this yourself.How you do this is:
·Hookup a section of RGB pixels, say 50 or 100 to your pixel controller
·Using your sequencing application, the built in test function of your controller or a test tool like xLights, output a solid white to all the lights on the section you are testing.I’d recommend leaving it on for 15 to 30 minutes to let it warm up, which will result in a slightly higher power drop.
·Using a multi-meter/VOM, measure the voltage coming directly out of the controller to the lights – record this voltage
·Again, using the mutli-meter/VOM, measure the voltage coming out the last pixel on the string – record this voltage
You should notice a drop in voltage at the end of the pixel section.How much of a drop is bad?Well that depends.Usually less than 10-20% is ok, over that amount you could run into problems either with a dimming of the lights at the end of the string or in the worst case, if the voltage drops low enough, below that required by the IC chip to operate, your pixels will have “random” failures as the voltage fluctuate from high to below what is required for the IC to operate.This is why running 12v pixels can sometimes be less fraught with issues than 5v pixels.For example, look at these hypothetical voltage drop examples:
·Input:12v / Output:7v | 5v total drop – A total drop of 41%
·Input:5v / Output: 3v | 2v total drop – A total drop of 40%
So, even though the 12v pixels dropped a higher voltage, it was still the same percentage of drop when compared to the 5V dropping to 3v.Now this is where it gets a bit complicated.Say that the power has dropped to 3v at the end of that hypothetical string of 5v pixels – normally a pixel chip requires around 5v of power to operate but some pixels actually can take lower levels of power and still maintain operation and a the same level of light output.For example, WS2801 pixels can run on voltages from 3.3 to 5.5v DC (12v strings that use 2801 drop the power down by using resistors) and maintain a constant current to the LED (thus ensuring they are all the same brightness) but TM1804’s are constant voltage and the LEDs will dim over long runs without power injection.There are also side issues such as power losses on 12v pixels but that maybe a mute issue for those in snowy areas that find value in a “heated” pixel.
We would always recommend testing your build before and after construction because nothing will be more frustrating than putting all this up only to learn that you’ll need to re-wire it because of power issues.
What do you do if the power drop is too great?We’ll you’ll need to split the power about half-way down the line, and then using a higher gauge wire (14 AWG and up is always a good start) from the power supply up at the start of the string, carry the power down to the split – the data will pass right through (it is regenerated at each pixel).This diagram shows and example of the power injection:
Controller Placement
We’ve already touched upon the three different types of controllers – centralized, de-centralized and a combination approach.Now it comes down to the actual placement of those controllers.There are three additional major items to consider here, other than the controllers themselves – where the DMX is coming from (and how much it costs), the overall cost of the controller(s) and distances from controller to the strings they are driving.
DMX Signal Source – There are two major methods of DMX signal generation:
·Serial based DMX output dongles – these devices (such as the ActiDongle) connect to a PC via a USB port and appear usually a as “COM” port or similar interface.Each dongle outputs 512 DMX channels to an RS-485 line (see our blog article on RS485.)These are used with controllers that take a DMX over RS485 connection.
·Ethernet based DMX output – some controllers (AlphaPix 4 and the AlphaPix 16) take a direct Ethernet connection into the controller and use the E1.31 protocol which is basically DMX running over TCP/IP (running over Ethernet.)While somewhat more complicated to setup, E1.31 offers huge numbers of DMX universes in a single data cable.This means that you could have as many as 16,320 DMX channels on a single CAT5 Ethernet cable to a single controller.
Controller Costs – In almost all cases, it is less expensive to hook up a E1.31 based controller to drive pixels (as low at $129 USD) vs an RS485 based dongle ($50) and a pixel controller ($40) - the difference being that the E1.31 controller could handle 2,720 pixels and the RS485 based controller could only handle 170 pixels.
Here are some of the thought processes I go through when looking at this design:
·If the controller supports forward or reverse addressing, I'm not limited to putting the controller on the left side with the strip going to the right (left right addressing is easier and more common.)So when I look at the house I see at the front corner of the garage two different start/ends occur which could allow me to place one dual output controller there along with a power supply.I also see the same thing above the front door where the top of the bottom sections of the house come to two points. All HolidayCoro pixel controllers support forward and reverse addressing.
·I know I’ll need four outputs since we have already determined that we need four separate sections of strip.I also can tell that I need to keep the strip under about 30ft to avoid power problems so for the section on the top of the house (30 LED/10 ICs or Pixels per Meter) I’ll need to do power injection but for the other sections I should be ok for power injected at one end of the strip. Again, you'll need to do your own testing here to determine maximum length.
·I could place a single, centralized controller on the wall above the front door but I need to consider that it will have long lengths from the controller to the start of the strings (much more so for the string on the front of the garage) and while I could use ghost pixels to repeat the distance, I still would be using a large controller in an awkward spot and I would have a controller with a lot of extra outputs that I won’t use (in this design.)
·I could use single or dual output controllers and place them at the corner of the garage and above the front door and that would remove my issues with distances but would still mean that I would have controllers in an awkward spot directly in the front of the house.
So, what was the final design decision?A centralized controller.This was selected for the following reasons:
·Ghost/Phantom/Null pixels can be used to repeat the signal from the centralized location where the controller is located to the start of the string.That resolves the issue with distance (within reason.)
·The additional outputs of the controller (16 outputs total) can be used for other elements within the display.
·In this design the power requirements (324 watts) match up well with a single 350 watt power supply. This isn't to say that more power could be required - test, test, test!
·This allows a E.131 connection without additional dongles – so one cable for all the channels required.
Here is a photo showing the final layout:
·The green lines are power (12v DC in this case) using a minimum of 16 AWG wire
·The purple lines are data (SPI) signals from the controller to the strip
·The green circles are where power injection on the strip occurs
·The blue square is the 350 watt power supplies (or supply)
·The red square is 8 to 16 output pixel controller
·The orange lines are the pixel strip
·Black dots are estimates for locations of null/ghost pixels
So there you have it – while this is a very lengthy article for a blog posting, it doesn't encompass EVERY feature, function or possibility – this article should help you work out the major issues in completing a project of similar nature.You will also notice that we have not even covered the software side of this project – setup of E1.31, configuration of the channels within a sequencing application or even issues involved with sequencing 1,308 channels which is a project unto itself.We hope to tackle this issue in future blog articles, so stay tuned!
Please note that due to the number of requests we receive, HolidayCoro is unable to provide free design services due to the amount of time it requires to evaluate each individual solution.We do however, offer consulting services if you should need them.
Thank you, David HolidayCoro.com 27-Mar-2016 Here is a follow up video that shows a design process using RGB nodes:
I'm a firm believer in understanding "why" something does what it does and what effect those systems have on performance and reliability. It is only when a person understands the underlying systems and processes that the "mystery" of why issues occur can be understood. One area of confusion I see with customers comes up over and over and that is the relationship that the DMX and LOR protocols have with RS-485. First, lets start with some basics:
Both DMX and the LOR controllers use a protocol for communicating between a PC (usually) and a controller (AC or DC) to make lights turn on or off at a specific time. A protocol is nothing more than a definition of how two systems talk with each other - another example of a protocol would be TCP/IP or even the English language - it is nothing more than a set of rules that both parties/controllers agree on.
DMX Protocol: The DMX protocol was defined in the early 1990's by the theatrical lighting community and is codified in a specific standard usually called DMX512 or E1.11.
LOR Protocol: The LOR protocol was designed by Dan at Light-o-Rama in the late 1990's. This protocol is unique to LOR and for the most part is not publicly used by other controller vendors. The reason this protocol was created instead of using the existing DMX protocol was because of it's need to run very slowly (19,200 kb/s) compared to DMX's 250,000 kb/s as a result of a need to run on cheap, poor quality flat telephone cable (this is why you have a phone jack on an LOR controller.)
Both DMX and the LOR protocols run "over" RS-485. RS-485 is a signalling protocol that defines the physical and electrical nature of the way each system will talk with each other. Examples of this would be the Ethernet standard or how the letters of the English language are written. RS-485 doesn't concern itself with what protocol is running over RS-485 but simply how the controllers interact at the physical and electrical levels. This is why you can use a LOR "dongle" and output either the LOR or DMX protocols because the dongle is nothing more than a RS-485 output device - it is the software (LOR S3) that defines the protocol that is going out over the RS-485 connection.
There are some pros and cons to both the LOR and DMX protocols and we've covered those in previous presentations. What is more important than the protocol (DMX, LOR) is RS-485 - this is where I see the majority of confusion, misunderstanding and problems. Here are some common questions and answers to issues surrounding RS-485:
You can't "Tee" or split DMX/LOR signals
Again, this doesn't have anything to do with DMX or LOR - splitting a signal from an output dongle does have everything to do with RS-485. On LOR controllers, there are two RJ45 plugs (excluding the one for telephone wire) for CAT5 cable. You'll notice that there isn't really an "in" and "out" - why? Because it doesn't matter. As the signal comes from the output dongle to the first controller, then to the second and so on, all that is happening is that each controller is "tapped" or "tee'd" off a single data line. Each controller simply listens to all the data on that RS-485 data line and waits for data that indicates that it should do something (dim, shimmer, etc). This is the exact same with DMX controllers (in most cases) - the difference on DMX controllers being that some of them have XLR plugs which have a different connection for input vs output but in MOST cases, the controller is just connected to the wire. (Yes there are some repeating/regeneration controllers but that is often the exception than the rule.)
Have you seen DMX controllers that only have only an input but no output? Have you wondered if you need to split the signal to each one or have a separate output dongle for each one?
Since there really isn't an input and output for the signal in most cases, all you need for a controller to work is to "tee" the controller off of the data line. That could be that each controller is just manually soldered to each input and then the cable continues on to the next controller or by using a passive CAT5 splitter/tee. So, if you have a controller that only has an input, it's not a problem - just split the signal outside of the controller as opposed to running the signal into the controller and then back out. This is the same method we use on most HolidayCoro DMX controllers.
I hear there is a limit to how many DMX controllers you can hookup to a single DMX output dongle.
It depends. Remember here that what really is at play is RS-485, not DMX. So DMX doesn't limit you at all to how many controllers you can have hooked up to a single dongle - RS-485 limits you. What are those limits - well it is a number of factors:
Power output of dongle - The RS-485 standard allows up to 19 volts of power for signal (-7 to +12). So, the more voltage that is used for the signal output, the better. This is somewhat like the difference between just yelling at someone a long distance away or using a bull horn - clearly the more powerful and amplified signal using the bull horn will go much feature and be much clearer to the recipient and will also be able to more easily overcome any background noise. While the RS-485 standard allows up to 19 volts, nearly all USB based devices are limited to the USB voltages - about 5v. Why does this matter? Two reasons - noise and power loss - see below for more details.
Length and gauge of signal cable - Since cable itself induces loss, as your cable length increases or the gauge of the cable rises (see our blog entry on cable sizes and lengths), the overall strength of the signal also drops. Lower power means there is a lower signal to noise ratio and you start to experience problems.
Type of cable - When selecting wiring for RGB elements, you have lots of choices out there. Excluding the gauge of the cable, the other major factor of cable selection is how the cable is constructed. There are two basic methods - twisted and non-twisted. Why does this matter? Twisted pairs of wires inherently does a much better job of rejecting EMI (noise) - this is the reason that all CAT5 cables are designed with twisted pairs of wires. If you selected a cable (alarm wire, SPT cable, etc) without a twisted pair for the data signal, you risk greater EMI interference over straight cable.
Number of devices - Each controller exerts a "load" on the RS-485 line. This basically means that as each new controller is added to the RS-485 line, it "sucks" some of the signal off, reducing the overall voltage on the line. Once you get too many on the line and the voltage drops too much, the controllers are unable to determine the actual signal from any noise on the line. So, how many can you put on a line? It depends the the RS-485 chip used in the controller, some less efficient chips might limit you to 30 controllers, some might limit you to 100+ controllers on a single line. How can you tell if you have too many controllers on the line? Checkout this video article that shows about 50+ controllers connected to a single DMX output dongle. You'll see that the only real way to know where you are pushing up against a limit is if you have the proper diagnostic equipment - namely a scope. This is why we sell our pocket scope - so you can determine exactly what the RS-485 signal quality looks like. We recommend this article by Maxim, the company that produces 485 chips if you would like in-depth details.
Termination - While "technically" all RS-485 connections should be terminated with a resistor, the reality is that it is often not needed and in fact, LOR doesn't even terminate their controllers due to the inherent design of their system (slow speeds and forced bus connections/tapping.) When and where you need termination mainly varies based on the wiring scheme (serial/bus, long taps, location of output dongle on the line, etc) - the best place for technical details on the do's and dont's of termination is in the Maxim article. You can also watch our video showing a real world lighting display, over a scope, showing what effects adding or removing a terminator will do to the signal. Be aware that termination is no free ride and won't fix a bad network design. Terminators are resistors and as a result, the "suck up" and reduce the level of signal - so while it might clean up the signal, you might end up with too little signal to make a difference.
With all this said, the vast majority of networks are simple and even when improperly constructed with the wrong wire, bad termination, long taps and other errors, RS-485 (and thus DMX and LOR) still continues to work. Why? Because RS-485 was designed just for this - to work in environments like factories with bad wiring, lots of EMI and long runs of cable. When you really need to be concerned about these issues is when you get dozens of controllers, over long lengths of cable and with bad cables or connections. If you are one of those people, we *highly* recommend getting a scope because without one, you'll only be guessing as to what the quality of your network is or where a problem might exist.
