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Introduction When USITT adopted the DMX512 standard for digital data transmission between consoles and DMX512 receiver devices, it provided a "lowest common denominator" which would allow any console to control any dimming system. After several years of field shakedown, this is mostly true. Most consoles and dimmers can be interconnected and "speak the same language." The DMX512 specification did not directly address the subject of signal distribution. Cables and cabling were to follow the EIA (Electronic Industries Association) EIA-485 specification, which is the electrical interface used by DMX512. This specification describes a "daisy-chained," point-to-point wiring scheme using shielded-twisted pair data cable. No cable stubs or branches are allowed. This was not expected to be a problem, since systems then consisted of a DMX512 console and one or more DMX512 dimmer banks located in a dimmer room. A single DMX512 cable ran from the control console to the dimmer room, and "daisy-chained" from rack to rack. The installation of the DMX512 cable was the responsibility of the system manufacturer. If problems occurred, it was up to the field engineer to correct them. Touring systems were more complex. Dimmer racks might be located all over the performance area, widely separated from each other. The DMX512 cable runs might need to exceed the 1000 foot-per-link limit. "Daisy-chain" distribution might be inconvenient or unwieldy. Electrical problems might be encountered because of the slight differences in earth grounds at different points in the building. When DMX512-controlled moving lights and color scrollers became available, the problems of DMX512 distribution became severe. All of the problems seen in touring shows were present, with some new ones added. Different manufacturers' have tried to deal with these problems by adding active DMX512 repeating circuitry, DMX512 distribution amplifiers, or other circuitry that is supposed to make things better. Unfortunately, this has not always turned out to be true. Just as a hapless TD was connecting the last piece of DMX equipment, the "Prince of Darkness" would descend and . . . ZAP! No light! Technical directors and electricians found themselves trying to fix and understand problems that were beyond their training, tools, and experience. A $10 voltmeter can be used to test a dimming system that uses analog control voltages. It is useless for troubleshooting DMX512. Specialized DMX512 test equipment does exist, but it costs fifty times what a voltmeter does. Most theatre technicians can't afford this investment, unless DMX512 troubleshooting is all they do. This article is intended to be a practical, "how-to" guide to cabling and distributing DMX512 for an IPS dimming system. The information can be applied, however, to any DMX512 installation. Some topics include:
DMX512 101
Q: What is a DMX512 signal? A: DMX512 signals are digital, differentially-driven, serial high-speed data. "Digital" means the signal is sent as two voltage levels; zero volts for a logic zero, and 2.5 volts to 5 volts for a logic one. "Differentially driven" means that the signal is sent on a wire in the pair, and its opposite is sent on the other wire. The receiving circuitry only looks for differences; any noise picked up on both wires will cancel. "Serial" means the data bits are sent one bit at a time, sequentially. "High speed" means that the bits are sent at 250,000 per second. In audio terms, this would be 125kHz, which is five times higher than the highest audio frequencies.
A: The DMX512 standard specifies the cable pinout and the type of connectors to be used. Consoles or any devices which output DMX data are to use female 5-pin XLR connectors. Receiving devices (dimmers, moving lights, distribution amplifiers) are to use a male 5-pin XLR. These connectors were not originally designed for digital data, but were chosen for DMX use because of their ruggedness, familiarity, and presumed availability. A cable segment has a male on one end and a female on the other, and can be plugged together, just like mic cable.The pinout is shown below:
All cables used with IPS must follow this pinout. The signals on pins 2 & 3 are the DMX512 output data, and the signals on pins 4 & 5 are used for IPS talkback data. Not all vendors' DMX512 cables will have the second data pair. It is required for IPS talkback. The shield should connect to pin 1 at both ends of the cable. Do not connect the shield to the body of the connector. It is within the scope of the specification to use the optional second data link for transmitting a second 512 dimmer link in a large console. If this is true for the console you are using, the second link must be "split" externally. Unfortunately, some vendors have used the optional second data link (pins 4 & 5) in nonstandard ways. These include:
A: The cable used to carry a DMX512 signal can't just be any piece of wire that happens to be lying around. The cable must be an EIA-485 data communications rated cable. This cable has twisted pairs, with a total shield and low capacitance per meter between conductors, characteristics that are required by EIA-485. Twisted pair cable has a constant impedance and a greater ability to reject external electrical noise. The total cable shield blocks external noise and magnetic fields from disrupting the data, just as a shield on a microphone cable prevents hum and noise pickup. Low capacitance prevents a cable from acting like a filter for high frequencies, which in DMX512 is the signal itself. Industry standard cable types include Alpha 5272, Belden 8132, and TMB Proplex 224, which is a cable specifically made for DMX512 data. You can't use telephone wire, unshielded intercom cable, microphone cable, or zip cord for DMX512 data and expect your system to work reliably. The maximum length of a DMX512 link is 1000 feet.
A: If the wires are broken, nothing. In the case of short circuits, the EIA-485 drivers should have protected themselves by shutting down. If the cable was shorted to a voltage greater than the common-mode voltage (the AC line, for example), your system could be toast!
A: Sometimes, yes. If the IPS TLK pair is broken, the dimmers will be working fine, but the dimmer information on the console or Status Monitor may be garbage, or not be there at all. If the "Data 1+" signal is disconnected, something interesting may happen. The "+" may float to an intermediate level, or be biased to a valid "one" level if a "fail-safe" circuit is present. The receiver will still recover the data, but it may not do so reliably. This is a subtle failure, and is why cables should be "rung out" with an ohmmeter to make sure that all five signal lines have continuity. If the reverse happens and the "-" is floating, the data is inverted and properly-designed receivers will ignore it. Of course, a broken wire could also result in data that is sometimes OK and is sometimes total garbage! You could have quite a light show, unfortunately!
A: "Daisy-chained" cable means that the wire run starts at the DMX512 transmitter (a console or distribution amplifier) and runs to the "DMX512 IN" on the first DMX512 receiver, from the DMX512 OUT on the first receiver to the DMX512 IN on the second, from the DMX512 OUT on the second to the DMX512 IN on the third, and so on. The cable cannot branch or split, and cannot have taps or stubs. It must look like one long piece of cable. This type of wiring is required because of the high signal speeds. Branches, taps, and stubs will cause signal "reflections" and "ringing" which will garble the data. No DMX512 "twofer's" are allowed!
A: Just like video cables, DMX512 cables must be terminated in their characteristic impedance to eliminate ringing and signal reflections on the data link. When a high speed signal reaches the end of a cable, and if the cable is not terminated correctly, the signal waveform will reflect back along the cable, producing "ghost" signals that garble the data. Termination provides impedance (usually a resistor) equal in value to the characteristic impedance of the line. The matched termination absorbs all the energy in the line, eliminating reflections. Termination, simply put, makes the line look like it is infinitely long. Signals can't reflect back on an infinite line, right? For EIA-485 circuits, each end of the link must be terminated with a 120 ohm impedance. Failure to properly terminate the line may cause data errors. Unfortunately, not all vendors are using the EIA-485 termination resistor value of 120S. Some vendors are using termination values that may make line reflections worse. Many DMX512 devices provide a termination switch, much like those found on video monitors. The last DMX512 unit on the cable must have the termination switch in the "on" position, with all others "off." No cable may be plugged into the DMX512 OUT connector of the terminated unit. Some DMX512 devices will disconnect the data from the DMX512 OUT connector if the termination switch is "on." The last DMX512 device (IPS or other) unit on a link must have the termination switch "on" and must not have a cable plugged into the DMX OUT connector.
A: The DMX512 line driver circuit may be overloaded, and may shut itself off because of the overload. The DMX512 link will not be working. In the case mentioned above, (a unit that cuts the data off), all units downstream will show a "no data" error. IPS does not cut off the signals to the DMX OUT connector if the termination switch is "on."
A: A termination plug may be made from a male 5-pin XLR connector with a 120 ohm resistor soldered between pins 2 and 3, and another 120 ohm resistor soldered between pins 4 and 5. Plug it into the DMX512 OUT connector on the offending unit.
A: The EIA-485 spec allows 32 "unit loads" (UL) on a link with a 120S termination at each end. Most EIA-485 receiver circuits present between 0.6 and 1 U.L. to the line. IPS presents 0.6 U.L. to the line, and up to 45 IPS bars or blocks may be on a single link. If non-IPS equipment doesn't say, assume 1 U.L. per receiver, or call the manufacturer and find out. Some DMX512 devices (some color scrollers and dimmer banks) may buffer and retransmit the DMX512 signal. This is intended to eliminate the 32 U.L. limit per link. Find out what happens to the DMX512 data if the unit is turned off or fails. The unit should have some means to bypass the buffer and send the data straight through.
A: DMX512 distribution amplifiers can be used. More about them later.
A: That depends on the device. The DMX512 specification requires a dimmer to hold its level for 1 second after loss of data. What happens next, is up the manufacturer. IPS will hold the level for 5 minutes then fade to black in about 15 seconds. Other dimmers will go to black, while some may hold at level for anywhere from a few seconds to forever, or may switch to a special "loss of data" preset. What happens to a color scroller or moving light is again, up to the manufacturer. When the data cable is unplugged from the console, the device may be vulnerable to random operation from noise pickup. IPS DMX512 data receivers terminate the line with a "fail-safe" circuit that ensures that a floating line always appears inactive. Random noise pickup could give unpredictable results on non-IPS devices, including dimmers flashing on, or moving lights and color scrollers changing position and color. Try to avoid floating cables, if possible.
A: Again, this probably depends on the equipment. A floating cable shouldn't cause any damage to an IPS system unless it contacts high voltage. Some people, unfortunately, have made multi-cables that bundle DMX512 wiring with power wiring! This is bad practice, and might make it possible for the high voltage to short to the DMX512 lines. This could destroy most of the electronics in a system, not to mention placing lighting operators at risk of electric shock! Another problem with floating cables occurs in areas that have a static electricity problem. A person can build up a static charge of 35,000 volts under the right (wrong?) conditions. If this discharges into the cable, or even the metal body of the connector, it can inject tremendous voltages into the DMX512 receiver circuitry, destroying it instantly. IPS DMX inputs are protected against ESD voltages up to about 10,000 volts.
A: If two pieces of widely-separated equipment (a console and a dimmer bank, for example) are connected, any differences in the circuit common voltage can produce a difference voltage on the signal lines. This voltage would appear across both DMX512 lines as a common-mode voltage (CMV). EIA-485 can operate with a CMV range of +12 to -7 volts. Voltages beyond this range may cause erratic operation of the EIA-485 circuitry, resulting in data errors. IPS products have optically-isolated DMX512 sections that eliminate CMV problems if no other equipment in the system violates the rules by connecting DMX512 signal common to earth ground at more than one point.
A: Common-mode voltage problems can range from very occasional data errors to wholesale system failure and destruction. When common-mode voltage is present, but is within the EIA-485 specification, the system may work fine. If the CMV is shifting, the system may move in and out of correct operation. If the CMV is greater than the EIA-485 spec, the DMX512 circuitry may be damaged. The ultimate CMV failure is line voltage shorted to DMX512 !
A: Take your $10 voltmeter and set it for DC volts, and unplug the cable from the first DMX512 receiving device. Put one probe on the DMX512 common pin in the cable (pin 1), and the other on the DMX512 common pin on the DMX512 receiving device. Measure the voltage. If it's greater than + or - 7 volts you have common mode voltage problems. If you measure a small or zero voltage, you're probably OK. Set your meter to the AC volt scale, and measure again. If you see more than 5 volts, you still have trouble. A DC voltmeter blocks AC voltages, so a low DC reading could be hiding a large AC voltage. If you measured voltages larger than the guidelines, what can you do about it? This gets sticky. If the CMV problems are due to each piece of equipment tying DMX512 common to earth ground, and the earth ground is at a different potential around the building, you (probably) can't very well fix the earth ground. Perhaps the equipment gives you the option disconnect the tie between DMX512 common and earth ground. Try it. Be aware that this might be placing the system (and you) in grave danger in case of a line-voltage short. Check with the equipment manufacturer to find out how they solve CMV problems. If you can't float the equipment, then you will need an isolated DMX512 distribution amplifier.
A: Distribution amps will solve many problems, so let's look at CMV first. An isolated (such as the SA-104 & 108) dist. amp will have no hard electrical connection between the input DMX512 and the various outputs. The signals are coupled from link to link with optical couplers. Each link finds its own DMX512 signal common level. Since there is no connection between any of the signal commons, no ground loops exist, and any differences in absolute ground voltage are irrelevant. Your system would look something like Figure 2. This example assumes that all four dimmer banks are in different locations, and that they all have CMV problems. If dimmer bank 1 and 2 are next to each other, then they probably can be connected using the standard DMX512 IN / DMX512 OUT connectors. The input to dimmer bank 1 would go to output 1 from the dist amp. Remember to terminate the last DMX512 device on each link! IPS dimmers must use IPS distribution amps that have the proper circuitry to combine the talkback data. When using IPS dimmers with SA-104's and SA-108 distribution amps, no IPS dimmers may be connected to the FROM CONSOLE link. IPS dimmers must be on a branch output. This is also true of the FROM CONSOLE link when distribution amplifiers are cascaded to get more outputs. Conventional dimmers or other DMX devices that do not support talkback may be connected to the FROM CONSOLE link. The SM-101 Status Monitor, on the other hand, must be connected on the FROM CONSOLE link between the console and the first distribution amp to properly display all the IPS dimmers in the system.
A: If you have more than 32 U.L. on your link. The dist. amp lets you break your DMX512 "load" into separate branches. The SA-104 dist. amp can drive 32 U.L. per output, for a total of 128 U.L. If that isn't enough, the SA-108 has eight outputs. Both may be cascaded for more outputs.
A: Running the link as a daisy-chain is a hassle. A dist. amp lets you change the daisy-chain into "hub" or "star" arrangement with separate links. This also limits the number of DMX512 devices that can lose data if a cable gets damaged or disconnected. By intelligently laying out the DMX512 distribution, you can limit the damage to small areas if someone trashes a cable.
A: DMX512 provides 512 addresses, and few people will have 512 DMX devices. IPS dimmers have a restriction that the starting address may only be set in groups of three, and come with either three or six dimmers per unit. Dimmers may not be set to the same address, or overlapping addresses. Doing so will hopelessly scramble talkback data, although the dimmers will respond to whatever addresses are set. If the system has non-IPS dimmers or DMX color scrollers, it would probably be a good idea to pick a separate range of addresses for IPS, the dimmers, and the scrollers. This makes bookkeeping easier, since you will know that a "100 series" number refers to a color scroller, while a "300 series" number is an IPS dimmer. You can pick these ranges for maximum convenience. Set the addresses on the non-IPS equipment according to the manufacturer's instructions.
A: Maybe so, but it's a jungle out there, and knowledge is power. We can't cover every possible mix of IPS and non-IPS equipment, but here is a basic checklist to follow when setting up the DMX512 distribution for an IPS system. Various system examples, including mixtures of IPS and non-IPS will be shown with diagrams. While this checklist is written with an IPS system in mind, the basic rules are the same for any DMX512 installation.
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