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About DMX512

   

  DMX512 specifics
  DMX512 pinout and cabling details
  History and future of DMX512
  Summary

  DMX GENERAL
  About DMX512
  DMX devices
  Potential DMX problems
  DMX compared to other protocols
     
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  DMX in LD2000
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T

he communications standard "DMX512" covers digital multiplexed signals. It is the most common communications standard used by lighting and related stage equipment.
        DMX512 provides up to 512 control "channels" per data link. Each of these channels was originally intended to control lamp dimmer levels. You can think of it as 512 sliders on a lighting console, connected to 512 light bulbs. Each slider's position is sent over the data link as an 8-bit number having a value between 0 and 255. The value 0 corresponds to the light bulb being completely off while 255 corresponds to the light bulb being fully on.

DMX512 specifics

DMX512 data is transmitted at 250,000 bits per second using the RS-485 transmission standard over two wires. As with microphone cables, a grounded cable shield is used to prevent interference with other signals.
        There are five pins on a DMX connector: a wire for ground (cable shield), two wires for "Primary" communication which goes from a DMX source to a DMX receiver, and two wires for a "Secondary" communication which goes from a DMX receiver back to a DMX source. Generally, the "Secondary" channel is not used so data flows only from sources to receivers.
        DMX512 is connected using a daisy-chain methodology where the source connects to the input of the first device, the output of the first device connects to the input of the next device, and so on. The standard allows for up to 32 devices on a single DMX link. Although each device has an input and output connector, these are merely wired together -- no re-transmission or amplification is performed by each device.
        Each receiving device typically has a rotary switch which sets the "starting channel number" that it will respond to. For example, if two 6-channel dimmer packs are used, the first dimmer pack might be set to start at channel 1 so it would respond to DMX channels 1 through 6, and the next dimmer pack would be set to start at channel 7 so it would respond to channels 7 through 12.
        The DMX512 communications protocol is very simple and robust. It involves transmitting a reset condition (indicating the start of a new "packet"), a start code, and up to 512 bytes of data. Data packets are transmitted continuously. As soon as one packet is finished, another can begin with no delay if desired (usually another follows within 1 ms). If nothing is changing (i.e. no lamp levels change) the same data will be sent out over and over again. This is one of the best features of DMX512 -- if for some reason the data is not interpreted the first time around, it will be re-sent shortly.
        Not all 512 channels need to be output per packet, and in fact, it is very uncommon to find all 512 used. For example, most simple lighting consoles only output 16 channels or less. The fewer channels are used, the higher the "refresh" rate. It is possible to get DMX512 refreshes at around 1000 times per second if only 24 channels are being transmitted. If all 512 channels are being transmitted, the refresh rate is around 44 times per second.

DMX512 pinout and cabling details

5-pin XLR female, for DMX outputStandard DMX connectors use XLR connectors with five pins:

  • Pin 1: Shield (ground)

  • Pin 2: Primary Data Complement (-)

  • Pin 3: Primary Data True (+)

  • Pin 4: Optional Secondary Data Complement (-)

  • Pin 5: Optional Secondary Data 2 True (+)

A DMX output connector is always female, and a DMX input connector is always male. Some manufacturers use 3-pin XLR connectors, eliminating Pins 4 and 5.
        The DMX512 signal is transmitted via the industry standard interface EIA485, more familiarly known as RS485. RS485 is a balanced connection. The standard wiring is a twisted-pair, shielded, low-capacitance data cable designed for RS-485 -- never use standard microphone cable. Recommended cables are Belden 8227, Belden 9156, Belden 43906 (European DMX Cable Version).
        Data is transmitted in serial format asynchronously with the transmission speed of 250 Kbps. Voltage on both pins ("+" and "-") should be between +12 volts and -7 volts (measured to ground). EIA485 defines that the signal voltage between the two wires should be at least 200 millivolts. Higher voltage on the "+" pin and lower voltage on the "-" pin results in a digital "1". Higher voltage on the "-" pin and lower voltage on the "+" pin results in a digital "0". The ground wire is only a reference point and often used for shielding.
        DMX devices such as lights are connected in a daisy-chain fashion: from the controller to light #1, to light #2, to light #3 and so forth. According to the standard, a DMX512 controller can only drive up to 32 loads (e.g., one light = one load). But improvements in technology have reduced the load a single light puts on the circuit, so you may be able to drive as many as 128 lights (each one being 1/4 load) from a single controller such as the QM2000 board. To control additional loads (lights), a DMX splitter is required.
        The final device in the daisy-chain must be terminated. Terminating plugs contain a 120 ohm resistor soldered across pins 2 and 3. The terminator functions by absorbing signal power which would otherwise be reflected back into the cable and degrade the data.

History and future of DMX512

DMX512 was created in 1986 by the United States Institute for Theatre Technology (USITT) as a standardized method for connecting lighting consoles to lighting dimmer modules. It was revised in 1990 to allow more flexibility.
        The Entertainment Services and Technology Association (ESTA) has assumed control over the DMX512 standard. At this writing (May 2000), ESTA is making revisions to clarify and further extend the standard. Special care is being taken so that existing DMX512 equipment will work under any new standard. So there is no need to hold off on implementing the existing DMX512 standard, or on buying existing DMX512 devices -- these will work fine under any new revision.
        ESTA refers to the current draft revision as the "BSR 1.11 -- DMX-512/2000" standard. The draft is being taken through the rigorous process for establishing consensus so it may become an American National Standard. The draft may be available for public review sometime between August and December 2000.

Summary

  • DMX512 is a method of connecting a single controlling source to multiple receivers

  • Serial data can be sent up to 4000 feet over microphone-like cables

  • Up to 512 devices, or functions on a device, can be controlled with 8-bit resolution

  • All channels are continually being "refreshed", which increases safety

  • Although the DMX512 standard is being updated, existing equipment will still work the same under any new revision.

Next page: DMX devices >

   
  This page last updated: Thursday, November 30, 2000 01:32 PM

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