Sign In
Register
Discussion
|
Quick Start
DMX is the most widely used lighting control protocol, using one cable (usually with a 5 pin XLR connector) it can control up to 512 Dimmers or a number of intelligent fixtures.
The basic idea of DMX is that you connect each unit together in a daisy chain, starting at the desk and going from fixture to fixture, or fixture to dimmer in any order, up to a maximum of 32 units per physical cable segment. For multiple cable segments a buffer or repeater must be used to guarantee signal integrity.
Each unit is then assigned a 'start address', which specifies the first DMX channel that that unit will respond to, each channel after that is then offset from the start address, so a 6 channel dimmer with a start address of 13 will operate on channels 13, 14, 15, 16, 17 and 18. The next unit would then be addressed to start at channel 19 (or higher).
DMX can only be split (or T'd) by the use of an appropriate buffer unit, otherwise all units should just be chained together, using the 'DMX in' and 'DMX out' (or 'DMX thru') sockets. Note that the items do not need to be chained in the order that they are addressed.
At the end of the chain a termination plug should be inserted to avoid corruption of the signal. Care should be taken to avoid double terminating, which sometimes happens when a unit has a termination switch turned on, and a termination plug inserted.
Details
DMX (Digital Multiplex) (or DMX512-1990, to give it its full name) is the accepted standard for controlling lighting equipment. 5-pin XLR connectors are the standard. DMX systems should always be terminated by the use of a DMX Terminator, and wired with suitable DMX Cable
Some manufacturers also use 3-pin XLR connectors, but this is in contravention to the USITT standard for DMX512. Some manufacturers take this a stage further by swapping pins 2&3; the aim of this can only be to sow the seeds of confusion amongst technicians everywhere. Other deviations from standard practice include omitting the terminator at the end of cable runs, and using XLR microphone cables in place of cabling that meets the RS-485 transmission standard. Such practices may be acceptable for small lighting rigs like mobile discos, with short cable runs and only a few lights, but are likely to give rise to occasional unexpected 'glitches' and are not to be recommended.
The DMX standard was originated by the United States Institute for Theatre Technology but is now maintained by the Entertainment Services and Technology Association and its Control Protocols Working Group. The official standard is approved by the American National Standards Institute under it's official designation E1.11-2004.
The standard reference work on the subject is 'Recommended Practice for DMX512' by Adam Bennette - Second Edition - ISBN: 9780955703522 - jointly published by PLASA and USITT, and is essential reading for every lighting engineer.
Pinout
XLR Pinout
| Pin Number | Usage |
| 1 | Screen / Ground |
| 2 | Data negative |
| 3 | Data positive |
This is the pinout as defined in the standard for 5-pin connectors (pins 4 and 5 are also mentioned in the standard, pin 4 for an extra data -ve, pin 5 for a data +ve, but are seldom used. Vari*Lite however have made use of this second pair on some of its fixtures, Compulite use it for dimmer status reporting and some Avolites Pearls feature a second universe on pins 4 and 5). This layout also holds true for the majority of 3-pin connectors, but not all, with those where it doesn't hold have the two data pins reversed (some older Martin kit being a notable offender for this).
CAT5 Pinout
The following pinout is the official one which can be used when using CAT5 type cabling for DMX distribution, table 4 in E1.11 - 2004 (modified so that the pins are in numerical order) or the image below that (as the pins appear physically on the connector).
| Pin Number | Cable Colour | Usage |
| 1 | White/Orange | Data 1+ |
| 2 | Orange | Data 1- |
| 3 | White/Green | Data 2+ (optional) |
| 4 | Blue | Not assigned |
| 5 | White/Blue | Not assigned |
| 6 | Green | Data 2- (optional) |
| 7 | White/Brown | Data 1 common |
| 8 | Brown | Data 2 common (optional) |
Note that it also says:
External (user accessible) IEC 60603-7 8-position modular connectors are permitted only on patch and data distribution products and only when installed in controlled access areas.
They are not permitted to be used on fixtures, by the standard. However that was 2004, now it's 2009 so things may have changed.
Thanks to Boatman's posting in the Forum.
Refresh Rates
Table 6 in E1.11-2004 states that the minimum duration for the "BREAK" is 92us and the "MARK" after BREAK is 12us. Each data slot occupies a minimum of 44us and the minimum "BREAK" to "BREAK" time is 1204us.
Paragraph 8.7 states that there is no minimum number of data slots. However, using the above timings, the minimum packet size is sufficient to contain a "START" code and 24 "DATA" slots, although the latter do not have to be filled. That allows a maximum refresh rate of 830 updates/sec.
If all 512 data slots are used then the packet duration is 22.676ms which gives the accepted standard refresh rate of 44 updates/sec.
There may very well be a problem with fixtures that will not respond to faster refresh rates but that is because they don't comply with the E1.11 specification and not because the specification doesn't allow it. The specification also allows much slower refresh rates down to 1 update/sec, but some fixtures may decide that the DMX signal is lost with such a slow rate.
Here is a formula for calculating the maximum obtainable refresh rate in updates/second from the number of data slots used:
RefreshRate = 106 / (((DataSlotsUsed + 1) * 44) + 102)
Note that this simple formula will produce an out-of-spec result if less than 24 data slots are used.
History
In the 1980s, a number of manufacturers, most notably Martin, created RS-422 based control protocols for their equipment. Intelligent or automated lighting was being developed at this time and it was realised that existing analogue control techniques were unsuitable. The three-pin XLR connector was chosen due to its popularity for sound equipment, and the most common set of audio pinouts were adopted - pin 1 ground, pin 2 data positive, and pin 3 data negative.
After the USITT standardised DMX512, existing fixtures could be made compatible by means of a firmware upgrade to receive the new protocol. However, there was no way the polarity of pins 2 and 3 could be changed without hardware modifications. Hence the situation where some old designs of equipment have pins 2 and 3 swapped. Newer equipment that uses the non-standard 3 pin XLR connectors normally copies the 5 pin standard for the first three pins.
Discussion