HAZARD ALERT -- POWER on DMX512 DATA LINES
If you use DMX512 equipment as part of your lighting system, this is important to you.
In 1997 a warning was issued that some equipment did not comply with USITT's DMX512/1990 standard, as they placed between DC voltage on Pins 4 or 5 of their DMX512 connectors, in violation of the USITT DMX512 and DMX512/1990 Standards. In some cases, this DC voltage is even outside EIA-485's maximum voltage range (EIA-485 is the electrical standard that DMX512 is built on).
Most products comply with the DMX512/1990 Standard -- this non-compliant voltage is not present on the data lines.
What the Standard says:
The standard clearly states that Pins 4 & 5 are for an optional second data link (1) and must conform to the electrical requirements of the EIA-485 standard (2). The standard also states the rules for marking the equipment indicating compliance (3). Placing a voltage on Pins 4 and/or 5 of the 5 Pin XLR connector compromises a DMX512 system and is a clear violation of the USITT DMX512, DMX512/1990, and ANSI E1.11 (DMX512-A) Standards. Connecting DMX512, DMX512/1990, and DMX512-A compliant equipment that uses the secondary data link (Pins 4 & 5) to non-compliant gear may disable and/or damage compliant equipment.
In the Mean Time . . .
It should be pointed out that there are manufacturers that also do not use Pins 4 & 5 in compliance with the standard, although it is unclear whether such equipment is in production anymore. Only one or two may be using a voltage outside of the EIA-485 specification.
So, if you use compliant DMX512 equipment that uses Pins 4 & 5 in accordance with the standard, be aware that there is equipment out there claiming compliance with DMX512, but does not comply and may in fact damage your equipment. If a 5 Pin XLR is used for something other than Fully Compliant DMX512, it must not be labeled DMX512 (and should not even be labeled DMX).
Note that the use of a 5 Pin XLR connector on entertainment lighting equipment has become synonymous with DMX512, even if not identified as DMX512. Therefore, we recommend that 5 Pin XLR connectors only be used in accordance with the standard and that manufacturers not fully compliant work to bring their products into full compliance.
1. Pin assignments as called out in the standards are as follows:
PIN 1 - Signal Common (Shield)
PIN 2 - Data (Dimmer) Drive Complement (Data 1 -)
PIN 3 - Data (Dimmer) Drive True (Data 1 +)
PIN 4 - Optional Second Data Link Complement (Data 2 -)
PIN 5 - Optional Second Data Link True (Data 2 +)
2. The standards require adherence to EIA-485 with regards to all electrical characteristics. EIA-485's maximum voltage range is stated as -7 to +12 volts dc; +18 to + 25 volts dc is outside this specification.
3. The standards state rules regarding the marking of equipment -- Only Equipment conforming to the Standards may be so marked.
Note - Compliance with these Standards is the responsibility of the manufacturer and such marking and identification does not constitute certification or approval by USITT or ANSI Accredited Standards Committee E1.
However some products are labeled DMX512 but do not comply. Some of these non-compliant products have identification by a pin connection table adjacent to the connector and have a modified circuit which is limited in current at the male 5 Pin XLR connector and is not linked through from male to female. This circuit is claimed to be less likely to cause damage.