This FAQ is about the DMX512 standard (a standard that describes data transmission between controllers and lighting equipment and accessories) and is hosted by USITT. The information herein does not represent official positions or formal interpretations by USITT or ESTA. Many questions can be answered by referring to the PLASA/USITT Recommended Practice for DMX512. Most of the information applies to all versions of the standard. The ESTA Technical Standards Program's Control Protocols Working Group is developing an updated Recommended Practice as a companion to DMX512–A.
• In the case of the protocol implementer, a PICS serves as a checklist to reduce the risk of failure to conform to the standard through oversight.
• In the case of the supplier and buyer, or potential buyer, a PICS serves as a detailed indication of the capabilities of the product.
• In the case of the user, or potential user, a PICS serves as a basis for initially checking the possibility of working with another implementation (note that, while compatibility can never be guaranteed, failure to do so can often be predicted from incompatible PICS).
A common question is "can we use a good microphone cable?" The answer here is no. While there is some tolerance allowing for 100 ohm to 120 ohm cable, simply put, microphone cable is not at all suitable because of its high capacitance and incorrect characteristic impedance. It might work in some instances, but it is not appropriate for the electronics involved and it will fail at the most inopportune times.
Explain the second pair, and why I should install and use 2–pair cable.
DMX512/1990 says the optional data link is for data – that's all. It doesn't say anything about the direction of the data or what kind of data, other than it conforms to the electrical specification of EIA 485. DMX512–A discusses implementation more thoroughly than its predecessors.
Several companies use the second pair for various legitimate functions which may be proprietary, but are not always. Some companies do not wire pins 4 & 5 at all, some wire them through. You will have to check with the manufacturer directly or through their documentation. This ambiguity is addressed in the new ANSI version of the standard developed though the ESTA Technical Standards Program. Compliant legacy equipment should have no problem working with products compliant with the new version and vice–versa. Again, see the link above. It should answer most questions.
If you don't install 2–pair cable, you will may never be able to get any current or future enhanced functionality that may be offered in the future via the second pair. If you use a one–pair portable cable, even if you have 2–pairs in the permanent cable plant, you face the same shortcoming. However, it is likely cheaper to replace 1–pair portable cable with 2–pair version.
Why can't I use low voltage power on the second pair?
You may damage equipment. This has never been allowed. The original and 1990 standards state "This Standard shall follow EIA Standard EIA–485 . . ." The ANSI version prescribes "The electrical specifications of this Standard are those of EIA–485–A, except where specifically stated in this document. Where a conflict between EIA–485–A and this document exists, this document is controlling as far as this Standard is concerned." It continues: "Data 2- and Data 2+ of a DMX512 Port provide a secondary EIA–485–A data link." EIA–485 states in its scope "This Standard specifies the electrical characteristics of generators and receivers . . . for the interchange of binary signals . . ."
Several companies use the second pair for various legitimate functions which may be, but are not always, proprietary. Connecting different implementations of compliant equipment will not cause damage, but the enhanced functionality of the implementations will likely be lost. Some companies do not wire pins 4 and 5 at all; some wire them through. Users must check with the manufacturer(s) directly or through its documentation.
Connecting DMX512–A, DMX512 and DMX512/1990 compliant equipment that uses the secondary data link (Pins 4 and 5) to non–compliant gear may disable and/or damage compliant equipment. See DMX512 Hazard - Power on the Data Lines for more info.
Error Correction - Can I use DMX512 for control in hazardous applications?
No. DMX512 does not have any inherent error detection. In fact, the ANSI version added language stating: "There is no assurance that all DMX512 packets will be delivered" and "Since this Standard does not mandate error checking, DMX512 is not an appropriate control protocol for hazardous applications." Even with the data packet being refreshed constantly, there is no method for the receiver to determine that the information it received is information the transmitter (console) sent. One bad data packet, even if corrected in the next packet, can cause something to move or energize when it shouldn't. DMX512–A does offer a standard method for sending a checksum which can be part of an error correction routine (Annex Clause D4), but this is an optional implementation and cannot be relied on.
Why isn't DMX512 a command based protocol?
It could easily have been. However, several factors contributed to the current data streaming format:
1) The desire to keep it simple and develop it quickly.
2) DMX512 is an adaptation in part of Colortran's D192 (CMX) protocol with speed increase and other enhancements developed by the original USITT DMX512 project committee.
3) The trend in control at the time was towards several different serial protocols, and the DMX512 standard evolved from picking the best features of each while making sure that it was not compatible with existing equipment (thus putting all manufacturers on an equal footing).
In May of 2003, Charlie Richmond of Richmond Sound Design (USITT Fellow and past USITT Sound Commissioner) wrote: "As I see it, the difference between a data streaming protocol such as DMX and a command based one such as MIDI is a philosophical one. Commands make more sense being sent by a show controller to a lighting console and streaming data makes more sense being sent by an intelligent lighting console to 'dumb' dimmers."
Why do I have to terminate?
You have to terminate because missing or faulty termination is probably the biggest problem in DMX512 systems. EIA–485, the electrical basis for all versions of DMX512, requires the transmission line (i.e., the DMX512 data link) to be terminated. This is normally done with a resistance matching the characteristic impedance of the cable (see the Cable FAQ) which is ideally matched to the circuitry. Most consoles and the input/output ports of splitters have built in termination. Many receiving devices have a termination switch engaged when it is the end of the data link, disengaged when it is not at the end. Check the manufacturer documentation.
DMX512–A refers to preferred and multiple topologies - what does this mean?
A topology is a description of configuration of cabling and devices in a network. After wiring and termination errors, the most common problems in DMX512 systems are related to conflicts in topologies and data timings. The topology problem is often analogous to ground loops in sound systems (think 60 cycle hum). Different topologies provide different methods for isolating electrical noise in data systems. Most are valid but are not always compatible with each other and, in fact, may exacerbate a problem. The original and 1990 versions of DMX512 did not address this topic. The ANSI version (DMX512–A) does.
DMX512–A states "DMX512 systems should make use of earth ground referenced transmitting devices and isolated receiving devices. This approach provides for a single point solid ground/chassis connection at the source, and allows for variations in building ground potentials between transmitting and receiving devices."
DMX512–A also permits the use of isolated transmitter ports. They are often used in legacy systems where grounded receivers are used. When using isolated transmitter ports, DMX512–A requires declaration in the manual and PICS as well as marking of the equipment.
While also not the preferred topology, a non–isolated receiver topology illustrated in the ANSI standard is acceptable and exhibits better performance compared to other grounded or non–isolated topologies. This non–preferred topology also requires declaration in the manual and PICS as well as marking of the equipment.
A grounded receiver topology is allowed and usually found in the construction of entry level receivers where the cost of isolation might prove an untenable burden. It may be used by manufacturers of receivers who, for reasons beyond the scope of this FAQ, require a direct link between data link common and protective earth. Note that one of the most common noise problems occur in systems with more than one grounded receiver. This non–preferred topology also requires declaration in the manual and PICS as well as marking of the equipment.
Floating is both an input and an output topology and is also allowed but not preferred. It is often confused with the isolated DMX512 topology. It differs from an isolated topology in that there is no requirement of isolation between DMX512 inputs and outputs. This non–preferred topology also requires declaration in the manual and PICS as well as marking of the equipment.
There is another topology that is found in some equipment that is not acceptable under DMX512–A under any circumstances:
Disallowed Transmitter Topology
Disallowed Receiver Topology
What about DMX512 over Ethernet?
There are a number of products on the market that take DMX512 data and send that data over an Ethernet network where it is decoded by proprietary hardware and placed onto a DMX512 system. While the DMX512 data in these various implementations is universal, the methods used to carry the data over Ethernet are not compatible with each other. Most of these implementations should be able to exist on the same network, but that is not guaranteed; you must check with the manufacturers.
Several implementations are multi–manufacturer; some are proprietary. The following list is not exhaustive:
- Pathport - Developed by Pathway Connectivity; a DMX512 distribution and management system over Ethernet; implemented by a number of manufacturers
- Art–Net - Developed by Artistic Licence; a DMX512 distribution system over Ethernet; implemented by a number of manufacturers
- Other DMX512 over Ethernet solutions include (but are not limited to):
- ETC
- Strand Lighting
- Leviton / NSI / Colortran
- Sand Systems
What is RDM?
Remote Device Management is an open standard in development. This is an enhancement to USITT DMX512 for configuration, status monitoring, and management of DMX512–based systems. This standard (ANSI/ESTA 1.20, Entertainment Technology - Remote Device Management over USITT DMX512) was developed by the ESTA Technical Standards Program and is designed for interoperability between many manufacturers. Compliant DMX512 and DMX512–A devices are completely functional when RDM is present.
Will ACN replace DMX512?
No. ACN (The ESTA Architecture for Control Networks) is more formally known as ANSI/ESTA E1.17, Entertainment Technology - Multipurpose Network Control Protocol Suite. It is intended to provide the next–generation standard for manufacturer interoperable lighting control over high bandwidth networks that support UDP/IP (like Ethernet) and related protocols.
Many of the devices in an ACN system will still be DMX512 devices. At each Lighting Position a gateway (“black box”) converts from ACN to DMX512/RDM to daisy–chained legacy DMX512 devices and RDM enabled devices. While many devices such as Dimmer Racks and Media Servers will accept ACN directly, most end use devices will still be simpler, lower cost DMX512 or DMX512/RDM devices.