Sydney Mini

Intro to DMX communicationsMatt Edwards

DMX – What is it?

• In the good old days, computer control of dimmers relied on the use of a small dc voltage that was proportional to different dimming level for a light. This voltage ran along individual wires for individual channels and the most popular voltage was 0 – 10Vdc.

• This system suffers from two major problems:

1. It is prone to noise and earth loops if not wired properly over long distances.

2. It can be very non linear with the different kinds of lamps in use today.

DMX512

• The U.S.Institute of Theatre Technology first developed the DMX512 protocol in 1986 as a standard interface between dimmers and consoles.

• The “Standard” has evolved to enable additional equipment to be added like intelligent lights, colour changers, yokes, strobes, smoke machines, lasers and even confetti dispensers.

DMX Ports and Universes

• A single DMX port transmits the magnitude value information for a maximum 512 channels (or lesser) only. This port is known as a DMX universe.

No of UNIVERSE

CHANNELS

1 512

2 1024

5 2560

Physical Info

• The DMX data stream clocks out at the rate of 250Khz which means each bit is measured at 4 micro seconds widths.

• The RS485 standard uses two/three wires to transmit the digital HIs & LOs

Addressing

• Each Controller needs to have the Start Address set or programmed

DMX & RDM

• The original standard was a single directional data path from the Lighting Consol to the Dimmer.

• RDM (Remote Device Management) requires bidirectional communications between the console and the dimmer.

• In our community, there has been limited work done with RDM, but it may become more popular in the near future.

Renard

• Renard is a Serial communications protocol• Character Format

– Baud Rate can vary, current firmware is programmed for 57600.

– 1 Start Bit, 8 Data Bits, No Parity bit, 1 Stop bit (8N1) • Special Characters

– 0x7D - Pad byte, silently discarded by controller firmware, inserted by host PC to prevent Tx overrun

– 0x7E - Sync byte, start of packet marker. – 0x7F - Escape byte, used as prefix for encoding dimmer

levels that correspond to the special characters. • Packet Format

– Byte 0 - 0x7E (sync byte) – Byte 1 - Command/address byte (usually 0x80, see

below) – Byte 2-n - Dimmer values (0-0xFF, values 0x7D, 0x7E

and 0x7F have special encoding, all others are sent raw) • Both RS232 and RS485 are used

Renard Channel Limits

• Dependant on Baud rate and refresh rate

• DMX – 512 channels per Universe

Refresh Rate

Baud Rate 100ms

50 ms 25ms

115200 1150 286 286

57600 574 190 143

38400 384 190 94

Cable Distances

• The maximum distance between controllers depends on whether RS232 or RS422 is used as the physical communications method. – For RS232 the standards only specify

short distances (less than 15 m), (but past experiences in other situations suggests that it should work up to several times that distance, especially if

low capacitance cable is used.) – For RS422 it should work out to a

distance of more than 300m.

Main Differences

• Renard– RS232 & RS 485– Variable Baud

rate– Variable number

of channels– Assumed

addressing– Controllers

passing remaining channels to next controller

• DMX– RS 485 Only– 250kbit/s– 512 channels per

universe– Start address

programmed– All controllers on a

universe RX all channels

Renard Network

DMX Network

DMX &Renard Network

References

• Material for this presentation was taken from:– Ujjal’s DMX512 web site

http://www.dmx512-online.com/whats.html

– Christmas Wiki http://www.christmasinshirley.com/wiki/index.php?title=Renard