uC based DMX Transmitter

Simple Lighting Set

DATA FRAME

Each DMX512 transmitter sends 512 packets of 8-bit dimming values, between 0 and 255, where 0 represents the lights off and 255 represents the maximum intensity. The DMX512 protocol requires the transmitter to continuously repeat (at least once a second) the transmission of a frame as shown in the timing diagram in Figure1 and Table2.

STATE MACHINE
State1: SENDMBB - DMX Idle (Mark Before Break)
State2: SENDDATA - Send 512 bytes
State3: SENDMAB - DMX Idle (Mark After Break)
State4: SENDBREAK - Send Break

DMX512

DMX 512 is used to control intelligent lights in professional stage lighting.  It is an asynchronous serial communication and is based on the EIA/TIA-485 (aka RS-485).  RS-485 specifications allow only one driver to send data at a time, and up to 32 unit loads (12-kΩ load x 32=375Ω) can be placed on the bus.  Maximum distance is 1500 ft using a twisted-shielded cable (e.g. microphone cable) and two terminators, consisting of 120 Ω, 1/2W resistors.  As a rule of thumb, the data signaling rate (in bps) multiplied by the cable length (in meters) should not exceed 10^8.  For example, a system with a cable measuring 500 m should not transmit data at speeds greater than 200 kbps (10^8/500).

DMX 512 can use good quality microphone cables.  The 3-pin XLR connector wiring is shown below: 

DMX512 Connector: pin1=shield, pin2=Data-, pin3=Data+

 DMX 512 data is transmitted at 250 kiloHertz (kHz).  A single transmission (DMX Packet) includes synchronizing elements and channel data for up to 512 channels (1 universe).  RS-485 uses a daisy chain connectivity scheme and requires termination at both ends of the cable.

DMX512 Protocol