PyDMX is a module capable of sending DMX512 data via a driver. It was created to allow demonstrations on open days of DMX-based projects created at the University of York Department of Computer Science (UoY CS).
Idle | Break |MAB| Slot 0 | Slot 1 | ------\ /---\ /-\ /--------\ | | | | | | | | | | | | | | \-------------------/ \--------/ \-/ \--- - - - - - -
Diagram of the start of a DMX packet.
DMX512 (or commonly just DMX) is a relatively simple protocol. The DMX bus is in an Idle high state between packets. A packet starts with a break of 100 μs, followed by a Mark After Break (MAB) of 12 μs, this signifies the start of the frame and the start of the "slots".
S 0 1 2 3 4 5 6 7 E E \ /-\ /---\ /-- | | | | | | | | | | | | \-/ \---/ \------/
Diagram of a slot with value 0b10011000 or 152.
Each slot consists of 1 low start bit, 8 data bits, and 2 high stop bits. Each bit of a slot is 4 μs long. This corresponds to a baudrate of 250000 or 250 kbit/s (though due to breaks this is not entirely accurate). There are 512 usable slots (channels) per frame and 513 slots overall. Slot 0 is special as it signifies the type of frame being sent. 0x00 is the normal frame type and corresponds to "standard" lighting data.
A frame can contain any number of slots (beyond the type slot) up to the limit of 513 slots (including type slot). Typically the 512 usable slots are referred to as channels.
The idle period between packets must be at least 92 μs and the MAB must be at least 12 μs. Further a packet must not be longer than 1 second. There are no other requirements on timing, even on inter-slot breaks.
A light will typically take an "address" which is the channel index (starting at 1) which it will listen on. Some lights will use this as a start address and listen on some number of channels above it also if they require more than 8 bits of data.
For example a light might be set to address eight, but listen on channels 8 9, and 10. It could then use each channel as a component of an RGB colour value.
Importantly for writing software relating to DMX, the standard does not specify how to encode different types of data in slots. Therefore, each light manufacturer, or even light, does it differently. There is nothing stopping a manufacturer allowing you to select discontiguous addresses for each 8 bit value, or any number of more convoluted solutions.
The core of PyDMX has the following classes:
- DMXUniverse: represents a DMX universe
- DMXInterface: allows simple control of a DMX driver
- DMXLight: abstract base for lights
- DMXLight3Slot: represents a 3 slot RGB light
- DMXLight7Slot: represents a 7 slot RGB moving light
- DMXDriver: represents a DMX output driver
- Drivers are subclasses of this class
- Colour: represents a 24-bit RGB colour value
The core modules do not depend on any other python modules or external dependencies.
The FT232R driver is designed to work with the FTDI FT232R chip which is a USB to serial chip made by FTDI (Future Technology Devices International). It is specifically designed to work with the USB to DMX adapter board made by the UoY CS lab techs. It is untested with any other hardware configuration.
The FT232R driver requires the python module
pylibftdi which in turn requires the
libftdi shared library be installed. The driver also currently requires Linux due to its use of
libc via ctypes (this should be rectified in a later version).
Below is a basic example of sending an update to turn a light purple:
from dmx import Colour, DMXInterface, DMXLight3Slot, DMXUniverse PURPLE = Colour(255, 0, 255) # Open an interface with DMXInterface("FT232R") as interface: # Create a universe universe = DMXUniverse() # Define a light light = DMXLight3Slot(address=8) # Add the light to a universe universe.add_light(light) # Update the interface's frame to be the universe's current state interface.set_frame(universe.serialise()) # Send an update to the DMX network interface.send_update() # Set light to purple light.set_colour(PURPLE) # Update the interface's frame to be the universe's current state interface.set_frame(universe.serialise()) # Send an update to the DMX network interface.send_update()
A further example program is available in the
test.py file in the repository root.