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A simple and compact USB to DMX512 interface based on the STM32G431KB microcontroller (32 pin LQFP) is presented.

For the same firmware and almost the same electrical circuit 2 different hardware layouts are available: one compact small PCB and another less compact but easier to build Veroboard compatible design.

DMX commands and power supply are provided via USB. It is seen by a host PC as a MIDI interface or as a virtual COM port.
Accordingly, DMX control is possible by MIDI messages as well as by ASCII text based commands. As a special case, the MiniDMX protocol is implemented, which is supported by a number of DMX control software, for example "DMXControl3".

Assembly of the simple STM32G431 USB to DMX interface (breadboard and PCB). Compact PCB size 63x25mm.

The USB interface is "full speed USB2.0" grade. The host sided USB class selection between MIDI and virtual COM port is made by a jumper on the SWD connector (may be extended to a switch). The 5V supply (total current no DMX load ca. 30 mA, with terminated DMX bus up to 60 mA) is scaled down by a linear low drop regulator to 3.3V which supplies the microcontroller.

In MIDI as well as in ASCII mode,

• the commands are designed in a way that manual DMX control is possible with ASCII terminal software or with a MIDI sequencer.
• commands are provided to set any of the 512 DMX channels to any level 0..255 with different methods
• Hue, Saturation and Luminance of RGB lamps each can be set with one command (group of 3 DMX channels)
• perform soft fade transitions with fade time up to 12.7 seconds, adjustable in steps of 1/10 seconds
• a simple Chaser is implemented
• limit the number of transmitted DMX channels between 24 and 512
• read the actual levels of specific DMX channels back to the control software
• read the actual state of global paramters
• save up to 220 lighting scenes nonvolatile in the microcontroller flash
• At system start, all DMX channels are initialzed with zero level ("dark")
• reload any stored lighting scene by command with soft fade transition

When USB works as virtual COM port, the basic MIDI channel and user specific USB Vid/Pid can be configurated specifically. The user setup is stored nonvolatile in the microcontroller flash.

Because no RS-232 interface is installed on the board, the board is programmed (and debugged) via SWD ("Serial-Wire Debug") interface, preferably with ST-LINK, which is connected to the 3 pin socket next to the USB connector.

The source code is quite well suitable as a template for further own development, especially concerning DMX and USB. It is programmed directly on register level, no external libraries or drivers are used.

A detailled construction- and operation manual is provided here (PDF file).

Downloads:
The subsequently downloadable material is copyrighted (c)2020-21 by Wolfgang Schemmert.
Assembly of the devices, programming and use of the software is permitted for free by everybody for any purpose ("freeware"). For commercial use, restrictions of third-party software contributors (Segger GmbH, STM) must be respected.
All information is based on best knowledge, but "as is" and without any warranty. Any responsibility is excluded. Use for dangerous, life-threatening and medical applications is forbidden.

PCB layout for the small USB-DMX-G431 board
(1:1 TIF format, mirrored view "from component side" for PCB reproduction)

PCB layout for the USB-DMX-G431 Veroboard version
(1:1 TIF format. BOTTOM layer as well as component placement view are "from component side", i.e. BOTTOM layer shown mirrored "through the PCB" (as needed for PCB production))

Firmware "usbdmx-g431.hex" for the for the STM32G431 based USB-DMX interface  (24 October 2021)

Source code "usbdmx-g431v5.zip" for the STM32G431 based USB-DMX interface
(state 24 October 2021) This source code is provided as complete "Segger Embedded Studio" project (ZIP file, 322 kB).
Parts published by Segger GmbH are under license of Segger&|Rowley, parts published by STM are under license of STM, parts programmed by me are provided under GNU GPL3 license.)

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