Dmx512

One of my first projects when I started working for transtechnik (now ETC) was a DMX512 router, called E-Gate. References to that project can still be found on the ETC website. And even the windows software I wrote, called E-Gatekeeper, can still be downloaded, and it still works (even on Windows 10)

(E-gatekeeper screenshot image copyright by ETC GmbH)

The hardware was based on a Motorola (now NXP) PowerPC MPC860 CPU with a Xilinx FPGA. Back then (this was early 2000) the FPGA design was not done by me, but FPGA’s always fascinated me.

What is Wireshark? To answer this it is best to quote the description from the Wireshark website.

Wireshark is the world’s foremost and widely-used network protocol analyzer. It lets you see what’s happening on your network at a microscopic level and is the de facto (and often de jure) standard across many commercial and non-profit enterprises, government agencies, and educational institutions. Wireshark development thrives thanks to the volunteer contributions of networking experts around the globe and is the continuation of a project started by Gerald Combs in 1998.

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Open DMX USB is an open USB to DMX dongle hardware design developed by Enttec. The Open in Open DMX USB refers to the fact that everybody is free to use the design and produce its own USB DMX Dongle without paying any licenses. Since this fits the Linux philosophy I wrote a small Linux 4.X kernel driver to make it possible to use the dongle under Linux.

Getting the Driver

The source code of the driver is now hosted as dmx_usb_module on GitHub. You can use the following git command to clone and compile the sources

DMX512 is a serial protocol used to control lighting equipment like dimmers. It was developed in 1986 by the USITT and updated in 1990 and 2004. Detailed information on the standard can be found on the ESTA website.

The protocol is very straightforward; basically, it does nothing more than repeatedly sending a block of 512 bytes over a serial RS485 line. These 512 bytes, also called a universe, represent 512 different dimmer or parameter values (for example, color or gobo number). The simplicity of the protocol makes it possible to use small micro controllers to send/receive the DMX signal. One of the hardest things to get right is the bit timing; since DMX uses a 250kbit/sec speed, it is not possible to generate DMX with a standard PC serial port (apart from the speed standard, PC serial ports are RS232 and not RS485)