Mini USB-DMX interface

Posted on Feb 4, 2014

On the DMX line side a XLR connector should be used. Either a 5 or 3 pin connector. The pins/holes in a XLR connector are usually numbered so no need for a picture. For the 5 pin connector, only pins 1-3 are used. I used the following code to do some initial tests on the cirquit: dmx_tx_test. zip (Microsoft VC7 project). The code configures the device as a DMX transmitter and repetedly

Mini USB-DMX interface
Click here to download the full size of the above Circuit.

sends out DMX frames until a key is pressed. By setting all channels to either 0 or 255 it was easy to verify that the cirquit was transmitting DMX data using an ordinary voltmeter. The cirquit is not fully USB compliant. The USB spec says that a USB bus powered device must not draw more than 500 uA in USB Suspend mode. FT232RL (IC2) will correctly go into low power mode but MAX481 (IC1) will not. To set MAX481 in low power mode, pins RE and DE on MAX481 must be set to RE="1", DE="0". FT232RL provides both PWREN and SLEEP signals so USB Suspend mode could be handled with some additional gates. However, all sorts of crazy equipment (which definitely don`t comply with the USB spec) these days use the USB port as power source, so I don`t consider the USB Suspend mode violation too serious. But, here is room for improvements. There are some ready made (commercial) USB-DMX interfaces also using FTDI chips. While designing the cirquit I tried to make it comaptible with the Enttec Open DMX USB, which uses the older FTDI chip FT232BM. Practical tests shows that my cirquit is compatible with software supporting Enttec Open DMX USB, for example FreeStyler. When working with high power equipment like dimmers, be sure you know what you are doing! You are connecting your PC through the USB port to eqipment that switches kilowatts of power at several hundred volts. The above cirquit does not isolate the USB port from the DMX line. If...

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