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Logic RDM Sniffer

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Saleae Logic
The Logic as a 2 line DMX/RDM sniffer

The Saleae Logic makes an excellent DMX / RDM sniffer. With the Logic and a small amount of hardware you can build a sniffer for a fraction of the price of commercial units.


Parts

  • A Logic or Logic 16, but see the note below.
  • 485 driver circuit
  • XLR connectors.

Electrical

You need some hardware to convert from the differential 485 levels to a signal which the logic can understand.

At the basic level you only need an RS-485 receiver like the Maxim MAX485 or Linear Technology LT1785 to translate from the DMX/RDM differential levels to a TTL level that the Saleae Logic understands.

Example Circuit

Connecting this circuit to the Logic is fairly simple. Power the board with a MicroUSB Cable or from the +5V, and COM pins on the TTL connector. Then attach the Logic's grey Ground wire to the COM pin on the sniffer circuit, pull the DIR pin low to put the board in RX mode and connect your Logic's channel 1 (generally it's the black cable) to the RX pin. Finally Connect a DXM/RDM source to the board and make sure the green LED is flickering to show data is receiving on the TTL side.

For more information check RDM Sniffer Board

Software

Saleae Logic

The Logic App decoding DMX

Saleae provides the Logic App which works on Windows, Mac and Linux. The application has a plugin to decode DMX, but it doesn't understand RDM data. The tool is best for analyzing the signal timing rather than the high level messages.

OLA Sniffer

OLA has a text-based DMX/RDM sniffer that works with the Logic (it could easily be extended to work with the Logic 16). It can be found in tools/logic directory of the OLA sources.

Limitations:

  • the sniffer doesn't understand DUB responses yet since it looks for breaks.
  • it may struggle with signal timings that are at the edge of the E1.11 standard. If you experience problems with the sniffer please email the Open Lighting Mailing List