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Reversing Device Protocols

The VOX devices abuse the MIDI protocl and its System-Exclusive Messages as a transport layer for a custom protocol. You can find more details in the protocol documentation for the VT20/40/100X device.

Supporting Software

  • Wireshark with USBPcap
    • you can filter for the relevant messages with sysex.manufacturer_id == 0x42
  • HxD

Workflow

For Host-to-Device communication, i recommend:

  • connect your AMP to your PC
  • open Wireshark and ToneRoom
  • manipulate a parameter in ToneRoom and look at the messages in Wireshark
  • you should be able to easily find the patterns. It should be very similar between all the different devices, so you can look at existing protocol documentation for more info.

For Device-to-Host communication the ToneRoom+WireShark approach works, too. But IMO it sucks because you have to click a bunch for every message in WireShark. Alternatively:

  • use this application instead of ToneRoom
  • set the log-level in the sidebar to "DEBUG". The application will now log all incoming messages to the browser console.
  • Now turn knobs on your device and look at the incoming message. Figuring out the protocol should be straightforward then.