MIDI Thru ports only pass incoming signals through to another device.
They are not a secondary output.

You need to wire things up as:

  • DT Out > ST In
  • ST Thru > P800 In.

Then you can control the P800 via the DT.
Alternatively, MIDI Thru boxes can split a MIDI Out to multiple DIN connections for up to 16 devices.

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