Using Arduino Uno, CNC shield and TMC2208 in UART

[AndyBerllan]

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Is it necessary to add 1K Ohm on a Y cable for the Tx line or does the CNC shield already have this?

[jasonmnemonic]

Yes to 1kOhm. You will need to look at their datasheet on which line. I cannot remember if it is at Tx or Rx side.