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EDIT:

I want to interface with a legacy board through a high density connector. There are some USB lines on the connector that I do not wish to use.

These lines are connected to an FT232RQ on the legacy board. On each line there is 1k of series resistance and the data+ line has a 1.5k pull up to 3.3V.

On my new board should I tie these two lines directly to ground, together, or leave them totally unconnected?

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Your question is unclear due too much handwaving and dangling references, so I'll state what I'm answering. I seems you have a existing board that contains a USB device. You want to connect to this board, and are asking what to do with the USB lines since you won't be using the USB connection to communicate with the board.

Don't do anything. Just leave then unconnected.

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  • \$\begingroup\$ A restated the question, please let me know if you think it is more clear. You understood the question correctly when you restated it in your answer. \$\endgroup\$
    – Nick
    Apr 16, 2015 at 19:16
  • \$\begingroup\$ Why is it better to leave them unconnected rather than terminate them into the common ground between the two boards? Is it just to prevent sinking ~2mA through a series resistor and pull up resistor? \$\endgroup\$
    – Nick
    Apr 16, 2015 at 22:05

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