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I have a PCB with a serial TX UART used for programming & logs.

Most of the time nothing is connected & the ~50mm TX trace is unconnected - basically an antenna.

Are there any best practices for reducing the EMI caused by an unconnected TX port? Is it even necessary? It's 115200bps.

I already have a solid ground plane underneath it.

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When something is connected, the receiver will be a high-impedance CMOS input, which behaves pretty much the same as an open end. So there is no practical difference between connected and unconnected.

How much you need to care about EMI depends on the board layout, the environment, and how fast the signal edges are. Anyway, to reduce EMI, you can add source termination. Assuming that the trace has a characteristic impedance of about 50 Ω, and that your driver's output impedance is lower, add a series resistor of about 22 Ω or 33 Ω directly at the TX pin.

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  • \$\begingroup\$ Also the tx line will transmit any noise found on the vdd that powers the pin driver. That could be far worse problem than the actual data signal \$\endgroup\$ Mar 4 at 22:50

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