Mysterious RX pulses on UART connect on OS X

Arduino IDE 1.6.8, Arduino Due, Mac OS 10.11.3

I am seeing eight mysterious pulses on the RX line when I connect to the serial port using multiple client libraries (Python, JavaScript as well as the built-in Serial Monitor in the IDE). About 78-79us apiece, sampled at 1MS/s with a Logic Pro 16.

These eight pulses when interpreted at 57600 baud will jam the Firmata firmware. And they happen on every connection.

This is using a new install of the Arduino 1.6.8 IDE and with multiple sketches (the normal "Blink" sketch will reproduce this also).

Repro steps on my machine:

  • Install any sketch
  • Start a logic analyzer if you want to catch it
  • Go to Serial Monitor. I have mine configured for 57600 baud, Newline line ending, but it doesn't matter
  • If you want, close and repeat step 3
  • Note pulses each time you connect to the serial port

Any suggestions for diagnosing this? It sounds like it's serial driver-level in some way.

I'm trying to get another environment to see if I can narrow down what the issue is.