depending on which USB port I plug my USB-serial adpator, tty.PL2303-1B1 or tty.PL2303-3B1 shows up. (The third character from left is "1" or "3").
Is there a way to avoid this? (like, having the same tty showing up for both USB ports)
Simple Answer: No, It's a feature ;-).
Complex answer1: Download the sourcecode and edit the code to construct the tty suffix (method nl_bjaelectronics_driver_PL2303::createSerialStream()).
Complex answer2: Check the Apple documentation for serial ports. There is code to detect
serial ports independent of the filename.
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