9.1 Serial Port Names: ttyS4, tts/2, etc.
Once upon a time the names of the serial ports were simple. Except for some multiport serial cards they were named /dev/ttyS0, /dev/ttyS1, etc. Then around the year 2000 came the USB bus with names like /dev/ttyUSB0 and /dev/ttyACM1 (for the ACM modem on the USB bus).
A little later with kernel 2.4 came the "device file system" (devfs) with a whole new set of names for everything. The use of the device file system was optional and many still continued using the old system. Then in 2003-4, it was claimed that devfs had unsolvable problems and will be replaced with what's called "udev". Although udev doesn't provide all the functionality of devfs it does handle hot plugging. With all this confusion, most distributions use neither devfs nor udev.
If you use devfs or udev, ttyS1 becomes tts/1, ttyUSB1 becomes /usb/tts/1, and ttyACM1 is /usb/acm/1. Note that the the number 1 above is just an example. It could be replaced by 0, 2, 3, 4, etc. One may use devfs but have the conventional names linked (via symlinks) to the new names. So they use the new system with the old names but may also use some of the new names for some devices. It's even possible ?? to use the new names for the old (non-devfs) system.
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