I'm running El Capitan (10.11.2) and have had all sorts of problems with my USB-Serial cable after upgrading to El Capitan.
I have the latest drivers for my TrendNet TU-S9 USB-Serial Cable dated 11-10-2015. I've also tried getting the PL2303 drivers (dated 10-22-2015) from Prolific.
I unplugged the cable, uninstalled the old drivers per the instructions, rebooted, installed the latest drivers, rebooted, and plugged in the cable.
Under System Information-USB, I do see USB-Serial Controller D in the list of USB Devices. In /dev, I do not see tty.usbserial. Also, I do not see USB-Serial Controller as a possible interface in Network within the System Preferences. I just see Bluetooth PAN, FireWire, Ethernet and WiFi as possibilities when I try to add a new interface.
I've uninstalled and reinstalled several times and just can't get it to work. It is as if something is not letting the drivers be installed. I'd be grateful for any possible suggestions.