I’m trying to connect the USB serial port adapter to the U port of the CerbBee. Do I need a level shifter, or can I just connect TX (pin2) from the Adapter to RX (pin5) on the Bee?
If your device is TTL level then you do not need it but if your device (which probably is) RS232 level then you do need a shifter. If it has a d-sub connector then it is likely RS232.
I like to keep a few CP2102 based USB-TTL units around. At a few bucks per unit from eBay, they’re a bargain. Then you only have to worry about signal integrity over 16 feet…