How to get Beaglebone Black serial debug

I’m new in beagleboad black. I only know that that beaglebone can get serial information by just plugging mini usb to computer, but beaglebone black can’t. So I search on Internet and get from http://circuitco.com/support/index.php?title=BeagleBone_Black_Accessories that I use Adafruit 4 Pin Cable (PL2303) and I would like to get serial debug information from plug green line to P9-4 and white line to P9-5 and black line to GND. That’s right I can get the serial information from terminal like putty, but I can’t get right word. Just like I setting wrong baud-rate, I get much of disorder and nonmeaning word. I set 115200 baud-rate in putty and it works in the original version beaglebone. I have no idea it dosen’t work in Beaglebone black, anything wrong I did? Thanks.

http://circuitco.com/support/index.php?title=BeagleBone_Black_Serial

Gerald