Dear Charles Linquis,
Thank you for your explanation. really help a lot to me. I am really appreciated.
Looks like the Lookdown command is only find one byte of character, for example,

Lookdown,serstring,["A"],String
Debug DEC String,10,13

So, this command will reply result in String i.e. decimal value "0", right?

serin2,PORTC.4,16390,1000,NotFound,[WAIT ("CB2E")]

This command will look for "CB2E" string only if the incoming data stream is huge and don't care the position of this string, right?
Yes, for my condition is that i want to detect this "CB2E" string only then carry out next job. Sorry to confuse you, i am now using hardware serial port to do the debugging.

TQVM