Thank you all, guys.

I have a chat with a software developer from Goodson, he show me how to check the connection in between PC & the printer:

1. Go to DOS (Start - Run: cmd)

2. Type in:

mode com1 9600,n,8,1
--> The printer will send RS232 connection detail and display on DOS

dir>com1
--> send root directory detail to and print on the thermal printer

3. He also told me to to print these characters "ABCDEFG" by:

type in: ABCDEFG then follow by ASCII command of LF & CR
but I could not figure out exactly what actual command I should type in:
C://ABCDEFG 0x0A 0x012
???

4. Setup XOn/XOff for RS232 connection with the printer. May be able to cheat with hardware connection???

5. If I want to send data from the MCU (8051) to the printer to print this information:

Data output 1: Value 1
Data output 2: Value 2
Data output 3: Value 3
Data output 4: Value 4
Data output 5: Value 5

Whereas Value 1 to Value 5 are ADC1 to ADC5 readings collected by the MCU.

Please advice if possible. Thanks!