Unable to make Serial communication both ways

Question

This is my first attempt at understanding how serial communication on pi work, so I am testing this code:

 '''
UART communication on Raspberry Pi using Pyhton
http://www.electronicwings.com
'''
import serial
from time import sleep

ser = serial.Serial ("/dev/ttyS0", 115200)    #Open port with baud rate
while True:
    received_data = ser.read()              #read serial port
    sleep(0.03)
    data_left = ser.inWaiting()             #check for remaining byte
    received_data += ser.read(data_left)
    print (received_data)                   #print received data
    ser.write(received_data)                #transmit data serially 

I connected my pi ( pi 3) via usb-ttl converter to my PC ( windows). On windows I started putty and on pi I executed the above code.

It works but only in one direction. That is I can send any data from PC to the pi, but whatever I am typing on pi is not at all appearing on PC (putty). What can be done to make this duplex communication possible? (Note: I can send serial data both ways via putty, so I understand there is no issue with serial hardware)

Update

   ls -l /dev/ttys0
   crw-rw---- 1 root dialout 4, 64 Mar 7   23:14  /dev/ttyS0    

   stty </dev/ttyS0 
   stty: 'standard input': Input/output error

   stty -F /dev/ttyS0 -a
   stty: : /dev/ttyS0  Input/output error

Update 2

Answer 1

I'm not sure what is your problem, because the setup as described is a bit too vague. However, before using the serial devices on a Raspberry, you might (depending on your HW and OS version) need to setup your OS and boot config to let this happen.

Generally, you need to check/edit 3 things:

• If serial is enabled in the Kernel OS config with raspi-config
• Make sure nothing else is already using the serial in /boot/config.txt
• Disable the kernel console to use the serial in /boot/cmdline.txt

Do this:

sudo raspi-config

# => Interfacing Option 
#    => Serial
#       => NO
#       => YES  <-- change to yes

sudo nano /boot/config.txt

# Add this line to disable BlueTooth (which also uses a serial line)
dtoverlay=pi3-disable-bt

sudo reboot

# Now edit your boot command line to prevent the linux console to
# steal control of it.
sudo nano /boot/cmdline.txt

# Remove anything like these: 
#   "console=serial0,115200" or "console=ttyAMA0,115200"

sudo reboot