You can check whether the serial port connections are working correctly using a
serial loopback.

A simple loopback consists of connecting RXD and TXD together. These are pins 8 and 10 on the Raspberry Pi GPIO header, or pins 2 and 3 on the standard RS232 D9 connector on the USB-RS232 adapter:

An RS232 full loopback cable also connects pin 4 (DTR) and pin 6 (DSR) as well as pin 7 (RTS) and pin 8 (CTS) on the RS232 adapter. However, this is not required for most situations, unless these signals are used. By default, no pins are allocated on Raspberry Pi specifically for these additional signals:

Create the following serialTest.py script:

#!/usr/bin/python3 
#serialTest.py 
import serial 
import time 
 
WAITTIME=1 
serName="/dev/ttyAMA0" 
ser = serial.Serial(serName) 
print (ser.name) 
print (ser) 
if ser.isOpen(): 
  try: 
    print("For Serial Loopback - connect GPIO Pin8 and Pin10") 
    print("[Type Message and Press Enter to continue]") 
    print("#:") 
    command=input() 
    ser.write(bytearray(command+"rn","ascii")) 
    time.sleep(WAITTIME) 
    out="" 
    while ser.inWaiting() > 0: 
      out += bytes.decode(ser.read(1)) 
    if out != "": 
      print (">>" + out) 
    else: 
      print ("No data Received") 
  except KeyboardInterrupt: 
    ser.close() 
#End 

When a loopback is connected, you will observe that the message is echoed back to the screen (when removed, No data Received will be displayed):

If we require non-default settings, they can be defined when the serial port is initialized (the pySerial documentation at https://pyserial.readthedocs.io/en/latest/ provides full details of all the options), as shown in the following code:

ser = serial.Serial(port=serName, baudrate= 115200,  
    timeout=1, parity=serial.PARITY_ODD, 
    stopbits=serial.STOPBITS_TWO, 
    bytesize=serial.SEVENBITS)