Python3 Two-Way Serial Communication: Reading In Data - python-3.x

Question

I am trying to establish a two-way communication via Python3. There is a laser range finder plugged into one of my USB ports and I'd like to send/receive commands to that. I have a sheet of commands which can be sent and what they would return, so this part is already there.
What I need is a convenient way to do it in real-time. So far I have the following code:
import serial, time
SERIALPORT = "/dev/ttyUSB0"
BAUDRATE = 115200
ser = serial.Serial(SERIALPORT, BAUDRATE)
ser.bytesize = serial.EIGHTBITS #number of bits per bytes
ser.parity = serial.PARITY_NONE #set parity check: no parity
ser.stopbits = serial.STOPBITS_ONE #number of stop bits
ser.timeout = None #block read
ser.xonxoff = False #disable software flow control
ser.rtscts = False #disable hardware (RTS/CTS) flow control
ser.dsrdtr = False #disable hardware (DSR/DTR) flow control
ser.writeTimeout = 0 #timeout for write
print ("Starting Up Serial Monitor")
try:
ser.open()
except Exception as e:
print ("Exception: Opening serial port: " + str(e))
if ser.isOpen():
try:
ser.flushInput()
ser.flushOutput()
ser.write("1\r\n".encode('ascii'))
print("write data: 1")
time.sleep(0.5)
numberOfLine = 0
while True:
response = ser.readline().decode('ascii')
print("read data: " + response)
numberOfLine = numberOfLine + 1
if (numberOfLine >= 5):
break
ser.close()
except Exception as e:
print ("Error communicating...: " + str(e))
else:
print ("Cannot open serial port.")
So in the above code I am sending "1" which should trigger "getDistance()" function of the laser finder and return the distance in mm. I tried this on Putty and it works, returns distances up to 4 digits. However, when I launch the above Python script, my output is only the following:
Starting Up Serial Monitor
Exception: Opening serial port: Port is already open.
write data: 1
read data:
and it goes forever. There is no read data or whatsoever.
Where am I mistaken?

Answer 1

Apparently much more simpler version of the code solved the issue.
import serial
import time
ser = serial.Serial('/dev/ttyUSB0', 115200, timeout = 1) # ttyACM1 for Arduino board
readOut = 0 #chars waiting from laser range finder
print ("Starting up")
connected = False
commandToSend = 1 # get the distance in mm
while True:
print ("Writing: ", commandToSend)
ser.write(str(commandToSend).encode())
time.sleep(1)
while True:
try:
print ("Attempt to Read")
readOut = ser.readline().decode('ascii')
time.sleep(1)
print ("Reading: ", readOut)
break
except:
pass
print ("Restart")
ser.flush() #flush the buffer
Output, as desired:
Writing: 1
Attempt to Read
Reading: 20
Restart
Writing: 1
Attempt to Read
Reading: 22
Restart
Writing: 1
Attempt to Read
Reading: 24
Restart
Writing: 1
Attempt to Read
Reading: 22
Restart
Writing: 1
Attempt to Read
Reading: 26
Restart
Writing: 1
Attempt to Read
Reading: 35
Restart
Writing: 1
Attempt to Read
Reading: 36

Answer 2

It seems to me that your ser.timeout = None may be causing a problem here. The first cycle of your while loop seems to go through fine, but your program hangs when it tries ser.readline() for the second time.
There are a few ways to solve this. My preferred way would be to specify a non-None timeout, perhaps of one second. This would allow ser.readline() to return a value even when the device does not send an endline character.
Another way would be to change your ser.readline() to be something like ser.read(ser.in_waiting) or ser.read(ser.inWaiting()) in order to return all of the characters available in the buffer.

Answer 3

try this code
try:
ser = serial.Serial("/dev/ttyS0", 9600) #for COM3
ser_bytes = ser.readline()
time.sleep(1)
inp = ser_bytes.decode('utf-8')
print (inp)
except:
pass