Python3 Two-Way Serial Communication: Reading In Data

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?

Try to add `ser.close()` before `ser.open()`. – Michael Jul 31 '17 at 19:22 — Michael, Jul 31 '17 at 19:22
Unfortunately that did not change anything. – Schütze Aug 01 '17 at 08:20 — Schütze, Aug 01 '17 at 08:20

score 7 · Accepted Answer · answered Aug 02 '17 at 10:37

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

score 1 · Answer 2 · answered Aug 01 '17 at 15:48

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.

I've tried both ways, and it resulted with read data line being printed for 5 times, but again, without a value. At this point I am also doubting if writing function properly works, because I do not see the the effects of the commands on the laser. The device may not even be receiving the command, let alone replying. — Schütze, Aug 02 '17 at 07:42

score 1 · Answer 3 · answered Sep 11 '19 at 07:04

1

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

answered Sep 11 '19 at 07:04

Reyhaneh Torab

360
5
9

1

please try to be more descriptive in your explanation – borchvm Sep 11 '19 at 07:21

Python3 Two-Way Serial Communication: Reading In Data

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