Develop Reference

Sending serial commands to a Kangaroo motion controller with pyserial

Hi everybody, I need help with following problem please:
I try to drive a Dimension Engineering Kangaroo/Sabertooth motor controller from a Python script using pyserial.
Everything works so far except that the motor output stops after some time and the USB/serial connection becomes 'unresponsive'. The faster the rate at which the serial commands are sent, the quicker this happens.
This is how I initialize the serial connection:
# open serial port
Saber = serial.Serial("COM1", 19200, timeout=1, write_timeout=0)
# starting the motor
Saber.write("1,start\r".encode("gbk"))
Then the position commands are sent using a loop:
while True:
pos = 2000 # just an example, the script calculates a new required position every iteration
command = "1,p" + str(pos) + "s500\r"
Saber.write(command.encode("gbk"))
time.sleep(.1) # the lower this break, the faster the device becomes unresponsive...?
After the device stops, I have to disconnect it / power it down, to be able to open a serial connection again.
Could it be that some sort of buffer gets filled and 'locks' the port?
I have tried to free the read/write buffers using:
Saber.reset_input_buffer()
Saber.reset_output_buffer()
But that does not solve the problem :/ Any help is greatly apprechiated! Thanks!

How to check simultaneously for serial input and input from keyboard (simultaneous use of readchar and serial library)

I am trying to code the following using python3 in a raspberry pi:
1) wait for a 14 digits bar code (barcode scanner connected through usb port and input received as keyboard data)
2) after a barcode is read, wait for serial communication (device connected to usb port and sends serial commands. could be one, or more....) the idea is that all commands received are going to be associated with the scanned barcode
3) the process of waiting for serial commands has to stop when a new barcode is read. THIS IS THE PART I HAVE NOT FIGURED OUT HOW TO DO IT
After some research, I decided to use the "readchar" library for the barcode scanner and the "serial" library for the serial communication received. Both of them work by themselves but the problem is when I try to detect both things at the same time.
In the following code, I managed to read a barcode and then wait for 5 lines of serial communication to finally repeat the process and read a barcode again. The program works as it is right now BUT the problem is that I don't know how many lines of serial communication I will receive so I need to somehow detect a new barcode while also waiting to receive the serial communication.
import readchar
import time
import serial
ser = serial.Serial(
port='/dev/ttyUSB0',
baudrate = 115200,
parity=serial.PARITY_NONE,
stopbits=serial.STOPBITS_ONE,
bytesize=serial.EIGHTBITS,
timeout=1
)
print("Waiting for barcode...")
while 1:
inputStr = ""
while len(inputStr) != 14: #detect only 14 digit barcodes
inputStr += str(readchar.readchar())
inputStr = ''.join(e for e in inputStr if e.isalnum()) #had to add this to strip non alphanumeric characters
currentCode = inputStr
inputStr = ""
print(currentCode)
ser.flushInput()
time.sleep(.1)
# Wait for 5 lines of serial communication
# BUT it should break the while loop when a new barcode is read!
count = 0
while count < 5:
dataRead=ser.readline()
if len(dataRead) > 0:
print(dataRead)
count+=1
print("Waiting for barcode...")
If I add a condition to the while loop that reading the serial communication using (ser.readline()) so that if a character is read from the scanner (readchar.readchar()) then it messes thing up. It is like if readline and reacher can not be in the same while loop.
Doing some research I think I need to use Asynchronous IO, or threads or something like that, but I have no clue. Also I don't know if I could keep using the same libraries (serial and readchar). Please help

I cannot be sure (I don't have your barcode reader and serial port device) but based on what you say I don't think you need threads, you just have to rely on the buffers to keep your data stored until you have time to read them.
Simply change the condition on your second while loop to:
while serial.inWaiting() != 0:
This way you will make sure the RX buffer on your serial port will empty. This approach might or might not work depending on the speed and timing of your devices.
You could also try to add a short delay after the buffer is emptied:
import serial
import time
ser=serial.Serial(port="/dev/ttyUSB0",baudrate=115200, timeout=1.0)
time.sleep(1)
data=b""
timeout = time.time() + 1.0
while ser.inWaiting() or time.time()-timeout < 0.0: #keep reading until the RX buffer is empty and wait for 1 seconds to make sure no more data is coming
if ser.inWaiting() > 0:
data+=ser.read(ser.inWaiting())
timeout = time.time() + 1.0
else:
print("waiting...")
This keeps trying to read from the port for 1 second after the last byte is received, to make sure nothing else is coming. You might want to play with the duration of the delay depending, again, on the speed and timing of your devices.
Again, I don't have your devices, so I'm in no position to judge, but the way you read characters from the barcode/keyboard looks far from optimum. I doubt readchar is the best approach. At the end of the day, your barcode reader is probably a serial port. You might want to dig into that and/or find a more efficient way to read several keyboard strokes in one go.

I found this answer in another question:
How to read keyboard-input?
I have tried it and it works! I´ll also give a try to the method proposed by Marcos G.

pySerial - updates too slow

I'm on Windows, reading data via Serial from another device at 115200 baud rate. The data coming in is from a microcontroller which has a sensor connected to it sending integer sensor (gyroscope) readings ranging from 1 to 25.
I used PuTTY to connect to it and I can read those values perfectly and they update almost instantly when I move my gyroscope.
However, when I use the python code below, it takes almost 20-25 seconds for it to update after I move the gyroscope. Why is it taking so long? How do I fix it? I've tried all sorts of things like changing timeout, adding sleep delays, nothing works.
import serial
ser = serial.Serial(COM1, 115200, timeout=0)
ser.flushInput()
while True:
DATA = ser.readline()
VAL = DATA[0:len(DATA)-1].decode("utf-8")
print(VAL)
EDIT:
import serial
import time
ser = serial.Serial(COM1, 115200, timeout=0)
ser.flushInput()
while True:
DATA = ser.readline()
VAL = DATA[0:len(DATA)-1].decode("utf-8")
print(VAL)
bufClear = ser.read(ser.inWaiting())
time.sleep(0.5)
Now I can get it to update quickly, however it seems to cut out some info. Sometimes. For example if I move my gyroscope to a value that corresponds to 22. The print output would be like
22
22
2
2
2
22

PySerial read() misses beginning of the frame data

I want to write a program that communicates with intelligent meters (electricity, heat, etc.) over Meter-Bus protocol. I have an M-Bus <-> RS232 converter and RS232 <-> USB.
When I test my Python script with heat meter, by sending it a command it responds me with a long but correct frame, then if I query it again soon after I get only partial frame respond. Waiting around 30s helps to get a whole frame correctly. This partial respond is always the same.
Heat meter respond frames
But with electricity meter it's always the same, although the whole, correct frame would be shorter than in case of heat meter, I always get only the same part of it (little over half of bytes and never the beginning). Electricity meter respond frames
There's also command that initializes slave - slave responds with '\xE5', even though I see Rx diode blinking I cant catch this single byte.
What I have tried:
I tried reading certainly more bytes than could be in my buffer like ser.read(500)
Tried using ser.inwaiting but it would return 0 until I used time.sleep(1) prior to it
Loading chunks of bytes from pySerial inWaiting returns incorrect number of bytes
Manipulating xonxoff parameter and timeout.
Everything with no effect.
import serial
import time
def ser_to_mbus():
ser = serial.Serial(
port='COM4',
baudrate=2400,
bytesize=serial.EIGHTBITS,
parity=serial.PARITY_EVEN,
stopbits=serial.STOPBITS_ONE,
timeout=2,
#write_timeout=1,
xonxoff=False,
rtscts=False,
dsrdtr=False,
#inter_byte_timeout=0.01,
)
# Should receive a hex response '\xE5' with command below.
#to_send = b'\x68\x0B\x0B\x68\x73\xFD\x52\x84\x11\x10\x00\xFF\xFF\xFF\xFF\x63\x16'
#to_send = b'\x10\x7B\x01\x7C\x16' # heat meter request frame
to_send = b'\x10\x7B\xFD\x78\x16' # electricity meter request frame
ser.write(to_send)
ser.close()
def mbus_to_ser():
ser = serial.Serial(
port='COM4',
baudrate=2400,
bytesize=serial.EIGHTBITS,
parity=serial.PARITY_EVEN,
stopbits=serial.STOPBITS_ONE,
timeout=1,
#write_timeout=1,
xonxoff=1,
rtscts=False,
dsrdtr=False,
#inter_byte_timeout=0.2,
)
time.sleep(1) # Otherwise ser.in_waiting is empty.
buffer_size = ser.in_waiting
frame = ser.read(buffer_size)
return frame
ser_to_mbus()
frame = mbus_to_ser()
print(frame)
I've been monitoring M-Bus frames with EMU and my serial port with Serial Port Monitor. Frames sent by EMU and received are always correct, the same with Serial Monitor. If I use my code to send and receive frames, Serial Monitor gets same results as I did in my terminal - it sees wrong, 'sliced', frames.
Devices respond only after they receive correct requests (I see an Rx diode blinking).
I don't know if there is a problem with threading, or with a buffer or maybe Python script is not fast enough to catch all bytes from a buffer?

Reading serial data in realtime in Python

I am using a script in Python to collect data from a PIC microcontroller via serial port at 2Mbps.
The PIC works with perfect timing at 2Mbps, also the FTDI usb-serial port works great at 2Mbps (both verified with oscilloscope)
Im sending messages (size of about 15 chars) about 100-150x times a second and the number there increments (to check if i have messages being lost and so on)
On my laptop I have Xubuntu running as virtual machine, I can read the serial port via Putty and via my script (python 2.7 and pySerial)
The problem:
When opening the serial port via Putty I see all messages (the counter in the message increments 1 by 1). Perfect!
When opening the serial port via pySerial I see all messages but instead of receiving 100-150x per second i receive them at about 5 per second (still the message increments 1 by 1) but they are probably stored in some buffer as when I power off the PIC, i can go to the kitchen and come back and im still receiving messages.
Here is the code (I omitted most part of the code, but the loop is the same):
ser = serial.Serial('/dev/ttyUSB0', 2000000, timeout=2, xonxoff=False, rtscts=False, dsrdtr=False) #Tried with and without the last 3 parameters, and also at 1Mbps, same happens.
ser.flushInput()
ser.flushOutput()
While True:
data_raw = ser.readline()
print(data_raw)
Anyone knows why pySerial takes so much time to read from the serial port till the end of the line?
Any help?
I want to have this in real time.
Thank you

You can use inWaiting() to get the amount of bytes available at the input queue.
Then you can use read() to read the bytes, something like that:
While True:
bytesToRead = ser.inWaiting()
ser.read(bytesToRead)
Why not to use readline() at this case from Docs:
Read a line which is terminated with end-of-line (eol) character (\n by default) or until timeout.
You are waiting for the timeout at each reading since it waits for eol. the serial input Q remains the same it just a lot of time to get to the "end" of the buffer, To understand it better: you are writing to the input Q like a race car, and reading like an old car :)

A very good solution to this can be found here:
Here's a class that serves as a wrapper to a pyserial object. It
allows you to read lines without 100% CPU. It does not contain any
timeout logic. If a timeout occurs, self.s.read(i) returns an empty
string and you might want to throw an exception to indicate the
timeout.
It is also supposed to be fast according to the author:
The code below gives me 790 kB/sec while replacing the code with
pyserial's readline method gives me just 170kB/sec.
class ReadLine:
def __init__(self, s):
self.buf = bytearray()
self.s = s
def readline(self):
i = self.buf.find(b"\n")
if i >= 0:
r = self.buf[:i+1]
self.buf = self.buf[i+1:]
return r
while True:
i = max(1, min(2048, self.s.in_waiting))
data = self.s.read(i)
i = data.find(b"\n")
if i >= 0:
r = self.buf + data[:i+1]
self.buf[0:] = data[i+1:]
return r
else:
self.buf.extend(data)
ser = serial.Serial('COM7', 9600)
rl = ReadLine(ser)
while True:
print(rl.readline())

You need to set the timeout to "None" when you open the serial port:
ser = serial.Serial(**bco_port**, timeout=None, baudrate=115000, xonxoff=False, rtscts=False, dsrdtr=False)
This is a blocking command, so you are waiting until you receive data that has newline (\n or \r\n) at the end:
line = ser.readline()
Once you have the data, it will return ASAP.

From the manual:
Possible values for the parameter timeout:
…
x set timeout to x seconds
and
readlines(sizehint=None, eol='\n') Read a list of lines,
until timeout. sizehint is ignored and only present for API
compatibility with built-in File objects.
Note that this function only returns on a timeout.
So your readlines will return at most every 2 seconds. Use read() as Tim suggested.