Python Serial Read Timeout Example
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I've got a python program which is reading data from a serial port via the PySerial module. The two conditions I need to keep in mind are: I don't know how much data will arive, and I don't know when to expect data.
Based on this I have came up with the follow code snipets: #Code from main loop, spawning thread and waiting for data s = serial.Serial(5, timeout=5) # Open COM5, 5 second timeout s.baudrate = 19200 #Code from thread reading serial data while 1: tdata = s.read(500) # Read 500 characters or 5 seconds if(tdata.len 0): #If we got data if(self.flaggotdata is 0): #If it's the first data we recieved, store it self.data = tdata else: #if it's not the first, append the data self.data += tdata self.flaggotdata = 1 So this code will loop forever getting data off the serial port. We'll get up to 500 characters store the data, then alert the main loop by setting a flag. If no data is present we'll just go back to sleep and wait.
The code is working, but I don't like the 5s timeout. I need it because I don't know how much data to expect, but I don't like that it's waking up every 5 seconds even when no data is present. Is there any way to check when data becomes available before doing the read? I'm thinking something like the select command in Linux. EDIT: Just thought I'd note that I found the inWaiting method, but really that seems it just change my 'sleep' to a poll, so that's not what I want here. I just want to sleep until data comes in, then go get it.
Ok, I actually got something together that I like for this. Using a combination of read with no timeout and the inWaiting method: #Modified code from main loop: s = serial.Serial(5) #Modified code from thread reading the serial port while 1: tdata = s.read # Wait forever for anything time.sleep(1) # Sleep (or inWaiting doesn't give the correct value) dataleft = s.inWaiting # Get the number of characters ready to be read tdata += s.read(dataleft) # Do the read and combine it with the first character. #Rest of the code This seems to give the results I wanted, I guess this type of functionality doesn't exist as a single method in Python.
# read up to ten bytes (timeout). Do also have a look at the example files in the examples directory in the. Python-m serial.tools.list_ports will print.
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I'm using PySerial to read from serial port like in the code below. CheckReadUntil read output of the command that I send to serial port until the sequence of symbols readUntil are in the serial output. Self.ser = serial.Serial(comDev, 115200, timeout=10).
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#Function that continue to read from Serial port until 'readUntil' #sequence of symbols appears def CheckReadUntil(self, readUntil): outputCharacters = while 1: ch = self.ser.read outputCharacters += ch if outputCharacters-len(readUntil):readUntil: break outputLines = '.join(outputCharacters) return outputLines However, if there is no sequence readUntil (for any reason), I'm just stuck in the function CheckReadUntil forever. The setting timeout=10 sets up timeout so I'm stuck in a loop that iterates every 10 seconds and does nothing, just waiting. How it is possible to understand that there was a timeout event so I may exit the infinite loop?
Output length may be different.