Help reading data from serial RS485

[deanhystad]

Serial expects data to be bytes and you give it a str. I am surprised this didn't raise an exception. What version of Python are you using? In pre-3.0 Python str and bytes were the same. Now str is made up of unicode characters.

This is some serial code I extracted from a recent project. I did some editing, so this code may not run as is.

import serial

class SerialDevice():
    def __init__(self, port_name='COM1', baudrate=19200, timeout=0.1):
        self.port = serial.Serial()
        self.port.port = port_name
        self.port.baudrate = baudrate
        self.port.timeout = timeout

    def open(self, port_name=None, baudrate=None, timeout=None):
        if port_name is not None:
            self.port.port = port_name
        if baudrate is not None:
            self.port.baudrate = baudrate
        if timeout is not None:
            self.port.timeout = timeout

        self.close()

        self.port.open()
        if not self.port.is_open:
            raise IOError(f'Open failed on port {self.port.port}')

    def close(self):
        if self.port.is_open:
            self.port.close()

    def send_cmd(self, command):
        """Send command and read reply"""
        if not self.port.is_open:
            raise IOError('Port is not open')
        self.port.write(f'{command}\r'.encode())
        reply = self.port.read_until(b'\r')   # My device uses /r as a terminator
        if not reply:
            raise IOError('Read timeout error')
        return reply.decode()

The main things you are missing are encoding the strs to bytes before sending/writing with a corresponding decoding on the other side, and a timeout. Every serial program that uses read (and especially readline) should specify a timeout.