如何检测RS232串口连接的打印机是否断开连接？

Your existing approaches have critical limitations:

• The serial.read() method blocks indefinitely if the printer sends no data, and Windows often keeps COM ports registered even after physical disconnection.
• WMI's Win32_SerialPort only lists ports recognized by the OS, not whether a printer is actively connected and responsive on that port.

Here are three reliable methods to detect printer disconnections:

Method 1: Monitor Serial Control Lines (DTR/CTS/RTS) #

Most RS232 printers maintain active control signals (like DTR or CTS) when connected. Poll these signals periodically to check connection status.

import serial
import time

def check_printer_connection(port='COM1'):
    try:
        ser = serial.Serial(port, timeout=1)
        # Check DTR line (adjust based on your printer's specs)
        is_connected = ser.dtr
        ser.close()
        return is_connected
    except serial.SerialException:
        # Port not recognized (likely disconnected)
        return False

# Poll every 5 seconds
while True:
    if not check_printer_connection('COM1'):
        print("Printer disconnected!")
    else:
        print("Printer connected")
    time.sleep(5)

Note: Verify which control line your printer uses (DTR, CTS, RTS) via its manual.

Method 2: Send a Test Command and Validate Response #

Send a known command to the printer (e.g., status query) and check for a valid response. This ensures the printer is not just connected but functional.

import serial
import time

def test_printer_response(port='COM1', baudrate=9600):
    try:
        ser = serial.Serial(port, baudrate=baudrate, timeout=2)
        # Example: Epson ESC/POS status query command
        ser.write(b'\x10\x04\x01')
        response = ser.read(10)
        ser.close()
        # Check for non-empty response (adjust based on your printer's expected output)
        return len(response) > 0
    except (serial.SerialException, TimeoutError):
        return False

while True:
    if not test_printer_response('COM1'):
        print("Printer disconnected or unresponsive!")
    time.sleep(5)

Look up your printer's specific command set (e.g., ESC/POS for Epson, StarPRNT for Star Micronics) for accurate test commands.

Method 3: Listen for Windows Device Removal Events #

Use pywin32 to monitor system events triggered when an RS232 device is physically disconnected.

import win32gui
import win32con
import win32api
import win32com.client
import time

class DeviceMonitor:
    def __init__(self):
        # Create a hidden window to receive device events
        self.hwnd = win32gui.CreateWindowEx(0, "STATIC", "", 0, 0, 0, 0, 0, 0, 0, 0)
        win32api.SetWindowLong(self.hwnd, win32con.GWL_WNDPROC, self.window_proc)
        # Set up WMI watcher for serial port removal
        self.wmi = win32com.client.Dispatch("WbemScripting.SWbemLocator").ConnectServer()
        self.watcher = self.wmi.ExecNotificationQueryAsync(
            "SELECT * FROM __InstanceDeletionEvent WITHIN 1 WHERE TargetInstance ISA 'Win32_SerialPort'"
        )
        self.watcher.OnObjectReady = self.on_device_removed

    def window_proc(self, hwnd, msg, wparam, lparam):
        if msg == win32con.WM_DEVICECHANGE and wparam == win32con.DBT_DEVICEREMOVECOMPLETE:
            print("Serial device disconnected!")
        return win32gui.DefWindowProc(hwnd, msg, wparam, lparam)

    def on_device_removed(self, obj, ctx):
        port = obj.TargetInstance.DeviceID
        print(f"Printer on {port} disconnected!")

    def run(self):
        while True:
            win32gui.PumpWaitingMessages()
            time.sleep(0.1)

if __name__ == "__main__":
    monitor = DeviceMonitor()
    monitor.run()

This method detects physical disconnections in real time without polling.

Key Tips: #

• Control line monitoring is lightweight but depends on your printer's signal behavior.
• Test commands are the most reliable way to confirm the printer is functional, not just connected.
• Device event monitoring works best for sudden physical disconnections.

内容的提问来源于stack exchange，提问作者José Timaure