Hey there! Let's work through this problem step by step. Since your USB-to-serial adapter works perfectly with the ASCII printer (you can send data via serial.write() and it prints), we know the adapter and basic serial setup are functional. The issue is almost certainly related to matching your lab device's specific requirements or adjusting your reading logic. Here's what to check:

1. Match Serial Parameters Exactly to Your Lab Device #

Your code snippet cuts off after serial.Serial('COM5'...—this is a big red flag. Serial communication relies on exact parameter matching between your computer and the device. The printer's settings (baud rate, parity, stop bits, etc.) might not match what your lab device expects.

You need to initialize the serial port with all parameters from your device's manual. For example, a common setup might look like this, but replace the values with your device's specs:

import serial
ser = serial.Serial(
    port='COM5',
    baudrate=9600,  # Critical: must match your device's baud rate
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    bytesize=serial.EIGHTBITS,
    timeout=2  # Add a timeout to avoid infinite blocking
)

Double-check every parameter—even a single mismatch (like using 19200 instead of 9600 baud) will prevent data from being read correctly.

2. Check if the Device Requires a Trigger Command #

Unlike a printer that responds immediately to any data sent, many lab devices don't send data automatically. They need a specific command (like a query string) to initiate data transmission.

Try sending a command to the device before reading. For example, if your device expects a GET_DATA\r\n query:

# Send the trigger command (convert to bytes, add line endings as required)
ser.write(b'GET_DATA\r\n')
# Read the response
response = ser.readline()
print(response.decode('ascii'))  # Decode ASCII data to a string

If you don't know the command, check your device's user manual. You can also test this with a serial debugging tool first to confirm what triggers data output.

3. Verify Flow Control Settings #

Some lab devices use hardware flow control (RTS/CTS) or software flow control (XON/XOFF) to manage data transmission. If your code doesn't enable the correct flow control, the device might hold back data.

Add flow control parameters to your serial initialization if your device requires it:

ser = serial.Serial(
    # Existing parameters...
    rtscts=True,  # Enable hardware flow control if required
    xonxoff=False  # Disable software flow control unless your device uses it
)

Again, refer to your device's manual to confirm the correct flow control setting.

4. Test with a Serial Debugging Tool First #

Before diving deeper into Python code, use a standalone serial tool (like PuTTY, SSCOM, or TeraTerm) to connect to your lab device. Set the correct serial parameters and see if you can read data.

• If the tool can't read data either: The problem is with the device itself (e.g., it's not in data output mode, wiring is loose, or it's powered off).
• If the tool can read data: The issue is in your Python code—go back to check parameters, reading logic, or command triggers.

5. Adjust Your Python Reading Logic #

How you read data matters. Here are common pitfalls to fix:

• readline() requires line endings: If your device doesn't send \n or \r\n with its data, readline() will block until the timeout expires. Try reading fixed bytes or all available data instead:

  ser.flushInput()  # Clear any leftover buffer data
  data = ser.read_all()  # Read all data in the buffer
  print(data.decode('ascii'))
  

• Timeout settings: A timeout of 0 is non-blocking (might return empty before data arrives), while None blocks indefinitely. Set a reasonable timeout (1-5 seconds) to give the device time to respond.

6. Confirm the Correct COM Port #

Even though the printer used COM5, sometimes Windows assigns a different port when you connect the lab device. Open Device Manager > Ports (COM & LPT) to verify the port number for your USB-to-serial adapter when connected to the lab device.

内容的提问来源于stack exchange，提问作者Martin Rune Hansen