diff --git a/SLM-stress-test/nl52_usb_probe.py b/SLM-stress-test/nl52_usb_probe.py
new file mode 100644
index 0000000..c2f789a
--- /dev/null
+++ b/SLM-stress-test/nl52_usb_probe.py
@@ -0,0 +1,165 @@
+#!/usr/bin/env python3
+"""
+RION NL-42 / NL-52 USB serial probe — zero dependencies (stdlib termios only).
+
+The NL-52's USB port enumerates as a virtual COM port ("RION USB to RS232C
+Converter"). On Linux that is almost always handled by an in-kernel USB-serial
+driver (ftdi_sio / cp210x / ch341) which creates /dev/ttyUSB*. This script
+opens that port and sends a few harmless REQUEST commands (no settings are
+changed) to confirm two-way communication.
+
+Protocol (NL-42/NL-52 Serial Interface Manual 55779):
+  Request:  "<Command>?"          + CRLF
+  Setting:  "$<Command>,<param>"  + CRLF      (NOT used here — read-only probe)
+  Reply:    result code "R+0000"  + CRLF, then data line(s) for requests.
+
+Before running:
+  1. On the meter: MENU -> I/O -> Communication Interface -> "USB"
+     (set this BEFORE plugging in the cable).
+  2. Connect a generic USB-A -> mini-B cable directly (no hub).
+  3. Find the port:  ls -l /dev/ttyUSB*   (and `dmesg | tail` after plugging in)
+
+Usage:
+  python3 nl52_usb_probe.py                      # defaults to /dev/ttyUSB0
+  python3 nl52_usb_probe.py --port /dev/ttyUSB0 --baud 115200
+  python3 nl52_usb_probe.py --cmd "System Version?" --cmd "Clock?"
+"""
+
+import argparse
+import os
+import select
+import sys
+import termios
+import time
+
+# Safe, read-only probe commands (all are pure requests).
+DEFAULT_COMMANDS = [
+    "System Version?",   # firmware version — proves the link end-to-end
+    "Clock?",            # current date/time
+    "SD Card Free Size?",
+    "DOD?",              # snapshot of currently displayed values
+]
+
+BAUD_CONSTANTS = {
+    9600: termios.B9600,
+    19200: termios.B19200,
+    38400: termios.B38400,
+    57600: termios.B57600,
+    115200: termios.B115200,
+}
+
+
+def open_serial(port: str, baud: int) -> int:
+    """Open a serial port in raw 8N1, no flow control. Returns an fd."""
+    if baud not in BAUD_CONSTANTS:
+        raise ValueError(f"Unsupported baud {baud}; choose from {sorted(BAUD_CONSTANTS)}")
+
+    fd = os.open(port, os.O_RDWR | os.O_NOCTTY | os.O_NONBLOCK)
+
+    attrs = termios.tcgetattr(fd)
+    iflag, oflag, cflag, lflag, ispeed, ospeed, cc = attrs
+
+    # Raw mode
+    iflag = 0
+    oflag = 0
+    lflag = 0
+    # 8 data bits, enable receiver, ignore modem control lines
+    cflag = termios.CS8 | termios.CREAD | termios.CLOCAL
+    # (no PARENB = no parity, no CSTOPB = 1 stop bit, no CRTSCTS = no flow control)
+
+    bconst = BAUD_CONSTANTS[baud]
+    ispeed = bconst
+    ospeed = bconst
+
+    termios.tcsetattr(fd, termios.TCSANOW, [iflag, oflag, cflag, lflag, ispeed, ospeed, cc])
+    termios.tcflush(fd, termios.TCIOFLUSH)
+    return fd
+
+
+def send(fd: int, line: str):
+    os.write(fd, (line + "\r\n").encode("ascii"))
+
+
+def read_reply(fd: int, timeout: float = 3.0) -> bytes:
+    """Read whatever arrives within `timeout` seconds (idle-gap terminated)."""
+    buf = bytearray()
+    deadline = time.time() + timeout
+    while time.time() < deadline:
+        r, _, _ = select.select([fd], [], [], 0.3)
+        if r:
+            try:
+                chunk = os.read(fd, 4096)
+            except BlockingIOError:
+                continue
+            if chunk:
+                buf.extend(chunk)
+                # Once we've seen a CRLF and there's a brief idle, stop early
+                deadline = min(deadline, time.time() + 0.4)
+    return bytes(buf)
+
+
+def main():
+    ap = argparse.ArgumentParser(description="RION NL-42/NL-52 USB serial probe")
+    ap.add_argument("--port", default="/dev/ttyUSB0")
+    ap.add_argument("--baud", type=int, default=115200,
+                    help="USB CDC usually ignores baud, but RS-232C needs a match")
+    ap.add_argument("--cmd", action="append", dest="cmds",
+                    help="Override probe command(s); repeatable")
+    ap.add_argument("--timeout", type=float, default=3.0)
+    args = ap.parse_args()
+
+    commands = args.cmds or DEFAULT_COMMANDS
+
+    if not os.path.exists(args.port):
+        print(f"[!] {args.port} does not exist.")
+        print("    Plug in the meter (Comm Interface = USB) and check: ls -l /dev/ttyUSB*")
+        print("    Also check the kernel saw it:  dmesg | tail -20")
+        return 2
+
+    try:
+        fd = open_serial(args.port, args.baud)
+    except PermissionError:
+        print(f"[!] Permission denied on {args.port}.")
+        print("    Add yourself to the 'dialout' group, or run with sudo:")
+        print(f"      sudo usermod -aG dialout $USER   (then log out/in)")
+        return 2
+    except Exception as e:
+        print(f"[!] Could not open {args.port}: {e}")
+        return 2
+
+    print(f"[*] Opened {args.port} @ {args.baud} 8N1 (raw, no flow control)")
+    print(f"[*] Sending {len(commands)} read-only request command(s)\n")
+
+    ok = 0
+    try:
+        for cmd in commands:
+            send(fd, cmd)
+            reply = read_reply(fd, args.timeout)
+            decoded = reply.decode("ascii", errors="replace").replace("\r", "\\r").replace("\n", "\\n\n")
+            if reply:
+                ok += 1
+                print(f"  > {cmd}")
+                for line in decoded.splitlines():
+                    print(f"      {line}")
+            else:
+                print(f"  > {cmd}")
+                print(f"      (no response within {args.timeout}s)")
+            print()
+            time.sleep(1.0)  # NL-series likes >=1s between commands
+    finally:
+        os.close(fd)
+
+    if ok == 0:
+        print("[!] No responses. Things to check:")
+        print("    - Meter's Communication Interface is set to USB (not RS-232C)")
+        print("    - ECO / Sleep mode is OFF (both disable the comm interface)")
+        print("    - Right port (try other /dev/ttyUSB* or /dev/ttyACM*)")
+        print("    - For RS-232C path, baud must match the meter's setting")
+        return 1
+
+    print(f"[OK] {ok}/{len(commands)} commands answered — two-way comms confirmed.")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())