diff --git a/bridges/mm_client_check.py b/bridges/mm_client_check.py
index 8bd86cb..76d971f 100644
--- a/bridges/mm_client_check.py
+++ b/bridges/mm_client_check.py
@@ -44,7 +44,11 @@ unit, this will connect and then see nothing — that is contention, not a fault
 from __future__ import annotations
 
 import argparse
+import errno
+import os
+import select
 import sys
+import termios
 import time
 from pathlib import Path
 
@@ -52,7 +56,72 @@ sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
 
 from micromate.client import MicromateClient, _content  # noqa: E402
 from micromate.protocol import ProtocolError  # noqa: E402
-from minimateplus.transport import SerialTransport, TcpTransport  # noqa: E402
+from minimateplus.transport import TcpTransport  # noqa: E402
+
+
+class StdlibSerial:
+    """Raw serial on stdlib `termios` — no pyserial.
+
+    `minimateplus.SerialTransport` needs pyserial, and a bench host is whatever
+    is to hand.  On a PEP 668 distro (Mint 22, Ubuntu 24.04, Debian 12) a plain
+    `pip install pyserial` is refused outright, so a diagnostic that depends on
+    it is one you cannot run at the moment you need it.  `bridges/mm_link.py`
+    and `scratch/fake_unit.py` already take this approach; this is the same
+    ~30 lines, and it means the tool runs on a stock Python 3 anywhere.
+
+    Not a general replacement for SerialTransport — no flow control, no
+    parity options, Linux/macOS only.  Enough for a Micromate, which is 8N1
+    with no handshaking.
+    """
+
+    _BAUD = {9600: termios.B9600, 19200: termios.B19200, 38400: termios.B38400,
+             57600: termios.B57600, 115200: termios.B115200}
+
+    def __init__(self, path: str, baud: int = 115200) -> None:
+        if baud not in self._BAUD:
+            raise ValueError(f"unsupported baud {baud}; pick from {sorted(self._BAUD)}")
+        self.path, self.baud, self.fd = path, baud, None
+
+    def connect(self) -> None:
+        if self.fd is not None:
+            return
+        self.fd = os.open(self.path, os.O_RDWR | os.O_NOCTTY | os.O_NONBLOCK)
+        a = termios.tcgetattr(self.fd)
+        a[0] = a[1] = a[3] = 0                               # raw in/out, non-canonical
+        a[2] = termios.CS8 | termios.CREAD | termios.CLOCAL  # 8N1, ignore modem lines
+        a[4] = a[5] = self._BAUD[self.baud]
+        a[6] = list(a[6])
+        a[6][termios.VMIN] = 0
+        a[6][termios.VTIME] = 0
+        termios.tcsetattr(self.fd, termios.TCSANOW, a)
+        termios.tcflush(self.fd, termios.TCIOFLUSH)
+
+    def disconnect(self) -> None:
+        if self.fd is not None:
+            os.close(self.fd)
+            self.fd = None
+
+    def is_connected(self) -> bool:
+        return self.fd is not None
+
+    def read(self, n: int) -> bytes:
+        if self.fd is None:
+            return b""
+        r, _, _ = select.select([self.fd], [], [], 0.05)
+        if not r:
+            return b""
+        try:
+            return os.read(self.fd, n)
+        except OSError as e:
+            if e.errno in (errno.EAGAIN, errno.EWOULDBLOCK):
+                return b""
+            raise
+
+    def write(self, data: bytes) -> None:
+        if self.fd is None:
+            raise OSError("port is not open")
+        while data:
+            data = data[os.write(self.fd, data):]
 
 
 class _Timed:
@@ -107,7 +176,9 @@ def main() -> int:
     )
     ap.add_argument("target", help="host:port for TCP, or a serial device path")
     ap.add_argument("--baud", type=int, default=115200,
-                    help="serial only; a Micromate's modem port runs at 115200")
+                    help="serial only; the USB-A/FTDI path runs at 115200. "
+                         "Ignored by the USB-B 'PC' port, which is CDC-ACM "
+                         "and negotiates its own rate.")
     ap.add_argument("--timeout", type=float, default=10.0)
     ap.add_argument("--download", action="store_true",
                     help="also download the first stored event (read-only)")
@@ -120,7 +191,7 @@ def main() -> int:
         inner = TcpTransport(host, int(port), connect_timeout=a.timeout)
         path = f"TCP {host}:{port}"
     else:
-        inner = SerialTransport(a.target, baud=a.baud)
+        inner = StdlibSerial(a.target, baud=a.baud)
         path = f"serial {a.target} @ {a.baud}"
 
     transport = _Timed(inner)