diff --git a/tools/pbhw.py b/tools/pbhw.py
index 2976130..02bdeeb 100755
--- a/tools/pbhw.py
+++ b/tools/pbhw.py
@@ -132,18 +132,28 @@ class Suite:
         got = erased.read_bytes() if erased.exists() else b""
         self.check("EEPROM erase leaves 0xff", got == b"\xff" * size, f"{len(got)} B")
 
-    def application(self, info, app: pathlib.Path, marker: str) -> None:
+    def application(self, info, app: pathlib.Path, marker: str,
+                    marker_wait: float = 2.5) -> None:
         rc, out = self.rig.pureboot("--flash", str(app), "--verify-flash", str(app))
         self.check(f"application flash + verify ({app.name})", rc == 0, self._brief(out))
 
         if marker:
             # The tool hands over as it ends its session, so the application is
-            # already running; opening the port does not reset a board whose DTR
-            # is unwired, so this simply listens.
-            data = self.rig.capture(seconds=2.5)
+            # already running — but only on a board whose DTR is unwired, where
+            # opening a port simply listens. Where DTR *is* wired to reset (an
+            # Arduino, most USB-serial dev boards), this open resets the part
+            # and the activation window comes first, so a marker emitted once at
+            # startup happens on the far side of a wait this cannot know the
+            # length of: the window is a compile-time constant and nothing on
+            # the wire reports it. Hence --marker-wait, and a fixture that
+            # repeats its banner (PUREBOOT_HEARTBEAT) rather than saying it once.
+            data = self.rig.capture(seconds=marker_wait)
             seen = marker.encode() in data
             sample = "".join(chr(b) if 32 <= b < 127 else "." for b in data[:40])
-            self.check(f"application runs (emits {marker!r})", seen, f"|{sample}|")
+            self.check(f"application runs (emits {marker!r})", seen,
+                       f"|{sample}|" if seen or data else
+                       f"nothing in {marker_wait:g} s — if this board resets when its port "
+                       f"opens, that wait has to outlast the activation window")
 
         back = self.work / "app-back.bin"
         rc, out = self.rig.pureboot("--read-flash", str(back))
@@ -190,7 +200,7 @@ class Suite:
     # ------------------------------------------------------------------- run
 
     def run(self, app: pathlib.Path | None, loader_image: pathlib.Path | None,
-            marker: str) -> int:
+            marker: str, marker_wait: float = 2.5) -> int:
         print("identity")
         info = self.identity()
         if info is None:
@@ -206,7 +216,7 @@ class Suite:
 
         if app:
             print("\napplication")
-            self.application(info, app, marker)
+            self.application(info, app, marker, marker_wait)
         else:
             print("\nskip  application checks (pass --app <image.hex>)")
 
@@ -232,6 +242,10 @@ def main(argv: list[str] | None = None) -> int:
                         help="the resident loader's .bin, to prove the slot survives an erase")
     parser.add_argument("--marker", default="",
                         help="text the application emits when it runs, e.g. APP")
+    parser.add_argument("--marker-wait", type=float, default=2.5,
+                        help="seconds to listen for it. On a board whose DTR is wired to "
+                             "reset, opening the port resets the part, so this must outlast "
+                             "the activation window (default 2.5)")
     args = parser.parse_args(argv)
 
     rig = pbrig.Rig(pbrig.Deployment.from_args(args))
@@ -239,7 +253,8 @@ def main(argv: list[str] | None = None) -> int:
           f"{' (autobaud)' if args.autobaud else ''}")
     print("this overwrites the application flash and EEPROM\n")
     with tempfile.TemporaryDirectory(prefix="pbhw-") as temporary:
-        return Suite(rig, pathlib.Path(temporary)).run(args.app, args.loader, args.marker)
+        return Suite(rig, pathlib.Path(temporary)).run(args.app, args.loader, args.marker,
+                                                      args.marker_wait)
 
 
 if __name__ == "__main__":