diff --git a/debug/README.md b/debug/README.md
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+# Debug code: TimePix TDC loss investigation
+
+**Not production code.** Temporary tools for finding why the second TimePix board loses its TDC
+during BEC scans (October 2026). Nothing in `superxas_bec` imports these files; delete this
+folder once the issue is resolved.
+
+## `serval_bec_sequence.py`
+
+Replays, directly against Serval, the HTTP sequence that BEC + the ADASItpx 2.2 IOC send for a
+`timepix_scan`, and prints Serval's TDC1/TDC2 rates per point. Needs only Python 3 on the Serval
+machine (p6-0008); BEC, the IOC and Accos must not use the detector while it runs. Restart
+Serval before each run.
+
+```bash
+python3 serval_bec_sequence.py --dry-run                                # print requests only
+python3 serval_bec_sequence.py --points 40 --no-tdc2                    # BEC sequence, TDC1 only
+python3 serval_bec_sequence.py --points 40 --no-tdc2 --configure-once   # config once, then start only
+python3 serval_bec_sequence.py --points 40 --no-tdc2 --software-trigger # one measurement, soft triggers
+```
+
+`python3 serval_bec_sequence.py --help` lists all switches (`--chain-mode`, `--one-measurement`,
+`--raw-base` for recording raw data, ...).
diff --git a/debug/serval_bec_sequence.py b/debug/serval_bec_sequence.py
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+#!/usr/bin/env python3
+"""Replay the Serval HTTP sequence that BEC + the ADASItpx 2.2 IOC send for a timepix_scan.
+
+No BEC, IOC or tpx3app needed: only Python 3 (stdlib) and a running Serval.
+
+Per scan point the IOC (asiTpx.cpp startMeasurement, release 2.2) does:
+  1. GET /detector/config, overwrite its own fields, PUT the whole config back
+     (Tdc = ["P0123", "P0123"] because BEC enables TDC1 and TDC2, rising edge, all channels)
+  2. GET /server/destination, set Raw[0] (and Image), PUT it back
+  3. GET /measurement/start  (nTriggers = 1, so Serval ends the measurement after one frame)
+BEC waits for the frame and starts the next point the same way.
+
+The script prints Serval's Tdc1/Tdc2 event rates per point. On this detector
+(2 boards x 4 chips) TDC1 at ~130 kHz reads ~1.04 MHz when all 8 chips see it and
+~520 kHz when one board loses it.
+
+Switches to find which part of the sequence triggers the loss:
+  --tdc2 / --no-tdc2       TDC2 "P0123" (BEC) or "" (Accos)
+  --merge / --no-merge     re-send Serval's full config (IOC 2.2) or only the IOC fields (IOC <= 2.1)
+  --chain-mode NONE        what the IOC sent before release 2.2 (BEC scans worked then)
+  --one-measurement        one measurement with nTriggers = points (Accos style) instead of one per point
+  --configure-once         send config + destination only before the first point, then only
+                           GET /measurement/start per point (no config back and forth)
+  --software-trigger       one measurement (SOFTWARESTART_TIMERSTOP, nTriggers = points), then
+                           GET /measurement/trigger/start per point (IOC: TriggerMode Software +
+                           cam1:TriggerSoftware); config is sent once
+  --raw-base               raw output, e.g. file:///data/SavedStream/ or tcp://connect@127.0.0.1:8451
+                           (a tcp target needs a listener; omit for no raw output)
+
+Examples:
+  python3 serval_bec_sequence.py --dry-run
+  python3 serval_bec_sequence.py --points 40 --raw-base file:///data/SavedStream/ --raw-pattern bec_seq_%Hms_
+  python3 serval_bec_sequence.py --points 40 --no-tdc2
+  python3 serval_bec_sequence.py --points 40 --one-measurement
+  python3 serval_bec_sequence.py --points 40 --configure-once
+  python3 serval_bec_sequence.py --points 40 --software-trigger
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+import sys
+import time
+import urllib.error
+import urllib.request
+
+
+class Serval:
+    def __init__(self, url: str, dry_run: bool):
+        self.url = url.rstrip("/")
+        self.dry_run = dry_run
+
+    def _request(self, method: str, path: str, body: dict | None = None):
+        data = None if body is None else json.dumps(body).encode()
+        stamp = time.strftime("%H:%M:%S")
+        print(f"{stamp} {method} {path}" + (f" {json.dumps(body)}" if body is not None else ""))
+        if self.dry_run:
+            return None
+        req = urllib.request.Request(self.url + path, data=data, method=method)
+        if data is not None:
+            req.add_header("Content-Type", "application/json")
+        try:
+            with urllib.request.urlopen(req, timeout=30) as resp:
+                text = resp.read().decode()
+        except urllib.error.HTTPError as exc:
+            sys.exit(f"{method} {path} failed: {exc.code} {exc.read().decode()}")
+        try:
+            return json.loads(text) if text else None
+        except json.JSONDecodeError:
+            return text
+
+    def get(self, path: str):
+        return self._request("GET", path)
+
+    def put(self, path: str, body: dict):
+        return self._request("PUT", path, body)
+
+    def dashboard(self) -> dict:
+        if self.dry_run:
+            return {"Measurement": {"Status": "DA_IDLE"}}
+        req = urllib.request.Request(self.url + "/dashboard")
+        with urllib.request.urlopen(req, timeout=10) as resp:
+            return json.loads(resp.read().decode())
+
+
+def configure_detector(
+    serval: Serval, args, n_triggers: int, trigger_mode: str = "AUTOTRIGSTART_TIMERSTOP"
+) -> None:
+    config = (serval.get("/detector/config") or {}) if args.merge else {}
+    config["ExposureTime"] = args.exposure
+    config["BiasEnabled"] = 1  # the IOC sends an int, not a JSON boolean
+    config["BiasVoltage"] = args.bias
+    config["ChainMode"] = args.chain_mode
+    config["TriggerPeriod"] = args.period
+    config["nTriggers"] = n_triggers
+    config["TriggerDelay"] = 0.0
+    config["TriggerIn"] = 0
+    config["TriggerOut"] = 0
+    config["TriggerMode"] = trigger_mode
+    config["Tdc"] = ["P0123", "P0123" if args.tdc2 else ""]
+    serval.put("/detector/config", config)
+
+
+def configure_destination(serval: Serval, args) -> None:
+    destination = (serval.get("/server/destination") or {}) if args.merge else {}
+    if args.raw_base:
+        raw = (destination.get("Raw") or [{}])[0]
+        raw["Base"] = args.raw_base
+        raw["FilePattern"] = args.raw_pattern
+        if args.raw_base.startswith("tcp://"):
+            raw.pop("SplitStrategy", None)
+        destination["Raw"] = [raw]
+    else:
+        destination["Raw"] = None
+    destination["Image"] = []
+    destination["Preview"] = None
+    serval.put("/server/destination", destination)
+
+
+def wait_for_measurement(serval: Serval, timeout: float) -> tuple[float, float, int]:
+    """Poll /dashboard until DA_IDLE; return max Tdc1/Tdc2 rates and dropped frames."""
+    tdc1 = tdc2 = 0.0
+    dropped = 0
+    deadline = time.monotonic() + timeout
+    time.sleep(0.5)
+    while time.monotonic() < deadline:
+        meas = serval.dashboard().get("Measurement") or {}
+        tdc1 = max(tdc1, meas.get("Tdc1EventRate") or 0)
+        tdc2 = max(tdc2, meas.get("Tdc2EventRate") or 0)
+        dropped = meas.get("DroppedFrames") or 0
+        if meas.get("Status") == "DA_IDLE":
+            break
+        time.sleep(0.5)
+    else:
+        print("  measurement did not finish in time")
+    return tdc1, tdc2, dropped
+
+
+def report(tdc1: float, tdc2: float, dropped: int) -> None:
+    full = 1.0e6  # rough "all 8 chips" TDC1 rate at the beamline
+    flag = "  <-- below ~1 MHz, a board may have lost the TDC" if 0 < tdc1 < 0.75 * full else ""
+    print(f"  Tdc1EventRate {tdc1:.0f}  Tdc2EventRate {tdc2:.0f}  DroppedFrames {dropped}{flag}")
+
+
+def wait_for_frame(serval: Serval, frames: int, timeout: float) -> tuple[float, float, int]:
+    """Poll /dashboard until FrameCount reaches frames; return max Tdc1/Tdc2 rates and drops."""
+    tdc1 = tdc2 = 0.0
+    dropped = 0
+    deadline = time.monotonic() + timeout
+    time.sleep(0.5)
+    while time.monotonic() < deadline:
+        meas = serval.dashboard().get("Measurement") or {}
+        tdc1 = max(tdc1, meas.get("Tdc1EventRate") or 0)
+        tdc2 = max(tdc2, meas.get("Tdc2EventRate") or 0)
+        dropped = meas.get("DroppedFrames") or 0
+        if serval.dry_run or (meas.get("FrameCount") or 0) >= frames:
+            break
+        time.sleep(0.5)
+    else:
+        print("  frame did not arrive in time")
+    return tdc1, tdc2, dropped
+
+
+def run_software_trigger(serval: Serval, args) -> None:
+    """One measurement, config sent once, one software trigger per point."""
+    configure_detector(serval, args, args.points, trigger_mode="SOFTWARESTART_TIMERSTOP")
+    configure_destination(serval, args)
+    serval.get("/measurement/start")
+    for point in range(1, args.points + 1):
+        print(f"--- point {point}/{args.points}")
+        serval.get("/measurement/trigger/start")
+        report(*wait_for_frame(serval, point, args.period + 30))
+        if point < args.points:
+            time.sleep(args.pause)
+    serval.get("/measurement/stop")
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
+    parser.add_argument("--url", default="http://localhost:8080")
+    parser.add_argument("--points", type=int, default=40)
+    parser.add_argument("--exposure", type=float, default=9.9979, help="s, BEC uses exp_time - 2.1 ms")
+    parser.add_argument("--period", type=float, default=10.0, help="s")
+    parser.add_argument("--bias", type=float, default=100.0)
+    parser.add_argument("--chain-mode", default="LEADER", choices=["LEADER", "NONE"],
+                        help="LEADER = IOC 2.2 and Accos; NONE = what the IOC sent before 2.2")
+    parser.add_argument("--tdc2", action=argparse.BooleanOptionalAction, default=True)
+    parser.add_argument("--merge", action=argparse.BooleanOptionalAction, default=True)
+    mode = parser.add_mutually_exclusive_group()
+    mode.add_argument("--one-measurement", action="store_true")
+    mode.add_argument("--configure-once", action="store_true")
+    mode.add_argument("--software-trigger", action="store_true")
+    parser.add_argument("--raw-base", default="")
+    parser.add_argument("--raw-pattern", default="")
+    parser.add_argument("--pause", type=float, default=1.0, help="s between points (BEC: ~1 s)")
+    parser.add_argument("--dry-run", action="store_true", help="print the requests only")
+    args = parser.parse_args()
+
+    serval = Serval(args.url, args.dry_run)
+    if args.software_trigger:
+        run_software_trigger(serval, args)
+        return
+    runs = 1 if args.one_measurement else args.points
+    n_triggers = args.points if args.one_measurement else 1
+    timeout = n_triggers * args.period + 30
+
+    for run in range(1, runs + 1):
+        print(f"--- measurement {run}/{runs}")
+        if run == 1 or not args.configure_once:
+            configure_detector(serval, args, n_triggers)
+            configure_destination(serval, args)
+        serval.get("/measurement/start")
+        report(*wait_for_measurement(serval, timeout))
+        if run < runs:
+            time.sleep(args.pause)
+
+
+if __name__ == "__main__":
+    main()