Laser latch + safety-chain gating scope-verified; interlock semantics pinned

fire_test.py phases A/B with the scope on the PSU LASER_ON pin, power
byte 0 throughout, HV unpowered, operator-executed unlock: latch LOCKED
severs FIRE entirely (pin flat + kernel laser_enable 0 under 40k
streamed fire bits); latch unlocked with the chain unarmed shows FIRE
live kernel-side while the PSU pin stays flat - the factory board
gates LASER_ON behind OK_2_FIRE exactly like the OpenGlow AND design.
interlock_circuit semantics pinned by the 13-to-7 transition: b0
LASER_ON monitor active low, b1 FIRE active high, b3 latch 1=locked.
The 1-tick FIRE drop measurement is deferred (SoC net inaccessible;
PSU pin needs an armed chain) - documented with both closure paths.

docs/BRINGUP.md

@@ -183,13 +183,32 @@ hardware I/O — host testing).
      after end-of-data** (PWMSAR retains the last value; the end-of-data
      backstop forces FIRE/step lines low, not the power setpoint) — the
      laser-off guarantee rests entirely on FIRE.
-   - **≤1-tick FIRE drop at underrun/end-of-data: OPEN** — needs a
-     scope on the FIRE net (bit-4 toggling with the latch locked: FIRE
-     is observable while LASER_ON stays gated off by the hardware AND).
-     Signal naming (per the OpenGlow LASER SAFING sheet): FIRE = the
-     CPU's per-tick request (kernel attr `laser_enable`); OK_2_FIRE =
-     chain verdict; LASER_ON = FIRE∧OK_2_FIRE to the tube (active low);
-     HV_EN = HV supply enable, driven by the safing hardware only.
+   - **Laser latch + safety-chain gating: scope-verified 2026-08-02**
+     (`scripts/bench/fire_test.py`, probe on the PSU-connector LASER_ON
+     pin; power byte 0 throughout, zero step bytes, HV unpowered,
+     operator at the power switch; phase B latch-unlock executed by the
+     operator). Phase A (latch LOCKED): 40,000 streamed FIRE bits →
+     pin dead flat AND kernel `laser_enable` stayed 0 — the latch
+     severs the FIRE drive entirely. Phase B (latch unlocked, chain
+     unarmed): kernel `laser_enable=1` mid-window, but the PSU pin
+     stayed flat and `laser_on`/`laser_on_sampled` stayed 0 — the
+     factory board gates LASER_ON behind OK_2_FIRE exactly like the
+     OpenGlow AND design (FIRE ∧ OK_2_FIRE, active high at the PSU
+     pin). **Interlock snapshot semantics pinned by experiment**
+     (13→7 during the unlocked FIRE window): b0 = SoC-side LASER_ON
+     monitor, active LOW (1 = not lasing); b1 = FIRE, active high;
+     b3 = latch, 1 = locked/0 = unlocked.
+   - **≤1-tick FIRE drop at underrun/end-of-data: OPEN (deferred).**
+     The SoC FIRE net is not probe-accessible on the bench, and the
+     chain-gated PSU pin cannot show FIRE without arming the full
+     chain (HV powered + pgood + HV_WDOG retriggered). Close it later
+     either by finding a probe point on the FIRE net (safing-logic
+     input) or as the first measurement of the eventual chain-armed
+     laser-milestone session, with the duty-0 discipline from this one.
+     Signal naming (per the OpenGlow LASER SAFING sheet, now confirmed
+     to match the factory board): FIRE = per-tick request (kernel
+     `laser_enable`); OK_2_FIRE = chain verdict; LASER_ON =
+     FIRE∧OK_2_FIRE to the PSU; HV_EN = HV enable, safing-driven only.
    - **Interlock readback semantics cross-check: OPEN** (see
      factory-laser-safety-readbacks notes).
 3. **Homing**: X/Y home switch GPIOs exist in the cnc pin map (unused so

scripts/bench/fire_test.py

@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+"""FIRE-line drop-timing test - runs ON the board. Usage: fire_test.py A|B
+
+Stream: power(0) first byte (duty forced to ZERO before any FIRE bit -
+the run-start reset would otherwise leave it at 100%), then
+1 s pads / 2.000 s FIRE bits / 1 s pads / 2.000 s FIRE bits ending
+exactly at end-of-data (the backstop edge the gate wants timed).
+No step bytes; motor_lock=15; streaming=0 (normal completion).
+
+Phase A: laser latch LOCKED - expects nothing on the LASER_ON pin.
+Phase B: latch UNLOCKED for the run (re-locked in finally). Refuses to
+run if laser_pgood reports the HV supply good. Duty is zero throughout.
+"""
+import fcntl, os, struct, sys, time
+
+TICK_HZ = 10000
+FIRE = b'\x10'
+PAD = b'\x00'
+
+
+def wr(attr, val):
+    with open('/sys/glowforge/' + attr, 'w') as f:
+        f.write(str(val))
+
+
+def rd(attr):
+    with open('/sys/glowforge/' + attr) as f:
+        return f.read().strip()
+
+
+def rd_pos():
+    with open('/sys/glowforge/cnc/position', 'rb') as f:
+        raw = f.read(32)
+    return struct.unpack('<5i', raw[:20])
+
+
+def snap(tag):
+    print('%s: state=%s laser_enable=%s laser_on=%s laser_on_sampled=%s interlock=%s'
+          % (tag, rd('cnc/state'), rd('cnc/laser_enable'), rd('cnc/laser_on'),
+             rd('cnc/laser_on_sampled'), rd('cnc/interlock_circuit')))
+
+
+mode = sys.argv[1].upper() if len(sys.argv) > 1 else 'A'
+
+stream = (
+    bytes([0x80]) +                # power = 0: duty zero before any FIRE bit
+    PAD * TICK_HZ +                # 1 s baseline
+    FIRE * (2 * TICK_HZ) +         # 2.000 s FIRE window (bounded by pads)
+    PAD * TICK_HZ +                # 1 s gap
+    FIRE * (2 * TICK_HZ)           # 2.000 s FIRE window ending AT end-of-data
+)
+
+print('phase %s: stream %d bytes = %.3f s' % (mode, len(stream), len(stream) / TICK_HZ))
+
+if mode == 'B':
+    pgood = rd('cnc/laser_pgood')
+    if pgood != '0':
+        print('ABORT: laser_pgood=%s (HV supply reports good) - refusing phase B' % pgood)
+        sys.exit(1)
+
+snap('pre ')
+wr('cnc/motor_lock', 15)
+wr('cnc/step_freq', TICK_HZ)
+wr('cnc/laser_latch', 1)
+
+fd = os.open('/dev/glowforge', os.O_WRONLY)
+try:
+    fcntl.flock(fd, fcntl.LOCK_EX)
+    os.lseek(fd, 1, os.SEEK_SET)
+    wr('cnc/enable', 1)
+    time.sleep(0.5)
+    os.write(fd, stream)
+    pos_before = rd_pos()
+
+    if mode == 'B':
+        wr('cnc/laser_latch', 0)   # UNLOCK for this run only
+        print('latch UNLOCKED for phase B run')
+
+    wr('cnc/run', 1)
+    t0 = time.time()
+    sampled_mid = False
+    state = ''
+    while time.time() - t0 < 20:
+        state = rd('cnc/state')
+        if not sampled_mid and 1.5 < time.time() - t0 < 3.0:
+            snap('mid (inside FIRE window)')
+            sampled_mid = True
+        if state != 'running':
+            break
+        time.sleep(0.1)
+    print('done: state=%s after %.1f s' % (state, time.time() - t0))
+finally:
+    wr('cnc/laser_latch', 1)       # re-lock unconditionally
+    fcntl.flock(fd, fcntl.LOCK_UN)
+    os.close(fd)
+
+pos_after = rd_pos()
+print('pos before=%s after=%s MOVED=%s' % (pos_before, pos_after, pos_before[:3] != pos_after[:3]))
+snap('post')
+print('underruns=%s faults=%s' % (rd('cnc/underruns'), rd('cnc/faults')))
+wr('cnc/disable', 1)
+print('safe state restored: state=%s latch=LOCKED' % rd('cnc/state'))