125000 bench: RX overrun confirmed (NAK=1082, lamps dead) — CPU is the wall

Board->PC near-perfect at 125k (framing 5) while PC->board shredded by
packet length (2-byte polls 60% delivered, 4-byte lamp cmds ~2/3 NAKed
as checksum-corrupt): the 2MHz HC11 RX ISR can't drain back-to-back
bytes at 80us. MAX232 acquitted; zero wedges even at 40% inbound loss.
62500 image built as the sweet-spot candidate (RIOv4_2_patched_62500.bin).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

rio-firmware/RIOv4_2-ANALYSIS.md

@@ -304,3 +304,17 @@ just survivable. To actually harvest the bandwidth, the next dial is
 host-side: drop the 55ms analog poll interval (the board now answers in
 ~4.6ms of wire time). Cabinet adoption still gated on game-side baud
 patches; 9600 patched chip remains the production standard meanwhile.
+
+### 125000 bench results (2026-07-18) — CPU wall found; RX overrun confirmed
+
+`testlogs/riomash-patched-125000-20260718-125310.log` (103s): zero wedges
+(all gaps self-recovered <=0.52s even here), but the board's RX drowns:
+NAK=1082 (~2/3 of the ~1650 lamp commands arrived checksum-corrupt —
+overrun-dropped bytes; lamps visibly dead), analog delivery 60.6% (2-byte
+requests survive better than 4-byte packets), both status exchanges
+failed. Direction asymmetry nails the cause: board->PC was near-perfect
+(framing 5), so the MAX232 leg is fine at 125k — the 2MHz HC11 RX ISR
+(~160 E-cycles/byte, 1-byte buffer) is the wall, exactly as the cycle
+estimate predicted. Verdict: 125000 NOT viable. Sweet-spot candidate:
+`RIOv4_2_patched_62500.bin` (320 cycles/byte, MAX232 in-spec, ACK-wait
+80 ticks = 12.8ms), sha256 below, awaiting bench.

rio-firmware/testlogs/riomash-patched-125000-20260718-125310.log

@@ -0,0 +1,29 @@
+== RIO mash test :: COM1 @ 125000 8N1, 300s, chip label 'patched-125000' ==
+   lamp echo ON (drives reply/lamp collisions), wedge threshold 2.0s, app auto-recovery DISABLED
+   log: C:\VWE\riojoy\riomash-patched-125000-20260718-125310.log
+   firmware: 4.2
+   counters before: (no status reply — reply path dead?)
+
+>>> MASH NOW: two hands, 8 lamp buttons, as fast as you can. <<<
+>>> Test runs 300s. A wedge alarm will beep + banner.   <<<
+>>> Ctrl+C ends the run early and still prints the summary.   <<<
+
+[  33.05s] progress: presses=147 (147/min) analog=344 maxGap=0.31s wedges=0
+[  63.12s] progress: presses=472 (449/min) analog=668 maxGap=0.37s wedges=0
+[  93.14s] progress: presses=741 (477/min) analog=999 maxGap=0.44s wedges=0
+[ 103.09s] operator stop (Ctrl+C) — ending run, snapshotting counters...
+
+== MASH SUMMARY [patched-125000] ==
+run              : 103s on COM1, lamps on, wedge threshold 2.0s
+firmware         : 4.2
+presses/releases : 827 / 824  (470 presses/min)
+analog replies   : 1134 (~1872 poll slots; 60.6%), sentinels 0
+framing / NAK    : 5 / 1082
+gap histogram    : <100ms:804  100-250ms:298  250-500ms:30  0.5-1s:1  1-2s:0  2-5s:0  >5s:0
+longest gaps     : 0.52s@95.7s, 0.44s@63.3s, 0.44s@86.0s, 0.38s@79.8s, 0.37s@56.4s, 0.37s@73.4s, 0.37s@81.5s, 0.34s@95.3s, 0.31s@40.8s, 0.31s@58.4s
+wedge events     : 0
+counters before  : (no status reply — reply path dead?)
+counters after   : (no status reply — reply path dead?)
+counter delta    : (incomplete — a snapshot got no reply)
+verdict          : PASS — no wedge; compare gap histogram + counter delta to baseline
+log file         : C:\VWE\riojoy\riomash-patched-125000-20260718-125310.log