31250 bench run: wedge patch holds under retry storm; window is byte-scaled

127s mash at 31250: zero wedges — but 5655 framing resyncs, duplicate
replies (215% of poll slots) and AbandonCount=65 reveal the board's
reply ACK-wait scales with byte time: ~5+ms at 9600 (USB beats it,
zero framing) vs ~1.6ms at 31250 (USB cannot). All 65 retry
exhaustions self-recovered through the patched give-up path — the
strongest field proof of the latch fix yet; unpatched firmware would
have wedged on the first. Verdict recorded in ANALYSIS.md: cabinets
run the 9600 patched chip; 31250 stays a bench branch until the retry
window is widened in firmware.

Tool: before-snapshot now retries once (can lose the post-DTR boot
race, as this run showed); evidence log archived.

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

rio-firmware/RIOv4_2-ANALYSIS.md

@@ -240,3 +240,27 @@ heavier mash is decisive. Remaining (cabinet): overnight idle soak +
 a native-game session. Both check snapshots also reported a board/lamp
 fault item — persistent across chips, likely a real tired lamp; inspect
 separately.
+
+## 31250-baud bench run (2026-07-18) — patch holds; retry window is byte-scaled
+
+`testlogs/riomash-patched-31250-20260718-003114.log` (127s mash, FTDI
+COM1 @ 31250, latency timer already 1ms): **zero wedges**, but
+framing=5655, analog replies at 215% of poll slots (duplicates), and
+AbandonCount=65 — a reply-retry storm. Reading: the board's reply
+ACK-wait window scales with BYTE TIME, not wall clock. At 9600 the
+window (~5+ms) hides USB turnaround entirely (zero framing in both
+9600 runs); at 31250 it shrinks to ~1.6ms, which USB cannot reliably
+beat, so most replies retransmit and 65 exhausted all 4 retries.
+
+Two conclusions:
+1. **Brutal confirmation of the wedge patch**: 65 trips through the
+   give-up path in 127s, every one self-recovered ($DFF0 stub). The
+   unpatched firmware would have latched dead on the first.
+2. **31250 needs one more firmware tweak to be clean**: widen the
+   reply-retry wait (the pacing/limit around the $D90E/$D9BE retry
+   machine) so the window stays >=10ms of wall clock at 31250. Until
+   then: cabinets run the 9600 patched chip (validated clean); 31250
+   stays a bench branch. Note the duplicate replies also mean the raw
+   reply rate overstates throughput — unique samples are still capped
+   by the host's 55ms poll; harvesting the speed needs a faster poll
+   AFTER the retry window is fixed.

tools/RioSerialMonitor/MashTest.cs

@@ -346,23 +346,30 @@ internal static class MashTest
         // --- helpers ---------------------------------------------------------
         async Task<Dictionary<RioStatusType, int>> SnapshotStatusAsync()
         {
-            lock (gate) { statusItems.Clear(); statusCollecting = true; }
-            try
+            // Two attempts: the first request can lose a race with the board's
+            // post-DTR boot (port open pulses DTR = board reset), or get eaten
+            // by a reply collision — seen on the 31250 bench run.
+            for (int attempt = 0; ; attempt++)
             {
-                await link.RequestVersionAsync();
-                await link.RequestCheckAsync();
-            }
-            catch { /* port trouble surfaces as an empty snapshot */ }
-            await Task.Delay(1200);
+                lock (gate) { statusItems.Clear(); statusCollecting = true; }
+                try
+                {
+                    await link.RequestVersionAsync();
+                    await link.RequestCheckAsync();
+                }
+                catch { /* port trouble surfaces as an empty snapshot */ }
+                await Task.Delay(1200);
 
-            lock (gate)
-            {
-                statusCollecting = false;
-                // Counters arrive one item per type; keep the last value per type.
-                var map = new Dictionary<RioStatusType, int>();
-                foreach (CheckStatus item in statusItems)
-                    map[item.Type] = item.Number;
-                return map;
+                lock (gate)
+                {
+                    statusCollecting = false;
+                    // Counters arrive one item per type; keep the last value per type.
+                    var map = new Dictionary<RioStatusType, int>();
+                    foreach (CheckStatus item in statusItems)
+                        map[item.Type] = item.Number;
+                    if (map.Count > 0 || attempt == 1)
+                        return map;
+                }
             }
         }