diff --git a/docs/micromate_protocol_reference.md b/docs/micromate_protocol_reference.md
index c3154be..f2c5bb3 100644
--- a/docs/micromate_protocol_reference.md
+++ b/docs/micromate_protocol_reference.md
@@ -2097,8 +2097,39 @@ without a matching string, and the USB enumeration path has not been
 disassembled.
 
 **The test that would remove the inference: try an FTDI cable on the same modem.**
-Either it works or it does not, and nothing is left to argue about.  Until then
-this is well-supported, not proven.
+Either it works or it does not, and nothing is left to argue about.
+
+✅ **Corroborated by purchasing history (2026-09-25).**  TMI has bought **two**
+kinds of USB-to-serial cable:
+
+| cable | chipset | works with a Micromate? |
+|---|---|---|
+| **Sabrent** USB 2.0 to serial | **FTDI** | ✅ driver present |
+| **Benfei** USB to RS-232 | **Prolific PL2303** | ❌ no driver |
+
+So both types are in circulation and indistinguishable by eye.  A unit handed the
+wrong one has no working modem port, and nothing about the cable's appearance
+says so.
+
+⚠ **Identify cables by chipset, not by looks.**  `lsusb` on any Linux box:
+FTDI is VID `0403`, Prolific is `067b`.  Note also that counterfeit FTDI chips
+are common in cheap cables — they carry FTDI's VID but may not behave like one,
+and an embedded host with a single driver is far less forgiving than Linux.
+
+### Does this explain the 2026-09-22 field outage?
+
+⚠ **Not on the timeline as reported.**  A Prolific cable does not fail
+*gradually* — it never enumerates, so the modem port never works at all.  The
+unit was described as working at first and degrading.
+
+There is one story where it fits: if the initial success was over a **different
+path** — the USB **PC** port, or a bench test before deployment — that would work
+regardless of which serial cable was attached.  The modem path would then have
+been broken from the moment it was deployed, and "it worked and then stopped"
+would be a recollection conflating two connection types days after the fact.
+
+Plausible, unverified, and recorded as such.  Identifying that unit's cable would
+settle it.
 
 ### How this was isolated — the method is reusable