harden(modbus-rtu): factory throws on unknown transport + document RTU FC-shape assumption

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
2026-07-24 15:33:49 -04:00
parent 4a8c9badce
commit fcc7ed698e
3 changed files with 16 additions and 4 deletions
@@ -105,8 +105,9 @@ public static class ModbusRtuFraming
        }
        else
        {
-            // Write-echo response (FC 05/06/15/16) or any other length-less fixed shape:
-            // fixed 4 payload bytes + CRC(2).
+            // Fixed 4-byte echo: correct for the write FCs this driver emits (05/06/0F/10). A future
+            // variable-length response FC outside 01-04 (e.g. FC23) would be mis-sized here and
+            // surface as a desync — revisit the FC-shape table if the driver starts emitting one.
            trailing = 4 + 2;
        }