Possible bug in check for valid cemi frame

[nanosonde]

While testing the coupler routing I got "invalid frame" messages in the console.

Inspecting this code:

knx/src/knx/cemi_frame.cpp

Line 369 in 52d3866

uint8_t apduLen = _data[1 + _data[1] + NPDU_LPDU_DIFF];

If I remove the "+1" then everything works as expected. Why do we have the "+1" there?

[nanosonde]

_data[0] is the cemi message code.
_data[1] is the additional info length which was 0 during testing
NPDU_LPDU_DIFF is 8

Looking at the cemi frame format, it seems that the "+1" is simply wrong.

cEMI Frame Format

+--------+--------+--------+--------+---------+---------+--------+---------+
|  Msg   |Add.Info| Ctrl 1 | Ctrl 2 | Source  | Dest.   |  Data  |   APDU  |
| Code   | Length |        |        | Address | Address | Length |         |
+--------+--------+--------+--------+---------+---------+--------+---------+
   1 byte   1 byte   1 byte   1 byte   2 bytes   2 bytes   1 byte   2 bytes

[nanosonde]

The NPDU_LPDU_DIFF starts counting from 0 and includes the message code field.

[thelsing]

You should be right. This is already included in the coupler pull request. Closing.