AUTOSAR COM
Invalid values
Sometimes a sender is alive but has nothing good to say: its sensor is broken. It needs a way to send "I don't know".
A reserved value means "invalid"
Staying silent would look like a timeout, which means something else: "the sender is gone". So sender and receiver agree on one special value of the signal that means invalid. For a speed that normally runs from 0 to 250, the value 255 could be reserved for it.
sender
Invalidate
The application tells COM "invalidate this signal". COM writes the reserved value into the packet. It is sent like any other value.
receiver
Recognise
COM sees the reserved value arrive and does one of two things, chosen per signal.
The receiver's two choices
choice 1
NOTIFY
Tell the application "the value is invalid". Do not store the reserved value: a read still gives the last good value.
choice 2
REPLACE
Quietly swap the reserved value for the signal's start value and carry on as if that had arrived.
Try it
The reserved value is 255 and the start value is 0.
Check yourself
Optional. Answer from memory.
The AUTOSAR words
| Plain word here | AUTOSAR word |
|---|---|
| Reserved value | Data invalid value |
| Invalidate | Signal invalidation (Com_InvalidateSignal) |
| The receiver's choice | Data invalid action: NOTIFY or REPLACE |
Source: AUTOSAR COM specification, R25-11, section 7.2.4 (SWS_Com_00099, 00680, 00681, 00717). Finer point: an invalid value still counts as an arrival, so it resets the timeout stopwatch.