AUTOSAR COM
The signal gateway
A control unit connected to two buses can pass a value from one to the other. COM can do this by itself, without any application in between.
Copy a signal from one packet to another
The engine sends the speed on bus A. A display on bus B needs it too. The control unit in the middle is told once, in its setup: "whenever this signal arrives, copy it into that signal of that outgoing packet". That fixed copy rule is the signal gateway.
- It works on single signals, so the two packets can look completely different.
- One received signal can be copied into several outgoing packets.
- The copy rules are fixed in advance. They do not depend on the values.
After the copy, everything is as usual
For the outgoing packet, a gateway copy counts exactly like an application writing the signal. So everything from lessons 2 to 4 applies: is the signal triggered or pending, does the packet have a timer, is there a minimum gap.
The gateway does not copy in the very moment a packet arrives. It has its own heartbeat and does all waiting copies at the next beat. This keeps the arrival itself short and fast.
Try it
Let packets arrive on bus A. The gateway's heartbeat runs every 2 seconds.
Three rules worth knowing
Only new values
If the received signal has an update-bit and it is not set, nothing is copied.
Timeouts do not block
Even if the signal was overdue before, a value that arrives is still passed on.
Byte order is fixed up
If the two buses store the number's bytes in different order, COM converts on the way.
Not the only gateway. If a whole packet should go from bus A to bus B unchanged, COM is not needed at all. The neighbour below COM forwards whole packets, which is cheaper. The signal gateway is for when the packets differ.
Check yourself
Optional. Answer from memory.
The AUTOSAR words
| Plain word here | AUTOSAR word |
|---|---|
| Copy rule | Gateway mapping (source and destinations) |
| Gateway heartbeat | Com_MainFunctionRouteSignals |
| Forwarding whole packets | I-PDU gateway in the PDU Router |
Source: AUTOSAR COM specification, R25-11, sections 7.1.5 and 7.10 (SWS_Com_00377, 00359, 00466, 00360, 00701, 00703). Finer points: members of signal groups can be passed on too, and stay consistent. Signals in packets that are too big for one frame (lesson 13) cannot be passed on this way.