AUTOSAR COM

Two send settings, switched automatically

A packet can have two send settings, for example fast and slow. COM picks one by looking at the values inside the packet.

Why two?

Some information matters more in one situation than another. While the car is moving, the speed should be sent often. While it stands still, once in a while is enough, and the bus stays free for other things.

So every packet can carry two send settings. Each one is a full setting from lesson 2: DIRECT, PERIODIC, MIXED or NONE, with its own timer.

used while the condition is true

Setting "true"

For example: timer, every second.

used while the condition is false

Setting "false"

For example: timer, every four seconds. Or NONE: send nothing at all.

Who decides?

A condition on a signal's value, such as "speed is not 0". COM checks it every time the application writes that signal.

Several signals in the packet may each have a condition. If at least one is true, the "true" setting is used.
If none is true, the "false" setting is used.
If no signal has a condition, the packet simply always uses the "true" setting. That is the normal, simple case.

When the setting switches, COM uses the new one at once and restarts the timer. If the new setting has a timer, the first packet goes out right away, so the receivers learn about the change without waiting.

Try it

The condition is "speed is not 0". Start the car, stop it, and watch the rhythm of the packets change.

Setting "false" (car stands still)

Things to notice. Each switch causes one packet right away. Writing the same speed again changes nothing, because the condition did not change.

Nothing is thrown away. The condition only chooses the send setting. COM never drops a value on the sending side. Dropping unwanted values is something only a receiver does, with the same kinds of condition. That is lesson 8.

Check yourself

Optional. Answer from memory.

The AUTOSAR words

Plain word here	AUTOSAR word
Send setting	Transmission mode
Condition on one signal	Transmission mode condition (TMC)
The combined true / false result for the packet	Transmission mode selector (TMS)

Source: AUTOSAR COM specification, R25-11, section 7.2.3.2 (SWS_Com_00676 to 00679, 00032, 00799, 00239, 00244, 00495).