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Acknowledgement of KNX group telegrams
#1
Hello,

i have a special question about the behavior of the group adress acknowledgement by homelynk/spycelynk/wiser/LM.

When i look to the KNX statistic in Spacelynk (see attached), i can see a lot of repeated telegrams there.
The first idea for the reason in another thread was, that there is a loop created with an other IP router on the KNX bus via LAN.
But the traffic and the repeats are the same, even with deactivated "KNX IP Features" in Spacelynk.

In my opinion these repeats are coming from the TP coupler for all the telegrams he lets into the KNX subline and which do only pass through his filter table because they are needed by Spacelynk.
They are repeated because there is no one else who would send an acknowledgement back to the line coupler, because there is no other recipient on this line but Spacelynk and Spacelynk does not acknowledge these telegrams by default.
So everyone should have this "problem" when there is a Spacelink installed in a mainline or in a subline and there are group telegrams coming into this line via couplers and there filter tables only to get to Spacelynk and no one else in this line. These group telegrams should all be repeated several times by the coupler.

But the question is now, how can it be realised, that Spacelynk does acknowledge a telegram when he is the only receiver for this telegram on the KNX line, so that the coupler has no need to repeat the telegram several times?
There is the option "ACK all group telegrams" in the options for the KNX connection, but this would in my opinion mean, that really all telegrams are acknowledged, even if the have nothing to do with the group adresses used in Spacelynk.
For example, when a switch sends a group telegram to an actuator in the same line, Spacelynk would acknowledge it, only because he receives it, but this acknowledgement should come from the actuator itself.

Maybe it would in the future (next firmware) be possible, to add an option in the objects list for especially these group telegrams, which shall be acknowledged by Spacelynk.
So there could be set a hook or something like that.
But maybe when this is realised, it could be done in this way, so that it could be seen at once when looking at the oojects list.
At the moment there is this option "Read during start-up" in the object settings, but this is really not practicable to check each object, whether the hook is set or not.
It would be much better to see this hook at once, when looking at the objects list itself.

Kind regards
Michael

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#2
Hi
The situation described above by KNXUser is a serious design issue. I've reported it few years ago and it looks there's still no solution at all. I think his idea with limiting ACKs is great.

Today I found new strange behavior of ACKing:
I've two LMs connected to the same line. The first one has "ACK All group telegram" turned on. The second one (backup) has it turned off.
My first LM (Version: 20180822) shows 4 repeats per hour
My backup LM shows (Version: 20211014) shows 4000 repeats per hour.
If I turn "ACK All group telegram" on at the backup LM then this counter drops significantly down.
It seems to me that internal changes require ACKing too.
LM5Lp, firmware: 2018.08.22 and 2021.12.15, FlashSYS v2, ARMv7 Processor rev 5 (v7l), kernel 4.4.151 and 4.4.259
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#3
Are you sure you haven't created a loop by connecting both LMs to a single TP line? You need to disable IP features or change multicast IP on one of LMs.
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#4
Hi Admin,
today I came across to a same strange behavior.
Photo with ACK disabled (60% of repetitions).
Photo with ACK enable (0% of repetitions).

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#5
If you get no repeats when ACK all is enabled it means that there are KNX devices on the bus that are sending telegrams that are not acknowledged. If LM is the only recipient of such telegrams then you can leave ACK all enabled.
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