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+--- Thread: rounding error (/showthread.php?tid=912)
rounding error - rocfusion - 22.07.2017
Hi Admin,
It's acceptable the Logic Machine has to store values with a rounding error. It's more worrying when stored values with rounded errors cannot returned back to the KNX bus with the correct value.
For example, a floating point number of 103 is stored as 102.96. I use rounding function to restore the value back to 103, now if I use grp.response or grp.write to return 103 back to KNX bus, I see in the ETS group monitor 102.96. Even if I try, grp.repsonse('1/1/1',103), then the ETS monitor displays 102.96
Can you please correct the grp.response and grp.write functions?
Thanks,
Roger
RE: rounding error - admin - 24.07.2017
Roger, can you provide a full example and which data type are you using?
RE: rounding error - rocfusion - 24.07.2017
Hi Admin,
I have tested with 2 byte floating point and the temperature variant. So create just an object of 2 byte floating point. then write an event script for this new object that looks like,
if(event.type='groupwrite')then
grp.write(event.dst,103)
elseif (event.type=='groupread')then
grp.response(event.dst, 103)
end
Then use ETS to monitor the LM, in both cases 102.96 is sent on the bus.
thanks,
Roger
RE: rounding error - admin - 24.07.2017
If you try writing 103 with ETS you will get 103.04. The thing is that it's not possible to encode 103 as a 2-byte float value, you either get 102.96 or 103.04. If you need better precision then use 4-byte float if your devices support it.
RE: rounding error - buuuudzik - 24.07.2017
Because of this thing every KNX display has an option to round a value to a specified number of digits after the decimal point.