rounding error

[rocfusion]

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

[admin]

Roger, can you provide a full example and which data type are you using?

[rocfusion]

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

[admin]

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.

[buuuudzik]

Because of this thing every KNX display has an option to round a value to a specified number of digits after the decimal point.