S7 Real Read value is wrong

Hello Experts,

I've started to experiment alot with NodeRED and I am currently trying to read values from a S7-1510SP CPU with the node-red-contrib-s7 and this works very well!

But it seems like trhe value that is read in real into the NodeRED system is some decimals wrong, if I would enter 70.5 into the system then it would be fine but if I would enter 70.3 it gives me 70.299999899... etc.

Anyway to fix this?

Maybe this is a matter for the person behind the s7 contrib but if you know a way of rounding the value that would be great.

It seems like you are experiencing a floating-point error; this is due to javascript's number representation (here is an in-depth explanation)

Long story short, if you know that your variable should have only a single decimal digit, you could take the reading and apply the "toFixed" function, with 1 as "digits" parameter; here is an example:

let myValue = 70.299999899;

let myFixedPrecisionValue = myValue.toFixed(1); // "70.3" => keep in mind that this is a string

let myFixedPrecisionNumber = Number(myFixedPrecisionValue); // 70.3

Be aware that loss of precision could arise from big numbers (more on that in the article linked)

This is great!

Thank you very much, for your explanations and examples.

I am now using this as a universal "real filter" for my real signals with 2 decimals and it is working like a charm with the rounding aswell.