How to read signed float values?

[mridah]

I want to read a signed float value. I'm using Modbus Read node with quantity set to 2.

The function I have is as follows:

let pay = msg.payload.data;

var sign = pay[0] + pay[1] >= 65535 ? '-' : '+'; 

pay[0] = pay[0] > 32767 ? pay[0] - 32767 : pay[0];
pay[1] = pay[1] > 32767 ? pay[1] - 32767 : pay[1];

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16BE(pay[0], 2); // high byte
buf.writeUInt16BE(pay[1], 0); // low byte

var val = Math.round(buf.readFloatBE(0) * 100) / 100;

msg.payload = parseFloat(sign + val);

This gives me the correct numeric value, however the sign keeps changing randomly. So I figured my logic to calculate the sign is wrong.

How can I read the actual float value with the correct sign?

[unborn-andy]

Hi .. I believe that readFloatBE should handle negative and positive values .. no need to calculate the sign yourself.
Use the node-red-contrib-buffer-parser nodes .. they are very convenient for these kind of conversions.

Test Flow of their use (note that modbus nodes have the buffer data in msg.responseBuffer.buffer )

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[grewek]

Hi,

The sign bit in a floating value is the highest bit of the number so in your case this would be bit 15 in pay[0]
so what you could do is use a bit mask and a shift:

var sign = pay[0] & (1 << 15) ? '-' : '+'

Short explanation:
(1 << 15)
First we are take the value 1(binary: 0000 0000 0000 0001) and shift it up 15 positions to the left (<<) which gives us (binary: 1000 0000 0000 0000) we can now isolate the sign bit by use the Binary AND operator (i will use the value shown in your screenshot):

1000 0001 1010 1110 | (33198)
1000 0000 0000 0000 | (32786)
--------------------
1000 0000 0000 0000

This should solve your sign issues

[github-actions]

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