Help converting CAN Payload in String format to engineering unit

Hello,

I have the following valiable in Matlab which holds the CAN data from a recorder in the following format:

Hexadecimal

The output of Row 1047 Should be -1951 normally and -1.951 after factoring it by 0.001. This conversion is done using this website.This is basically a force value of a Dynamometer. How can I do this conversion in Matlab? Any known function?

The closest example I found is here, but does not actually solves my problem.

The DBC is as follows:

[Stephen23]

T = "61,F8,FF,FF";
N = double(typecast(uint8(sscanf(T,'%x,')),'int32'))*0.001

N = -1.9510

[Harpreet Singh]

Thank you, pretty effecient code this.

Answer 1

[Arjun]

As per my understanding you have a table of values and you are specifically interested in converting the payload from a comma separated hexadecimal string to a number in engineering unit and then scale it using the scaling factor of 0.001. One way to do so is as follows:

• First parse the payload string and remove the comma using the split function to convert into 4x1 string array.
• Convert the string so obtained to decimal from hexadecimal using hex2dec function.
• Convert the data to bytes using the uint8 function.
• Use the typecast function to convert the bytes data to signed 32-bit Integer.
• Finally apply the scaling factor to get the desired result.

You can refer to the following code that corresponds to the above approach:

%String to be converted
payload_str = "61,F8,FF,FF";
%Conversion to bytes after removing comma and converting to decimal
payload_bytes = uint8(hex2dec(split(payload_str, ',')));
%Interpret as a 32-bit signed integer
raw_value = typecast(payload_bytes, 'int32');
scaling_factor = 0.001;
%apply the scaling factor
engineering_value = double(raw_value) * scaling_factor;
disp(engineering_value);

The code produced an output of –1.951 for the given inputs.

Please refer to the following documentation links for more information on the split, uint8, hex2dec and typecast functions respectively:

• https://www.mathworks.com/help/matlab/ref/split.html#d126e1564444
• https://www.mathworks.com/help/matlab/ref/uint8.html
• https://www.mathworks.com/help/matlab/ref/hex2dec.html
• https://www.mathworks.com/help/matlab/ref/typecast.html

I hope this helps!

[Harpreet Singh]

Hello Arjun, thank you for your response. I also resolved this the same way, but I have two approaches as sometime you will have signals spread "partially" across 2 bytes as seen below.

For CAN signals spread partially the code section below will work well.

%% Exaple for partial signals

payload_str = "DF,5A,3F,19,00,B0,1F,05";

%Splitting payload_str

payload_split = split(payload_str,',');

%Reversing (Intel/Little Endian) and appending relevant bytes

payload_required = append(payload_split(7), payload_split(6));

%hexstring to binary

payload_binary = hexToBinaryVector(payload_required);

%Trimming to data bits per the photo above

payload_binary_trimmed = payload_binary(:,1:end-3);

%Trimmed binary to decimal

decimal = binaryVectorToDecimal(payload_binary_trimmed);

%Decimal to engineering values through scaling and offset

scale = 0.08;

offset= -40;

Eng_Value = (decimal*scale)+offset;

All the hard coded values to index the "payload_required" and trim "payload_binary_trimmed" can managed by refering to DBC directly using the "canDatabase" function.

Your method is approcal one when the signal counumes complete bytes.

Thank you once again.

[Arjun]

Hi Harpreet,

Yes you are correct ! That was good value addition for me.