Getting error: "Invalid combination of start bit, signal size, and byte order", when packing message in big endian format using vehicle network toolbox

I am trying to transmit a 4 byte can message in big endian format using the vehicle network toolbox using the code below.

message = canMessage(0x123, false, 4);
pack(message, 0x12345678, 31, 32, 'BigEndian');

But when i run it i get the error below when packing the message.

Error using can.Message/pack
Invalid combination of start bit, signal size, and byte order.

Error in canDebug (line 2)
pack(message, 0x12345678, 31, 32, 'BigEndian');

I have tried fixing the issue using the following changes, all errornous:

message = canMessage(0x123, false, 4);
pack(message, 0x12345678, 32, 32, 'BigEndian');

Error using pack
Expected STARTBIT to be a scalar with value < 32.

Error in can.Message/pack (line 614)
        validateattributes(startBit, {'numeric'}, ...

Error in canDebug (line 2)
pack(message, 0x12345678, 32, 32, 'BigEndian');

message = canMessage(0x123, false, 4);
pack(message, 0x12345678, 0, 32, 'BigEndian');

Error using can.Message/pack
Invalid combination of start bit, signal size, and byte order.

Error in canDebug (line 2)
pack(message, 0x12345678, 0, 32, 'BigEndian');

Answer 1

[Muskan]

The start bit and the byte order are not independent. You can further refer to the following MATLAB Answer for a better understanding:

https://www.mathworks.com/matlabcentral/answers/2009642-how-to-use-bigendian-byte-order-when-using-the-pack-and-unpack-functions-in-matlab-r2023a?s_tid=answers_rc1-2_p2_BOTH

I hope this helps!

[Søren Lang]

I have been looking at that question, though if you compress the message to 2 bytes, as to keep the message short and to the point.

message = canMessage(500,false,2);
pack(message,int16(1000),16,16,'BigEndian')
value = unpack(message,16,16,'BigEndian','int16')

You get the error:

Error using pack
Expected STARTBIT to be a scalar with value < 16.

This is because bit position 16 does not exist when doing 0 indexing as is done with the vehicle network toolbox.

When doing big endian the byte array is reversed, eg.

[0x12 0x34 0x56] -> [0x56 0x34 0x12]

Thus the startbit will be given as the total amount of bits in the message minus 8 bits. Thus to run the above code, it should look like below.

message = canMessage(500,false,2);
pack(message,int16(1000),8,16,'BigEndian')
value = unpack(message,8,16,'BigEndian','int16')