Question

I am communicating with a machine over serial. Part of the protocol communication spec states that the control sum is an “arithmetic sum of bytes from <‘PS’> (included), <‘data’> to <‘CS’>”

The packet messages are structured as follows:

<‘PS’><‘data’><‘CS’>, where:

<‘PS’> – Packet Size

Length: 1

Value: 0x02 to 0x63

Max packet length is 99 bytes

<‘data’> – Data

Length: 1…90 bytes

Value: 0x00 – 0xFF

The length of the data part depends on the command.

<‘CS’> – Check Sum

Length – 1 byte

Value: 0x00 – 0xFF

Example:

ACK Packet: 0x02 0x01 0x03 where 0x03 is the checksum.

So how do I compute the checksum for these bytes in C++?

Answer 1

It looks like the checksum is a simple sum, modulo 256.

int sum = 0;
for (int j = 0;  j < number_of_bytes_in_message;  ++j)
   sum += message [j];

sum %= 256;  // or, if you prefer  sum &= 255;