Compute Arithmetic Sum to Communicate with Machine Over Serial

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;

Answer 2

Use an unsigned 8-bit type (uint8_t or unsigned char) as an accumulator, add each byte to it as you generate the packet, then send it as the checksum byte. Exactly how depends on how you intend to construct the packets.