title: How is the Checksum from Hex file calculated?
body: <p>as the title says I'm curious to know how the checksum value is calculated, from what I've read it calculated using 2s complement. Below is a 2 lines from the hex file which was loaded onto my Microcontroller, I've added spaces to make it easier to read, S315 appears on every line, the address on line 1 is 080C0000 followed by 16 hex values which represent the bytes, the values AA on line 1 and AB on line 2 are I assume the checksum values. </p>
  
  <p>For line 1 I've tried adding the following <code>15+08+0C+00+00+4D+53+53+70+6F+74+31+00+66+10+AE+19+7E+63+1F+78</code> which gives me <code>555</code> Hex or <code>‭010101010101‬</code> in binary. I've entered the binary value into an online 2s complement calculator but it always says "invalid binary"??</p>
  
  <p>S3 15 080C0000 4D 53 53 70 6F 74 31 00 66 10 AE 19 7E 63 1F 78 <strong>AA</strong></p>
  
  <p>S3 15 080C0010 00 00 00 00 45 85 63 EB FF FF FF FF 04 00 03 00 <strong>AB</strong></p>