Calculate the checksum for ASCII commands

Definition

1. Initial value:
   checksum = 0
2. For each byte:
   checksum = checksum + command byte
3. 
   checksum = checksum &0xff
4. Convert checksum to ASCII high byte and ASCII low byte

Example

• command = $012[Enter]
• checksum = 0x24 + 0x30 + 0x31 + 0x32 = 0xB7
• checksum & 0xff = 0xB7
• checksum ASCII high byte: ASCII(0x42) = B
• checksum ASCII low byte = ASCII(0x37) = 7
• command with checksum = $012B7[Enter]