Normally have a start colon, 1 count byte, 2 address bytes, 1 null byte, 16 data bytes, 1 checksum byte. Total of 21 bytes. Checksum is the sum of all bytes, modulo 256. Only the low byte of the sum is retained. Checksum is the value to make the byte sum=00.
      Ex.      :100079000218EC2026D4C29630B2FDD29632171D52

      Number of bytes = 10H
      Address         = 0079H
      Sum of bytes    = 0AEH
      100H-0AEH       = 52H
      Checksum        = 52H
Address bytes usually start at 0000H and increment by the number of data bytes, usually 16.(10H). Address changes may shorten a line. The file terminates with an end record:
      :0000000000   or)  :00000001FF