connect with a bizerba scale using rs232

I already create a software that sent and receive data from scales. In my effort to connect with Bizerba CS 300 TM I stuck because it is necessary to sent the checksum first. So I want help in order to sent the right checksum. [28/11/2018 20:22:17] Written data (COM1) (I ask for weight) 04 02 30 31 1b 30 30 30 31 30 30 03 ..01.000100. [28/11/2018 20:22:17] Read data (COM1) (received the requirement for checksum "E" is random from scale) 02 31 31 1b 32 45 33 03 .11.2E3. [28/11/2018 20:22:17] Written data (COM1) (This is the answer I must sent "51c4de66") 04 02 31 30 1b 35 31 63 34 64 65 36 36 03 ..10.51c4de66. How can I find this "51c4de66"? Another example: [28/11/2018 20:39:17] Written data (COM1) 04 02 30 31 1b 30 30 30 31 30 30 03 ..01.000100. [28/11/2018 20:39:18] Read data (COM1) 02 31 31 1b 32 30 45 03 .11.20E. [28/11/2018 20:39:18] Written data (COM1) 04 02 31 30 1b 34 37 31 31 63 63 64 62 03 ..10.4711ccdb. Dialog 6 attached.