the Plc program for new barcode reader

Hello.i want to replace the old barcode reader(symbol sl1220) with new type(symbol ms2204)and the barcode reader worked in host mode...
the plc send the read command packet to barcode through a1sj71uc24-r2 module,the command packet is :

length opcode messagesource status checksum
04H E9H 04H 08H ?(2 byte)

my question is that how to identify the checksum?


thanks for your any help
ronald