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stm32 response transmission fault


i am using a stm32f401ccu6 board to receive modbus rtu requests from a master device(windows laptop running simplymodbus) and prepare the response buffer and send it

so far i can see that the response buffer is being prepared as expected in the live expressions table, however whenever i am trying to send the message back to the master it is receiving the message with FF and 00 values at the beginning and end of the response 

for example if the response is 07 03 04 57 73 7A then the master is receiving FF 07 03 04 57 73 7A 00

also i am using a rs485 transceiver along with the uart lines and the hardware seems to be working fine as when i programmed the stm32 to be the master and the pc to be the slave it was working perfectly fine 

what could be the issue