RS485 receive working but no transmit

Hello Forum,

I'm using usart3 of an stm32f090 to establish RS485 comms (not using DMA) in half duplex mode. DE is controlled using a GPIO pin. To transmit I drive this GPIO pin high for the complete duration of the TX and write out all the data via the usart. As soon as transmission is complete, I set  the GPIO pin low for data reception. I can receive incoming data fine but cannot seem to TX any data. The usart is connected via a ST485 device with DE and RE wired together as specified in the datasheet.  Usart3 is set up the same as usart5 which is working fine for TX and RX in rs232 mode. Probably something very simple that I'm missing so any pointers would be greatly appreciated.

Kind Regards

Bob Carter