hello Darwin:
thanks for your reply, now i have another question: if the connection between the TL7705 AND 74HCT08 is O.K. we want to use the output of 74HCT08 to control the directiong signal of the RS485 chips. i want to know we should add a pull-up or a pull-down resistor at the output pin of 74HCT08? you can see the pictures below:
