Is the SN75176 an RS485 driver/receiver ?. I think it is but I am not sure. If the answer is yes, DE and RE are the lines that enable transmission (Drive Enable) and reception (Receive Enable).

The RE line use to be active low, and the DE line use to be active high. This is useful to control both lines with the same control signal: '0' to receive, '1' to trasnmit.
It is up to you how to use the RE line, but it is frequent to disable reception when transmitting (otherwise you will receive an echo of your own transmissions).
On the other hand the DE line MUST be active when you transmit, in order to generate a signal in the A-B lines.

If you connect DE directly to an 8051 I/O pin, remeber to clear the bit as soon as possible when the program starts from reset. When you have to transmit a sequence of bytes, begin the process setting DE to '1'. Do not forget to clear DE once the last byte is completely out (including the whole stop bit).

Regards,

Alfredo del Rio.