"I use max232 and SN75176" Are you trying to mix 232 and 485 on the same line? That is not possible.
"Is the SN75176 an RS485 driver/receiver ?." YES
NOTE: most (all I know) '51s generate the TI interrupt/flag in the middle of the stop bit. It is ESSENTIAL to wait a bit of time (depending on transfer rate) before clearing DE.

Have fun,

Erik

PS