If you are trying to make a RS485 - 232 convertor with the Max232 and the 75176, You can either use the RTS from the serial port to control the /RE, DE and set it before you send anything out the port, you would need to feed this thru the Maxim chip to convert from 232 to TTL levels for the 485 chip.

The other alternative would be to monitor the data from the output of the MAx 232 and when data arrives enable the DE and after a time period of inactivity disable it so that you can get the response back, this is the way that I do it - with just a 555 in there, as it means that you PC software doesn't need to worry about the RTS and it makes it more portable as some serial ports nowadays only have TX, RX, and GND.

Marshall