TTL output levels on SN75176 transceivier
The VOH value is specified for an output current that would be used by a TTL input. Your microcontroller has CMOS inputs, which have nearly infinite input impedance, so the actual voltage will be a little bit higher:



You could put a strong pull-down resistor on the RX signal line. However, there are no guarantees, and the graph above shows only typical values, so you should better use some 'real' level-shifting circuit or device. Or it might be a better idea to use a 3 V receiver chip with CMOS output.