Also I suggest a couple of resistor on the output wiring. One 1Meg pullup at pin 6, and one 1Meg pulldown at pin 7 (of the 75176). That prevents undefined reception at the receiver side, when there's no enable of transmission.
The transmit enable pin is hardwired to logic high in my example, so transmission is always enabled. So these bias resistors are not necessarily needed here, but one may add them.
The solution is a bit hacked, and delicate, but worthy the same.
Of course, it's a bit of a hack. The two common solutions on a µC would be either to set the break flag in the UART register or decrease the baud rate and send 0x00. Unfortunately both options are not available on the netduino/.NET Micro Framework. The Break property on the SerialPort class is only available in full .NET and changing the baud rate while leaving the port opened isn't allowed (at least when i tested that).