From the '328P datasheet re: USART:
"The dashed boxes in the block diagram separate the three main parts of the USART (listed from the top): Clock
Generator, Transmitter and Receiver. Control Registers are shared by all units. The Clock Generation logic consists
of synchronization logic for external clock input used by synchronous slave operation, and the baud rate
The baud rate generator goes to both the transmit and receive shift register.
You would need to use
// receive some data
Serial.end(); // or whatever the syntax is
Serial.begin (230400); // or whatever
// send some data
to able to change speeds, and connect only Rx to DMX and only Tx to the mac.
Simpler might be to use a '1284P chip with 2 serial ports.http://www.crossroadsfencing.com/BobuinoRev17