@Grumpy_Mike I'm using 115200 baud, so 1ms delay is enough for 14 bytes. If I use a 1ms delay after I use flush, then it should be at least 7-14 times longer than necessary (which is okay, because the other device on the bus won't start transmitting until a minimum of 1ms after the last byte has been received from the Arduino). I do have complete data format control, as I am writing the software for both ends.
@retrolefty that's not an option, due to the HW setup of the other controller (Lego NXT). I also want to be able to have more than 2 devices on the bus.