How can I send no data with the Simulink Arduino serial transmit block?

Apologies if I've missed something obvious -

I'm experimenting with the Arduino blockset in Simulink, sending serial data from an Arduino Uno back to my PC. I can transmit data just fine, but how do I not send anything? If I use a zero as the input to the Serial Transmit block then the Arduino just sends a zero over the serial port every time step.

I tried using a function call or an enabled subsystem to gate the signal but the output remains at its previous level when the subsystem is disabled, so the serial keeps sending the same character. There must be a simple way to do this, but I haven't found it yet.

Answer 1

[Multiplexer]

This is how the block is designed to be. Its meant to send 0 when there is no data in serial buffer. Even the Instruments Control block has no option to "not transmit anything".