How do you transmit a continuous stream of synchronous serial data without any “idle time”?

Sealevel synchronous serial adapters are capable of a constant output stream if the application program can provide a constant stream of serial data. If you are using either blocking (normal) I/O or overlapped I/O with only one write active at a time, then the adapter will run out of data to send when the application is processing the next write. The most efficient solution is to have multiple overlapped writes that are active simultaneously.