Synchronous transmit-receive (STR) was an early IBM character-oriented communications protocol which preceded Bisync.
Ownership of the line is established by the transmitting station sending an inquiry sequence (TL INQ).
Following this initiation sequence the transmitting terminal sends SOR1 TL followed by the first block of data.
This process continues with even/odd SOR and ACK, until the sending terminal has transmitted all the data.
The receiving terminal responds CL EOT and re-enters synchronize mode.