When using a serial interface
When DTR/DSR control is selected, the printer transmits only 1 byte after
confirming the host is ready to receive data (DSR signal is SPACE). If the host
computer is not ready to receive data (DSR signal is MARK), the printer waits
until the host is ready.
When XON/XOFF control is selected, the printer transmits only 1 byte without
confirming the condition of the DSR signal.
This command is executed when the data in the receive buffer is developed.