Dtr/Dsr Protocol - NCR 7158 Owner's Manual

Chapter 6: Communication Interface 6-7
XON character = hexadecimal 11.
XOFF character = hexadecimal 13.

DTR/DSR Protocol

The DTR signal is used to control data transmission to and from the
printer. It is driven low when the printer is ready to receive data and
driven high when it cannot accept any more data. Data is transmitted
from the printer after it confirms that the DSR signal is low.