Star Micronics DP8340R - Manual (Page 43)

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9-2. X-ON/X-OFF Mode
The printer transmits an X-ON (Control Code; DC1, Hexadecimal Value; 11H,)
signal after power is turned on, if there is no printer error being generated. When
this signal is received by the host computer, the host computer transmits the data
to the printer. The X-ON signal is output intermittently every three seconds until
the host computer receives and responds to this signal.
The X-OFF (DC3, 13H) signal outputting begins when the amount of empty
space in the buffer becomes less than 256 bytes. When the computer receives the
X-OFF signal, it will halt data transmission as soon as it can. However even at this
time the printer can receive data until the buffer is completely full. An X-ON
signal is output when the contents of the buffer goes below 256 bytes. The
increase of the empty area in the buffer is caused by printing. If the computer
causes a buffer overflow, a flag bit in the status register will be set. (See Status)
The X-OFF signal will continue to be output at a 3-second interval until the data
buffer becomes near empty (less than 256 bytes).