Seiko FB-900 - Manual (Page 174)

● Serial interface
In this protocol, the printer sends the X-ON(11)H and X-
OFF(13)H codes to the computer to indicate the printer state.
In the X-ON/X-OFF protocol, the printer sends X-OFF(13)H to
the computer once when the ready state changes to the busy or
offline state, once when 64 bytes of data have been received
since this point in time, and once when the free area in the buffer
becomes zero.
The printer sends X-ON(11)H once to the computer when the
busy or offline state changes to the ready state or the power is
turned on. If the ONLINE key is pressed in the offline state and
the free space in the buffer is greater than the CIN value, the
printer sends X-ON and enters the ready state. If the free space
in the buffer is less than the CIN value, the printer enters the busy
state without any operation. In the busy or offline state, the
SRTS and DTR signals remain active. X-ON and X-OFF signals
are sent to the computer even if the DSR or CD input signal is
inactive. In the X-ON/X-OFF (I) protocol, the X-ON signal is
sent when the power is turned on. In the X-ON/X-OFF (II)
protocol, the X-ON signal is not sent when the power is turned
on.
The X-OFF signal is sent once in any of the following conditions:
(1) The ready state changes to the busy state.
(2) The ready state changes to the offline state.
(3) 64 bytes of data are received in the state shown in (1) or
(2) above, provided the free area in the buffer is greater
than 64 bytes.
(4) The free area in the buffer becomes zero, regardless of the
online or offline state.
The X-ON signal is sent once in any of the following conditions:
(1) The busy state changes into the ready state.
(2) The offline state changes into the online state.
(3) The printer enters the online state when power is turned
on, provided the X-ON/X-OFF (I) protocol is selected.
7-9 — 7. Interface specifications —
X-ON/X-OFF (I)/(II) protocol