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Handshaking
protocol
X-on/X-off
Protocol
X-on/X-off protocol
is
a
system
in
which the
printer
transmits a code to the computer to indicate
that
it
cannot accept more
data,
and a second code when it
is
once again ready. This protocol can be performed under
either RS-232D, RS-422A, or 20-mA Current-Loop signal
levels.
The C82307* /C82308* interface card sends an X-on
(11H) code when it
is
ready to receive
data,
and an
X-off
(13H) when
it
is
busy. The X-on/X-off signals may be
transmitted from the TXD terminal (pin No.
2)
at
RS-232D
signal levels, through the SDA and SDB
terminals (pin No. 9,
10)
at
RS-422A signal levels, or
through the TTY-TXD terminal (pin No.
17)
at 20-mA
Current-Loop signal levels.
After the X-on flag has been set,
data
can be accepted
up to the maximum capacity
of
the buffer.
•
Transmit timing
of
the
X-off
signal
The
X-off
signal is
transmitted
when the vacant area
for bytes in the buffer drops to flag set level.
•
Data transfer after
X-off
signal
Data can be sent to the
printer
even
after
the
X-off
signal
is
transmitted as long as sufficient room for
data
remains
in
the buffer. However,
if
the
transmitted
data
exceeds the vacant
area
in the
buffer, it will be discarded. The X-off
character
will
be
transmitted
again when the remaining buffer
capacity
is
actually 0 bytes. At
that
time, a BS code
(08H)
and "/"
character
(2FH) will be sent to the
printer.
17