COMMUNICATION 7
2.2.1
Input Buffer
All characters received, except nulls and deletes, are temporarily stored
in
an
input buffer before further processing. Nulls and deletes are ignored and do not
occupy space
in
the input buffer. The input buffer has a 2047 character
capacity.
If the printer falls too far behind the incoming data, the input buffer overflows
and
data is lost. If characters are lost due to input buffer overflow, a single sub-
stitute control character (octal 032) is placed
in
the input buffer at the point of
loss. If a character is received with a parity error, the character is replaced
in
the
input buffer by the substitute control character (octal 032) thus causing the error
character (reverse question mark) to be printed. The 032 control character, thus
indicates loss
of
characters, or a character received with parity error.
When the printer is capable of printing, characters are fetched from the input
buffer
and
printed
or
otherwise processed
as
required. When the printer is
incapable
of
printing, the printer scans the input buffer for printer status request
control sequences even if the input buffer is full.
2.2.2 XON/XOFF Protocol
Using the XON/XOFF protocol, the data source becomes synchronized with the
printer
as
described below.
After successfully powering up and becoming enabled to send, the printer
sends
an
XON
control character and constantly monitors the number
of
empty
character positions
in
the input buffer. When the number is less than 128, the
printer sends
an
XOFF
control character, signaling the data source to temporar-
ily stop sending data. Meanwhile, the printer continues
to
take characters from
the input buffer and print or otherwise process them. When the number
of
empty positions
in
the buffer exceeds 224, the printer sends
an
XON
control
character, thus signaling that transmission may resume.
The printer also sends
an
XOFF control character when it is not ready due to
error conditions
or
operator actions. Running out
of
paper
or
detecting a print-
head position failure causes
an
XOFF control character
to
be
sent. The operator
actions of opening the cover
or
placing the printer off-line also cause
an
XOFF
control character to
be
sent.