Data
Communications
Receive Buffer
The
terminal's
receive buffer
is
a
first
in/first
out
(FIFO) storage
area
for accepting
data
from
the
remote device.
When
you
are
using
any
type
ofreceive pacing,
the
buffer
is
partitioned
into
a working buffer
and
a 63 byte
overrun
area.
In
particular,
the
buffer size
is
255 bytes,
thus
if
receive pacing
is
being
used:
the
working buffer is 192 bytes long
and
the
overrun
area
is
63
byteslong.
If
the
received
data
overflows
the
workingbuffer
and
intrudes
on
the
overrun
area,
the
terminal
will exercise
whatever
receive pacing mechanism is
currently
enabled
(send
an
XOFF, for example, ifXON/XOFF receive pacing is enabled)
at
that
time
to
temporarily
halt
the
flow of
data
from
the
remote device. When enough
data
has
been processed so
that
the
receive buffer isonly one
quarter
full (64 bytes),
the
terminal
then
signals
the
remote device to
resume
transmission
(by
sending
an
XON, for example,
if
XON/XOFF receive
pacing
is
enabled).
Receive Errors
When
receiving
data
from
the
remote device,
the
terminal
can
detect
the
following
three
types
of
error
conditions (in addition to
parity
errors):
1.
Character
overruns
- a
character
is received before
the
preceding
character
was
processed
by
the
terminal's datacomm firmware.
2.
Framing
errors
- no stop
bit
was detected
at
the
end of a character.
3. Bufferoverflows -
the
entire
allocatedbufferspace is filled (both
the
working
buffer
and
the
overrun
area).
The
last
character
in
the
buffer will be
overwritten
by a
((DEL"
character.
Note
that
if
the
remote device is using
the
selected form of pacing,
this
condition should
never
occur.
Receiver errors,
when
detected, cause a
<DEL>
character
to be displayed on
the
screen
at
the
point
of
the
error.
The
host
may
determine
if
a datacomm
error
has
occurredby inspectingbyte
5 of
the
primary
terminal
status
bytes
(refer to Section
8,
Status,
for information on
terminal
status).
The
host
computer will
not
be able to determine which
type
of
error
occurred.
Local/Remote Modes
The
data
communications portion of
the
terminal
operates independently ofRemote
and
Local
modes.
If
the
terminal
is switched from Remote to Local while
data
is
being
received from
the
remotedevice,
the
datacomm
portionof
the
terminal
continuesreceiving
data,
and
storing
it
in
the
buffer.
In
such a case,
any
data
received
in
Local mode which overflows
the
buffer is
discarded by
the
terminal's firmware. Then, when
the
terminal
reenters
Remote mode,
the
data
stored
in
the
buffer
in
Local mode will be processed
and
sent
to
the
screen.
(To
prevent
buffer
overflow
errors
when
you switch from Remote to Local mode to stop
data
from
being
trans-
ferred to
the
screen, XON/XOFF receive pacing should be used.)
7-12
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