Data Communications
In
a full duplex
environment,
this
terminal
can
also
participate
in
the
following forms of
~,
receive pacing:
~
1.
Terminal
Ready Pacing.
The
terminal
can temporarily
restrain
the
host
computer from
transmitting
by lowering
the
Data
Terminal Ready (TRlCD) line.
It
does
this
when
its
receive
~~working"
buffer is full. When enough
data
has
been processed so
that
the
receive
~~working"
buffer
is
only one
quarter
full,
the
terminal
restarts
transmission
from
the
host
by
raising
the
TRIeD
line.
This
type
of receive pacing can only be used
in
a
hardwired
configuration.
2.
Receiver Ready Pacing. When
the
Receiver Ready line is lowered,
the
terminal
does
not
in~erpret
received
characters
as
data;
it
discards them.
3. XON-XOFF pacing.
The
terminal
uses
the
ASCII control codes XON
«DCl
>)'
and
XOFF
«DC2»
to
start
and
stop
the
host computer from
transmitting.
Note
that
a single XON
code cancels
any
number
of XOFF codes.
4. ENQ-ACK
handshake.
This
is a Hewlett-Packard
handshaking
mechanism.
With
this
form
of
handshaking,
the
host
computer
transmits
a block of
data
and
then
sends
an
ASCII
<ENQ>
control code.
The
terminal
responds to
the
<ENQ>
by
sending
back
an
ASCII
<ACK>
control code
when
it
has
processed all of
the
data
preceding
the
the
<ENQ>.
The
general
interpretation
of
these
two control codes is
as
follows:
ENQ:
"Have
you
processed
the
data
up
to
this
point?"
ACK: ' ,
Ye
s,
I
have."
If
the
host
computer is
an
HP
1000 or
HP
3000,
it
does
not
send
any
data
following
the
<ENQ>
until
it
has
received
the
<ACK>,
or
until
a
timeout
period (several seconds)
has
elapsed.
The
above pacing mechanisms
are
responded to by
the
terminal
in
the
following
order
of
precedence:
1.
Hardware
handshaking
pacing (highest priority)
2.
XON/XOFF
receive pacing
3.
XON/XOFF
transmit
pacing
4.
ENQ/ACK
pacing (lowest priority)
NOTE:
If
both
XON/XOFF
transmit
pacing
and
XON/XOFF
receive
pacing
are
enabled,
the
receive pacing
has
priority,
so
that
if
the
hostcomputersendsXOFF, followed by
data,
the
terminal
can
still respond
with
an
XOFF before
its
buffer overflows.
This
al-
gorithm
shouldalso be used by
the
hostcomputer,
as
the
terminal
may
send
XOFF
and
follow
it
with
transmit
data.
If
both
parties
function
in
this
way,
then
deadlock is
prevented,
and
both
parties
should
prevent
buffer
overrun
at
all
times.
7-14
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