HP 54501A - Protocol Overview

Output
Queue.
The
output
queue
of
the
instrument
is
the
memory area
where
all
output
data
(<
response messages
>
)
are
stored
until
read
by
the
controller.
The
HP
54501A's
output
queue
will
hold
300
characters,
however
the
instrument
will
handle block
data
of
greater
than
300
characters where
appropriate.
Parser.
The
instrument's
parser
is
the
component
that
interprets
the
commands
sent
to
the
instrument
and
decides
what actions
should
be
taken.
"Parsing"
refers
to
the
action taken
by the
parser
to achieve
this
goal.
Parsing
and
executing
of
commands
begins
when
either
the
instrument
sees
a
<
program
message terminator
>
(defined
later
in
this
chapter)
or
the
input
buffer
becomes
full.
If
you
wish
to
send
a long
sequence
of
commands to
be
executed
and then
talk
to another
instrument
while
they
are
executing,
you
should
send
all
the
commands
before
sending
the
<
program
message
terminator
>.
Protocol
Overview
Protocol Operation
The
instrument
and
controller communicate
using
<
program
message
>
s
and
<
response message
>
s.
These
messages serve
as
the
containers
into
which
sets
of
program
commands
or
instrument
responses
are
placed.
<
program
message
>
s
are
sent
by
the
controller to
the
instrument,
and
<
response message
>
s
are
sent
from
the
instrument to
the
controller
in
response
to
a
query
message.
A
"query
message" is
defined
as
being
a
<
program message
>
which
contains
one
or
more
queries.
The
instrument
will
only
talk
when
it
has
received
a
valid
query
message,
and
therefore
has
something
to
say.
The
controller
should only
attempt to
read
a
response
after
sending
a
complete
query message,
but before
sending
another
<
program
message
>.
The
basic
rule
to remember
is
that
the
instrument
will only talk
when
prompted
to,
and
it
then
expects
to
talk
before
being
told
to
do
something
else.
When
the
instrument
is
turned
on
or
when
it
receives
a
device
clear
command,
the
input
buffer
and output
queue
are cleared,
and
the
parser
is
reset
to
the
root
level
of
the
command
tree.
The
instrument
and
the
controller
communicate
by exchanging
complete
<
program
message
>
s
and
<
response
message
>
s.
This
means
that
the
controller
should
always
terminate
a
<
program
message
>
before
attempting
to
read
a
response.
The
instrument
will
terminate
<
response
message
>
s
except
during
a
hardcopy
output.
Message
Communication
and System Functions
3-2
HP
54501A
Programming