Protocol Overview - HP 54501A Programming Reference Manual

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