HP 8508A - Page 60

Operation
Queues
Queues
permit
a
device
to
report
status
or
other
information
in
a
sequential
manner.
Each
queue
has
a
summary
message
bit
which
indicates
that
the
queue
contains
some
information.
This
bit
will
be
TRUE
when
the
queue
contains
any
information,
otherwise
it
will
be
FALSE.
Reading
a
queue
will
remove
a
piece
of
information
from
it,
and
a
queue
is
considered
empty
when
it
no
longer
contains
any
information.
Queues
can
betleared
by
reading
all
the
information
in
them.
_. The Output Queue
There are two queves in the status reporting structure of the Vector Voltmeter.
The
Output
Queue
is
a
“first-in,
first-out”
(FIFO)
queue.
It
stores
output
messages
until
they
are
read
from
the
instrument.
The
availability
of
data
is
summarized
in
the
MAV
bit
of
the
Status
Byte.
The
Output
Queue
is
read
by
addressing
the
instrument
to
talk
and
then
handshaking
the
bytes.
It
can
be
cleared
by
the
Reset
Command,
the
Device Clear Command (IEEE488.1) or by power on.
The
Vector
Voltmeter
also
has
an
Error
Queue,
which
operates
as
a
FIFO
and
stores
error
messages
until
they
are
_
read
from
the
Vector
Voltmeter.
The
Error
Queue
is
summarised
in
the
error
bits
(bits
2
to
5,
inclusive)
of
the
Standard
Event
Status
Register.
It
is
read
by
the
SYSTem:ERRor
?
command
and
can
be
cleared
by
using
the
Clear
Status
command
(*CLS).
When
the
Error
Queue
is
empty,
the
string
“0,
NO
ERROR”
is
returned.
Event Registers
Event
Registers
capture
changes
which
occur
within
a
device.
Each
bit
in
an
event
register
corresponds
to
some
device
condition.
Bits
become
TRUE
when
some
pre-defined
device
condition
transition
occurs.
The
event
register
bits
are
latched
so
that
once
they
become
TRUE
they
cannot
be
cleared
except
by
the
user.
There
are
two
means
of
clearing
an
event
register.
Reading
a
register
will
clear
it
and
the
Clear
Status
command
(*CLS)
will
clear
all
event
registers.
The
Vector
Voltmeter
uses
two
kinds
of
transition
filter
for
setting
these
event
bits
TRUE:
1.
Positive
Transition.
The
event
becomes
TRUE
when
its
condition
makes
a
FALSE
to
TRUE
transition.
2.
Negative
Transition.
The
event
becomes
TRUE
when
its
condition
makes
a
TRUE
to
FALSE
transition.
3-36 E1089
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