HP 8508A - Operation; Service Requests

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Operation
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SYSTEMS
Receiving
the
Pass
Control
Message
The
Vector
Voltmeter
does
not
respond
to
the
Pass
Control
message
since
it
cannot
act
as
a
controller.
Receiving
the
Abort
Message
'
The
Abort
message
is
the
means
by
which
the
controller
sets
the
Interface
Clear
(IFC)
bus
control
line
true.
When
the
Abort
message
is
received,
the
Vector
Voltmeter
becomes
unaddressed
and
stops
talking
and
listening.
Sending
the
Status
Bit
Message
_
The
Vector
Voltmeter
has
no
Parallel
Poll
capability,
and
makes
no
response
to
the
status
bit
message.
Other Messages
Other
messages
sent
by
the
Vector
Voltmeter
are
responses
to
the
query
option
of
some
commands
and
status
information.
In
general,
the
query
options
return
the
recommended
form
of
the
corresponding
command.
This
is
either
a
number
(where
the
command
sets
a
range
or
sends
data,
or
sets
the
function
ON
or
OFF),
or
a
string
(where
the
command
itself
is
a
string).
Status
functions
return
a
number
to
be
decoded
to
determine
conditions
which have set status bits.
Service Requests
HP-IB
devices
may
request
service
from
the
controller
by
asserting
the
SRQ
bus
control
line.
The
controller
usually
responds
to
this
signal
by
asking
each
device
on
the
bus
in
turn
for
its
Status
Byte.
If
bit
6
is
set,
it
indicates
that
the
device
requires
service
and
the
controller
looks
at
the
remaining
bits
in
the
Status
Byte
for
further
informa-
tion.
The
interpretation
of
these
bits
depends
on
whether
the
device
complies
only
with
IEEE488.1
or,
like
the
Vector
Voltmeter,
with
IEEE488.1
and
IEEE488.2.
Bit
6
has
two
distinct
functions.
Within
IEEE488.1
it
is
the
Request
Service
(ROS)
bit
and
within
IEEE488.2
is
the
Master
Summary
Status
(MSS)
bit.
IEEE488.2
also
defines
bit
5
as
the
Event
Summary
Bit
(ESB)
and
bit
4
as
the
Message
Available
Bit
(MAV).
:
When
dealing
with
service
requests,
a
key
operational
difference
emerges
between
instruments
which
comply
only
with
IEEE488.1
and
those
which
meet
TEEE488.2
as
well.
If
the
Status
Byte
of
a
IEEE488.1
instrument
is
read
(by
a
Serial
Poll
Enable
bus
command),
the
ROS
bit
is
cleared
as
a
consequence
of
the
read
operation
.
A
Serial
Poll
of
an
IEEE488.2
instrument
produces
the
same
effect.
IEEE488.2
provides
a
further
command
,
*STB?,
to
read
the
Status
Byte.
If
this
command
is
used,
the
state
of
the
MSS
bit
is
returned
and
the
bit
is
not
cleared
when
read.
These
differences
arise
because
TEEE488.2
defines
a
status
reporting
structure
which
extends
the
specifications
of
the
original
standard.
In
IEEE488.2,
the
Status
Byte
summarises
the
underlying
status
data
structures
and
the
concept
of
enabling
service
requests
is
defined.
E1089
3-31