HP 54501A - Page 76

Parallel
Poll
Parallel
poll
is
a
controller
initiated
operation
which
is
used
to
obtain
information
from
several
devices
simultaneously.
When
a
controller
initiates
a
Parallel
Poll,
each
device
returns
a
Status
Bit
via
one
of
the
DIO
data
lines.
Device
DIO
assignments
are
made
by
the
controller
using
the
PPC
(Parallel
Poll
Configure)
sequence.
Devices
respond
either
individually,
each
on
a
separate DIO
line; collectively
on
a
single
DIO
line;
or
any
combination
of
these
two
ways.
When
responding collectively,
the
result
is
a logical
AND
(True
High)
or
a
logical
OR
(True Low)
of
the
groups
of the status bits.
Figure
3-32
shows
the Parallel
Poll
Data
Structure.
The
summary
bit
is
sent
in
response
to
a
Parallel
Poll.
This
summary
bit
is
the
"ist"
(Individual
Status)
local
message.
The Parallel
Poll
Enable
Register
determines
which
events
are
summarized
in the
individual
status
local
register.
The
*PRE
(Parallel
Poll Register
Enable)
command
is
used
to
write
to
the
enable
register
and
the *PRE?
query
is
used to
read
the
register.
The
*IST?
query
can
be
used
to read
the
"ist"
without
doing a
parallel
poll.
Message
Communication
and System Functions
HP
54501A
3-34
Programming