A-34
HP
9895A
Disc
Memory
Command Set
Parallel Poll:
Unbuffered Read
Type:
Purpose:
Description:
The
parallel poll response
is
re-enabled after the operation
is completed, normally
or
abnormally.
If
less
than
the specified
number
of bytes
is
accepted by the
HP-IB controller,
the
parallel poll response
will
be re-
enabled
by the Untalk command.
An
extra byte (the value of 1) tagged with
an
EOI
will
be
sent
if
the
controller
requests
more
than
the
specified
number
of bytes. However, this request
is
not necessary
for normal operations.
Disc read.
The unbuffered
read
allows more
than
one
sector
to
be
transferred from the disc to the
bus
controller using a single
command.
FollOWing
reception of the Read command, parallel poll
response
is
disabled
and
the
status of the selected unit is
checked.
If
the unit can
be
accessed (requirements for
execution), the internal buffer is filled with a sector of data
from the disc, just
as
in the buffered read. Now
the
9895A
waits for the
Send
Data command,
then
begins sending
data
to the
bus
controller. When all the bytes from the
sector have
been
sent, the 9895A then reads the next sec-
tor
into the internal buffer
and
sends
it
to the
bus
control-
ler. This process continues until a termination condition
is
reached:
If
the unit
becomes
unavailable,
or
a sector ·cannot
be
found when the buffer is empty, or
if
a CRC error
or
D bit is
encountered,
the
sector in the buffer
is
sent followed by
an
EO!.
If
at any time during the operation the device notices
that
it
has
been
untalked
or
that
the
bus
controller has
sent
a
byte, the process
is
stopped.
FollOWing
any of the
above
terminations, status is
updated
and
parallel poll
response
is re-enabled.
If
there was
an
error in reading
the
data
from the disc, the target
address
is
left pointing to the sector in which the error occurred.
Otherwise, the target address points
to
the sector following
the last sector
read
from the disc. Occurrence of the
error
will
cause
a dummy byte tagged with EOI
to
be
transmitted
to the host system, thereby terminating the
read
process.
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