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SCSI OPERATION
The
VP415
toruai!'ls a
microprooeS$0r to
a =
the data on
a
LaserVision
disc aad make
ii
available to
a
n indllSIJ)' standard
SCSI
(Small
Cornpu1er
Sys1em) Interlace. This in1erfaoe
is
used 10
connect
lhe
LaserVision
player (the target) 10 a hose compu1er
(the iriitiator).
alk.wing the host to read data from the disc.
as
weU
as providing
the
mual
player control functions. The software ru•ning in the
mkropfOcc~r in the player is called L V-DOS,
and
it
is
wilh
LY-DOS
lhal the
ho§t
computer communicates via the SCSI interface.
AlaserVisiondisccurren1lyhasas1oragecapacityol)24Mbytes.ltis
subdivided into
a
number of volumes which arc managed by L V-DOS.
Thc following
diagram shows how
a
disc might
be
structured:
System table for L V-DOS
Volume directory
Volume'JIM'
Vofome 'HENRY'
_ _ _ _ _ _ _ _ _ _ _ JOOMbylei;
1bc
system
table: contains information for
interna
l use by LY-DOS
and
the
volume
directory contains an
entry
for each
votume
on
the disc. An
entry
for
a
volume contains the volume name, its
whereabouts
on the
disc, and
wme C!ther control information.
The
vohlmes
themselves contain
applications dc'ldopcrs
want to put in
lhem.
LV-DOS
dats not manipulate lhis data
and
therefore
docs
not
ca:re.whatfonna1itisi11.
8k>cli;son the
disc are not
interleaved:
consecutive
logkal
blocks
are
in
ronsei;:u1ivephyskalordcr.witha11in1erleavevalueofl.
De1enion
and
correction o
f
errors
are
carried
ou1
tu•ly
by the
resident
LY-DOS firmware
such
tha
t dala transfcrn:d
lo
the
initialor
is error
free.
Currently
the pre-mastering system must 5upply all
of
the data to
be
mas1ered
onto
the
disc (including
the
sysiem
1able and volume
direnOf)').
VOl.l/Mt:S
"The
LaserVision disc has a high
storage capacity,
...;th the ability to
store S40CO
pictures and 324 Mbytesof dala on each
side.
To divide rhe
total
capaci1y and
to
enable
the
storage
of more
than,
Ol'IC
indepe11dcnt
apphcation
o
r
interactive LaserVision
o
n
011C side
of
the
disc.
lhe
concept
or
volumes has
been adopted.
Thts
concept
is
totally
transparenl lo
the
host computer
since communtl;a1Km
between the
hos!
computer
and
the
LaserVision
player r~fers
tolog.ieal
pictures
and
logtl;al
blocks.
The
n
anslation
from
logical
to
phystl;al
pictures
and
data btocbiscarriedout by
LY-DOS.
After
having received the
Start-unit command from
the
host computer
(the iniliator),
LV-DOS reads information
from the
disc about the
physical
location
of data and pictures
of a specific votume.
Once Start-
unit is
completed,
the volumes are
automatically opened, provided
the
rde
vant digital data can be read from the disc.
Volumu on
disc
are
accessible
as
logic units. 1be
SCSI command
format allows
only 8 lo1ic units 10
be
spcci6cd in
a r;ornmand.
Currcn1ly.
LV-DOS suppons
up to
7 volumes (exdudi11g
1he directory
voturnc)onadi5'.
J7
Logic
unit numben. 0 - 6 will
be
assigned to the volumes in the order
they
arc
specified in
the-
volume directory. If there
are
less
1ha
nsevcn
volumes on a
disc,
1hc unu5td lo1ie u11its are closed by
definition. Logic
unit
7
is intended
for
absolu1e
F-code read/write
(110)
and
for
access
to
the volume directory.
nu:
S<.:SI IN'U:Rt"A<.:t:
The SCSI interface
is
usually used to connee1 a microcompu1er 10 one
or more floppy
andfor
Winchester disc drives. It is
a
bussed system
of
a
very general
na
lure and makes few assumptions about the pieces
of
equipment
being
conneeled.
It is therefore well-sui1ed 10 lhe
requireme11ts
of
LV-OOS. and allows the transport of both player
conlrol
commands
and
disc data over the same physical link.
The SCSI standard pt"Ovides a number of ways of communicatiilg
between the
ho&!
oomputer and its peripherals. However,
only
the
information necessary to use the SCSI interface in conjunction
with
the
VP415 is covered
here.
For full detail$
of
the SCSI
standard,
reler to
the ANSI stalkiard
XJT9.2.
Consider
a
sysiem
consistin& or a
sin&lc
initiator
(the
host
compuler)
a
nd
a number
of
larJCIS
(of
which
the
Laser Vision
player~ one):
Wincbesierdisc
CD-ROM
SCSI bus
Host <0mpu1er
LV,ROM
When
1he h0$C
computer
wants the
player to do
somelhing, ii
m1nl
fi!"j\
gain
control
of
lhe
SCSI
bus.
This
is
the
11rbilratiOP pa.-,
and
is only
necessary to allow
for
lhe possible
extension
o
f
the
syslem
lo
muhipte
initiator.. in
!be
fulure. If lhe
arbitration phase
fails,
some other device
has
control
of
the
bus,
and
the
host computer must wait until ii is free
andretry.
The hosl
compulermusi
then gain the attenlion of the target
with which
it wishes to communicate
(i.e. lhe
player). This it docs in
thealetdoa
pliuc, in
whk
h
ii outputs the
SCSI
addl'HI
of the player
(each device
on the bus
has
a unique address in the range
O -
7)
and
waits for a
response. Should the
selcc1ion
phase fail,
a
"No
response
from
player~
error
mus1 be
re1.umed
to the hi&her level software. The SCSI
address
ofchc player issc1
by
means
of
the SCSI ADDRESS dip
switchetat
the
rear
of
t
he
playH.
Sec SCSI address
setting, under
·Special
issues·
Iner
in
this
section
Assuming
that 1he
arbitralion
phase
is successful,
lhe hos! compuler
musl
lhen tell tlte player what
it
wants
to
do.
This
is
in
the
form of a
commaad
pMR,
in
which
the
h0$t
computer
sends a
w....d
deacriplor Med
containing
the
command that
it
wishes
the
player
lo
carry
out
and any additional information required (e.g
.
lhc block
number
to
be
read).
The command
ph;ue
is
followed,
where appropriate,
by a._.
la
or
dalll 0111
phMC
in
which
1he requested inform1.1ion is tra1"ferred from
the player to !he
host oompu1er
(e.e. when reading
a
disc
block.)or vice
veru.
On
a
read.
1here may
be
a delay of several
s«onds before
1hi1
data is
available, depending on
how long
ii
takes to ex«ule
the
command
in question.
Note:
that
both
parties must usually know
in
advancchowmuchda1ais
tobe1tansfened.
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