The only issue left is how to specify
what
byte
is placed
on the data
bus,
if 808 g,
2813
i'1ode0, or 280 Mode 2 interrupts are used.
If you wish
the byte to be FF (hexadecimal) then there is nothin~ more to do; leave
the
board
as is.
This works because the data bus is sensed as all lis
(G~US
FF
in hexadecimal)
if none of G~e lines are specifically set to 0.
To generate
a byte different Glan FF, ~~e 8itstrea~er II board
allows
you
only
to set
one of d1e lines
to 0; all others
must
be left as 1.
First,
install
a
jumper
from
the single pad in area~which
is SINTA) to pad 8-5.
Second,
install a jumper from A-8
(which is PINT) to pin 11 of chip U2l.
(This uses
one
of the spare inverters on the board.)
Third, install a jumper from pin
10 of chip U21 to pad B-4.
(This uses one of the spare open collector
N~~D
gates on the board.)
Pad L-l is the output of the NAND gate.
Since PINT is
inverted and N.~l\JDed
with SI~TA, L-l will be
10\'1
(0) only
if both
PINT
and
SINTA
are
active;
that is, only if the board requested an interrupt and if
the CPU accepted
it.
Last, install a jumper from L-l to any of ~~e pads L-2
to L-9.
These
pads are connected to the data bus.
Thus if L-l is low, it
will pull one of the data bus lines low.
The
resulting
byte
generated
on
the data bus is given in the table below:
Byte on Data Bus
L-l is jumpered to
FF
nothin~
FE
L-9
FD
L-8
FB
L-3
F7
L-S
EF
L-2
DF
L-4
BF
L-6
7F
L-7
Data Bus Bytes Generated by Jwnper in Area L
Table 10
Note
that
three
of ~~ese bytes correspond to 8080 restart instructions.
Therefore, t..,'1ose
tJ1ree restart instructions can be generated,
as
indicated
in Table 9.
If there is more than one 8itstrea~er II board in the crnnputer, each
can
be jumpered to place a different byte on the data bus.
If you do ~~is, then
software can determine
wh ieh 8i tstreame r I I boa rd is the source
of the
interrupt,
since
only
the specific board ~"hich produced ~'1einterrupt can
have
an effect
on the data
bus
(assuming
you
use the jumpering
scheme
reco~8ended
above).
TI~e byte on" the data bus thus indicates which board it
came from.
If you are using 230 Mode 2 interrupts, then you can generate up
to 9 different vectors, and thus use up to 9 different 3itstreamer II boards
whose interrupts can be separated in software.
Note
that
in many Vector
Graphic
systems, the Flash'w'-riter
board is wired to generate interr'Jpts,and
does
not
alter
the data
bus.
Thus
the Flashwriter's
characteristic
Summary of Contents for Bitstreamer II
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