Sage II - Exception Errors

SAGE
II
SERVICE
KIT
CPU BOARD
CIRCUIT
DESCRIPTIONS
PAGE
5
0F
SCHEMATIC
DIAGRAMS
(cont)
I|CK+
GENERATIOTI
The
755
requires
a
wrlte
clock
of
500
Khz for
double
denslty
and
250 Khz
for
slngle
density.
Both
clocks
require
a
pulse
wtdth
of
250
nsec.
The
presettable
counter
U70
page
2
ls
clocked
wlth
a 4
Mhz
lnput
whlch
generates
a
ripple
carry
of duratlon
of
one lnput
clock
perlod (250
nsec).
Ihis rtpple
carry ls
used
to
preset
the
counter
to
elther
0
or
8
dependlng
on how
the
MFI''I+
bit ls
set on the lnput
to
D3
of
the
preset
of
the
counter.
The
counter
then
counts
up
to
15
from
elther
0,
or
8
causlng
the
4l'Ihz lnput
clock
to
be divided
by either
8
or
16
generatlng
e
I
ther
500
Khz or
250
l(hz
clocks
with
the requlred
pulse
wldth
of
250
nsec.
XVII.O6
PAGE
6 OF SCHBUATIC
DIAGRA}IS
:
Thts
sectl-on
ls
titled
Communicatlons
Clrcult
and lncludes
the four
maln
lnput
/output
ports
of
the
Sage
II system.
The other
I/O
devlces in
the
system
are
used lnternally
as
software
/
hardware
lnterface
to
generate
control
signals.
However,
the
port
for
the
dip
swltches
could
concelvably
be
used
as a
16-blt
parallel
port.
The
ports
ln
thls
section
are
the
RS232,
the
serial
prlnter,
the
parallel
prlnter,
and
the
IEEE
488
or IIPIB.
RS
232
PORT
The serial
termlnal
port
ls
inplemented
uslng
an
8251A
progranmable
communlcatLon
lnterface
U57. The
Receiver
Ready
R:(RDY
slgnal
at
pln
14
ls
lnverted
by
74L504 buffer
V37-5 to
6
produclng
RXIIr 8o
interrupt
signal
whlch goes
to
lJ36-Z
on
page
1. Pin
15
of
U57
ls
TX1I
which is
also
an lnterrupt
slgnal
and goes
to
the lntelllgent
lnterrupt
controller
V73-20 on
page
2. The
buffered
data lines
BDO
through
BD7
are
connected
to
U56 at the
lower
left
corner
on
page
2. Ttre read
/
wrlte
signals
RD-
and WR-
are
from
page
1. Ttre
chlp select
pln
ll of
U57 originates
at
Ul9
pin
15
on
page
2. Concludlng
the
system
slgnals
is
SRES+
from
U44-9
page
l,
2
MHz* from
page
2 U70-14, and
BRI from
u7
5-l .
The
port
is
connected
to
J10
through
buffer
gates
U60
and
U6l. The
transmlt
buffer
1160 ls
an MCl488
or
75188
and is
powered
by
positlve
and
negative
12
volts
to
level
shlft
the
output
for
proper
RS232 levels.
U61
ls an
MCl489
or
75189.
66