Sage II - WCK+ Generation; Page 6 of Schematic Diagrams; RS 232 Port

SAGE
II
SERVICE
KIT
CPU
BOARD
CIRCUIT
DESCRIPTIONS
PAGE
I 0F
SCHEI'IATIC
(
cont
)
The
Sage
II does
not
and
later
were
lald
devlces
may be
used.
currently
use
2
walt
states
but
cpu
boards
c80002
out
to facllltate
a
mod so that
slower
perlpheral
If
the
serial
lnput
to
the
serial
shtft
reglster
U54
pln
2
is
low
due to
RF-
actLve,
then
DTACK
w111
not
be
generated.
This
prevents
the
processor
cornpletlng
a
cycle
durlng refresh
of the
RAM and
thus
loadlng
lncorrect
data.
I^lhenever
a
locatlon
1s
addressed
whlch
ls
not
decoded
in
the
Sage
II
memory
oaPr
DTACK- is
not
generated.
thls
causes
a bus
tlmeout
error
whlch
allows
an
orderly
recovery
from
the
addressing
of nonexlstent
locatlons.
OPERATION
OF
Ttre
watch
dog
decade
counter
the
8th
count.
BUS ERROR
TII{E
OUT
CIRCUIT
tl-meout
circult
ls lmplenented
in
tJ67
,
a
74LS390
dual
whlch
ls
wlred
for
BcD
count
sequence.
QD
goes
htgh
on
Address
strobe
(
AS-
)
ts
applted
to
U57-pln
2
causlng rhe
counter
ro
be
cleared
whenever
AS-
ls l-nactlve
(
hlgh
).
A bus
tlme
out
error
le
caused
when
AS- stays
low
for
longer
than
16
usec.
Thl-s
ls
caused
by
elther
a software
or
hardware
condltlon
whlch
prevents
DTACK- from
belng
applled
wlthin
15
usec.
I^lhen
AS-
becomes actlve
(
goes
low
)
the
counter
ls
enabled
and
is
clocked
at
pln
I
by
the
500
KHZ
slgnal fron
A7O-12
of
page
2,
If
DTACK-
ls not
recelved
withln
16
usec
causing
AS- to
go
high
and
reset
the
counter,
then
the
QD
output
at
pin
7
goes
htgh
on
the
etghth
count and
ln turn
causes
BERR-
to
be
generated
by
U65
at
pin
12.
The
7406
lnvertlng
buffer
used at
U65
is
open
collector
output
as
ls
lJsZ
of the
user/supervlsor
violatlon
clrcult
whlch
was
descrlbed
earlier.
Bus
error
exceptlon processlng
can
thus
be
inltlated
only
by the
watch
dog
tlmer
or
by
user
trespassing
the system
l/O.
If a
second
bus error
occurs
whlle
the flrst
ls
belng
processed
then
the
processor
w111 halt
assumlng
a hardware
failure.
READ
ONLY
IiEI(ORY
The
read only
memory
sockets
Ul7
and
18
have
been
wlred
wlth
strap
optlon
pads
on the
prlnted
circult
board
so that
they
may be
configured
for
a
variety
of ROM,
PROM,
EPR0MT
or
EAROM ln
sizes
from
l6K btt to
256K bit. The
straps
PrQrRrSrTrUrWrX,
and
Y are
documented
ln
the
Sage
II manual.
58