Tandy 2000 Service Manual page 266

Tandy@
Model
2000
Service l{anual
u28(L/2), U12(r/2), 3/4
U13,
1/4
U29,
and
L/5 U65.
Working
backward,
the output
SCNT
is the logical
AND
of
the
synchronized
CNT
(U29.I1)
and
the
enable
nffUn/
(refresh
inhibit- refresh not allowed during video
accesses).
The
synchronized
CNT
signal is the logical
OR
of
two
versions of
synchronized
CNT
- one
that is latched
and one
that reflects
the real time status of CNT.
The
latched version of
CNT
is
required so that
SCNT
becomes
true only
between
memory
cycles.
Once
a refresh cycle is started,
denoted
by
the
presence
of
LRFSH
(latched refresh), the latched version of
CNT
is
cleared
and
replaced by
the real-time version
which
will allow
SCNT
to go false
when
CNT
= 0.
(The latched
version will not
update
until next
ALE
which
will not
occur
until
CNT
=
0 which allows
the
memory
access
cycle to finish
and proceed
to the next ALE).
There
are
two
latches
instead
of
one because
of the special
sequence where
a
non-SELECTed
refresh cycle preceeds
a
normal
access. In this case,
the
delayed
strobe
D300
is active
when
the
new
access
is
started. This
causes
the
decoder
to latch a refresh
cycle
and hang-up
the
sequence
by failing to set RDY.
By
making
the
sync
chain two flip-flops' only alternate
bus
accesses
can cause
a refresh
cycle.
g
.2.8
Decoder
The decoder
accepts
the input timing signals from
the
timing
generator
and
the status signals
and
creates the timing
strobes
for the
MEMORY
TIMING
LOGIC. The decoders U30,
U31'
V42, and
part of U7I are
programmable
logic arrays
and
contain proprietary information.
Timing diagrams
are
shown
in Figure
9-4.
257