Radio Shack TRS-80 Service Manual page 33

5-Meg Hard Disk Service Manual
the actual access. This
pre-selection feature effectively
reduces RAM access time
to
the output enable and propagation
time
of the
RAM
for
read operations. This feature also
reduces the width
of the
minimum
WR* strobe pulse for write
operations
.
RAM Accessing
RAM access
is
initiated by RCS* which
is
the logical or
(by
U59) of
RDO* and WRO* which are generated by the Fast
10
decoders
(U44
and U45). Data
to
be'
read from RAM will
be
placed on the
10
bus whenever RCS*
is
low and WC*
is
high.
Data
is
written into
a
selected RAM cell
on the
trailing
edge of WC*
if
RCS*
is low.
During writes
,
both WC* and RCS*
will be low for at least 120 nanoseconds
so
that data setup
time requirements are
met.
Scratchpad Operations
Because the RAM address counters can
be pre-set, direct
reads and writes
to a
specific address are possible. This
function
is
used
for
scratchpad storage during program
execution. This mode of RAM access requires two or three
instruction cycles for each random access
to the
RAM
as
opposed
to
one
for
sequential access using the
post-decrement
feature.
MAC Control Port
Basic control
of the various functional sections
of the
controller
is
accomplished by
a
dedicated 6-bit control port
called MAC CNTRL
(U29).
MAC CNTRL enables CRC generation
(CRCIZ*), functions of the WAIT control circuitry
(WAEN*),
gating of read or write functions
(WRITE*)
,
control
of
CRC
check word output
(
1BLA*
)
f
and
AM detection
(SRCH).
MAC
CNTRL output states are latched into the port by
a
write
strobe (WR7). Additionally, any time MAC CNTRL
is
loaded
with
a
new byte, the lower two data bits
(100
and
101)
are
strobed into the upper two address counter/latch bits
(RA8
and RA9). All remaining ports are distributed among the
basic functional sections of the
controller and will
be
described
in
detail within the discussion
of those
functions
.
Serial Data Separation
The controller board contains circuitry which processes
incoming MFM data from the drive by
a
method called data
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