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Principles
of Operation
Address decoding
for
peripheral devices
on
the
microproces-
sor bus
is
performed by
another
16P8A
PAL
(U2). Periph-
eral
devices are
decoded
at
16-byte
intervals
in
the
address
range of
$3800-$38FF. These
include,
for
example,
a
6522 A- VIA,
a
65C2
1
peripheral interface adapter (PI A),
and
a
68B50
asynchronous
communication
interface
adapter
(ACIA).
6.3.3
Memory
The
32K
bytes
of instrument
operation software
are stored
in
U7, which
is
a
27256
EPROM.
The
revision
level
of
the
soft-
ware
is
displayed
on power-up.
U8
is
a
32K
byte
static
CMOS
RAM
chip
that
is
used
for
storing relay
setups
and
as a
scratchpad during
normal
oper-
ation.
Its
power
source
and
chip enable
lines
are
routed
through U3,
a
DS-12I0
NVRAM
controller.
Figure 6-3
shows a
simplified
schematic of
the
RAM
and
battery
back-
up
circuitry.
The
NVRAM
controller
performs
the functions
of
switching
the
RAM
power
source
between
Vcc
and
the
lithium
battery
(BTl).
It
also disables
chip enable
(CE)
to
the
RAM
when
Vcc
is
outside the specified
limits.
(See
the
paragraph
describing the
reset
circuitry.)
In addition,
if
the battery
power
goes
below
a
specified
limit
while
Vcc
is
not
present,
a
DS-1210
chip
normally
inhibits
the
second
chip enable
signal to the
RAM
after
Vcc
is
restored.
Since
this
feature
is
not used
in
the
Model
707A,
the
software always does a
dummy
read
of
memory
locations
$00()()-$000
1
on power-up
to
get past the
second
chip enable
cycle.
This
permits
the
.Model
707A
to
operate properly with
no
battery,
or
if
the battery
has
been
replaced.
6.4
Relay
control
circuitry
The
relay control
circuits
reside
on
the
backplane
board,
dig-
ital
board,
and each
switching
card.
The
backplane board
acts as a
passive
conduit
for:
•
Control
signals
from
the
digital
board
to
the
switching
cards
and
response
from
the
switching cards
to
the
digital
board.
•
Power
lines to
the
switching
cards.
•
Expansion
of analog
signals
among
the switching
cards.
Operations of
relay control circuitry
on
the
digital
board and
a
typical
switching
card are described
in
the
following
paragraphs.
6-4
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