Specifications
10
S.
The
Data
Acquisition Subsystem
The
main
component
of
the data
acquisition
subsystem
is
the
large
module
on
the daughter board
which
contains the
16
channel
multiplexer. sample-and-hold, analog to
digital
converter
and
related
control
circuitry.
The
AD212
;s
currently designed to
accept
at
least
nine
different
modules from
two
manufacturers.
These
are the
DT5701,
DT5702, DT5703. DT5710. DT5712.
DT57l4.
and
DT5716
from
Data
Translation. Inc.
and
the
MP6812
and
MP69l2A
from
Analoqic Corp. These
modules
are not
entirely
plug compatible
with one another
so
that
a
number
of
jumpers
are required
on
the
daughter board to
compensate
for the
incompatibilities.
There
are also a
number
of
optional
modes
of
operation the
selection
of
which
requires additional
jumper
areas.
The
specific
function
of
these
jumpers
will
be
'described
when
the
related
part
of
the
c1rcutt
is
discussed.
Sa.
The
Multiplexer
The
A/D
circuit
can
only convert
one
analog voltage to
its
digital
equivalent
at
a time. Consequently.
if
it
is
desired to
measure
more
than
one
voltage then
some
means
of
connecting
different
input signals to the
A/D
must
be
provided. This
is
done
by
the mUltiplexer
which
allows
selection
of
anyone
of
16
input
signals under
program
control.
These
input signals are designated
channel 0 through channel
15
and
connection to
them
is
done
on
connector
P1
on
the daughter board.
There are
two
basic techniques
for
determining
which
channel
is
selected.
The
simpler
of
the
two
is
to load the address
of
the
desired channel
into
the
module
before
each
conversion. This
is
accomplished
by
setting
bit
2 of the
commend
register
to a 1
and
loading the address into the
MUX
(multiplexer) address
register
before the
module
is
strobed. This holds the load enable input to
the
module
low
so
that
the internal counter
is
parallel
loaded
with the
least
significant
four
bits
of
the
MUXAODR
register
IC25
when
the
module
is
strobed. This technique
works
well for
random
sampling
of
the channels
or
for
repetitive
sampling
of
a
given channel.
If
repetitive
scanning
of
more
than
one
channel
is
desired, then the
second
basic technique
is
faster.
Here
the
multiplexer automatically advances to the next sequential channel
every
time
the
module
is
strobed. This
mode
is
enabled
by
setting
bit
2
of
the
command
register
to a
O.
The
~du1e.
as
it
comes
from
the factory,
would
then continuously sequence through
all
16
channels
repetitively.
The
module
outputs the address of the
current channel as signals
MXADOUT1. MXADOUT2,
MXADOUT4.
and
MXADOUT8.
These
signals are connected to
bits
0 through 3
of
the
status
register.
IC30.
so
that
the
program
can always read the address
of
the current
channel.
The
AD212
has
additional
circuitry
consisting
of
IC1D.
IC2D,
RP1D.
and
SW1D.
The
lowest channel
number
desired
is
loaded
into the
MUX
address
register.
As
each
strobe increments the
channel
number.
IC1D
compares
the
number
in
SW1D
with the value
on
the
MUXADOUT
lines.
When
the
module
reaches the upper channel
number.
IC1D
lowers the load enable input to the
module
so
that
the
next strobe pulse loads the lower
limit
channel
number
from
IC25
into the
module
rather than allowing the
module
to increment to
the next channel. This
act
causes
IC1D
to
raise
the load enable
input
and
the channel
number
incrementing resumes.