trlectrcal
I est lnstruments
MAC-20 Retrofit
Package
Section ll
-
Detailed Description
ln the measurement
system,
in this case
the MAC-20,
the signal from
the current
sensor
(typically
about 50 mV
for 1000 Amperes),
is connected
to an integrator,
consisting
of
precision
resistors
and
a capacitor. This
signal is
processed
by a variable-
gain
instrumentation
amplifier, and a
digitally
programmable
gain
circu-it. An analog
to
digital
converter
(ADC),
under microprocessor
control, reads this
signal to a
precision
of
'12
bits
(+/-
0.025o/o). This
circuitry is
contained
on a single PC
board, originally
devel-
oped for the
Electrical Test
lnstruments
ORTMASTER
system.
The heart
of the
MAC-20
consists of a microprocessor
core unit, based
on the Zilog
2180, with
associated ROM,
RAM, and
other circuitry,
and manufactured
by ZWorld
Engineering
under the trade
name
SmartCore. lt is
connected to a
motherboard, which
contains a regulated
power
supply,
some support circuitry,
and two
programmable
peripheral
interface (PPl)
lC's.
One PPI is
configured
as a Centronics
parallel
interface,
and controls the A/D
subsystem
described
above. The other
PPI interfaces
to the
keyboard
/ display PC
board, described
below.
Software contained
in the ROM,
(also
known as
"firmware"),
performs
the required real{ime
data
collection, measurement,
user
interface,
and
output control functions.
The
keyboard / display PC
board
contains virtually all of
the hardware for
the user
interface
(LED
displays
and keyboard),
as well as contact
sensing and output
initiation
circuitry. The
eight LED
digits are multiplexed,
and have limited
alphanumeric
capability
for more flexible
indication
of such conditions
as overrange. The keys
are scanned
at
roughly
30 times
per
second,
and incorporate LED's
to indicate status.
An audible
indicator
(beeper)
sounds when
a key is
pressed.
The contact
sensing circuit-
ry
uses the key scanning
system, with
a transformer for isolation.
The sensing
signal is
a repetitive
pulse
of low voltage
and low current,
and contact
sensing leads may be
handled without
fear
of shock. Two separate
optically isolated
signals are
provided
for
output initiation with
an electromechanical
contactor or
SCR. All functions of
the
keyboard / display
PC board are handled
by a single PPl.
MAC-20 Software
A complete description
of the software is beyond
the scope of this manual;
however,
the following brief
overview may enhance
your
understanding of the MAC-20.
The
SmartCore is based on a Zilog 2180
processor
running at
9.2'16
MHz.
lts instructions,
written in C
and assembly
language,
are
contained
in
a socketed EPROM,
and data is
stored
in
self-contained static RAM. The 21B0
contains a
programmable
reload
timer
(PRT),
which is set to
provide
an interrupt at
a
rate
of 1200
per
second. A watchdog
timer
provides
reset
control and
protection
from software hangup
conditions.
Much of the
processing
takes
place
in the interrupt
service routine
(lSR),
which is
executed 1200 times
per
second. This ISR reads
the A,/D converter,
processes
the
data, and determines the
presence
or absence of current. lt
also
functions
as the main
timer
for
pulse
time measurement,
and determines the repetiiion rate
of other
proc-
esses. The
multiplexing
of the LED display, and the scanning
of
the keyboard,
are also
Electrical Test lnstruments, lnc.
Page ll-2
Copyright(C)
1
994-1998
All Righls
Reserved
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