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.
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