Lesson 10: Interfacing and Interrupts
From "AMERICAN LABORATORY"
by David G. Larsen, Peter R. Rony, and Jonathan A. Titus
Interfacing a Digital Multimeter
THIS MONTH, we shall discuss
the interfacing of an 8080based
microcomputer to a
very versatile laboratory instrument,
the Keithley model 160B
digital multimeter with model
l602B digital output. After a year's
use, we have found this multimeter
to be an excellent example of what
manufacturers can do to facilitate
the interfacing of their instruments.
The model l60B is a general-purpose
3(1/2)-digit multimeter that can
function as a de voltmeter, dc
ammeter, or ohmmeter. A total of
26 different ranges exist for the
multimeter in its three modes of
operation. The lowest range scales
provide maximum readings of
1.999 mv, 19.99 nA, and 1.999 Q.
The 1.999 mv scale has an accuracy
of ±0.1% of reading ±l digit. Thus,
a display reading of 1.000 mv will
have an uncertainty of ±0.002 mv,
or 2 J.1v. The highest possible readings
associated with the three different
modes of operation are 1200
v, 1999 mA, and 1999 MQ, with
the megohm reading being accurate
to only ±30%. This multimeter can
be viewed as the digital complement
of the ubiquitous multirange
chart-paper recorder. The multimeter
is basically a sophisticated
analog-to-digital converter (ADC)
that can handle most laboratory
requirements for digital data acquisition
provided that the data acquisition
rate is no greater than one
data point per second. * Switching
between the 26 different ranges is
performed manually. We would expect
that, in the future, such
switching will be performed by a
built-in microprocessor operating
under the control of an external
computer.
Read More: Interfacing A Digital Multimeter.pdf [45.2KB]
Microcomputer Interrupts
THIS month's column is the
first of several that will focus
on the concept of an interrupt.
When used in the context of a
computer, an interrupt can be defined
as the suspension of normal
program execu tion in order to handle
a sudden request for service, i.e.,
assistance, by the computer. At the
completion of interrupt service, the
computer resumes the interrupted
program from the point where it was
interrupted;' This specific use of
interrupt is consistent with the general
meaning of the term: to stop a
process in such a way that it can be
resumed.
A given computer will typically
communicate with avariety of external
I/O "devices." If it is a minicomputer,
it may communicate with a
Teletype or alphanumeric keyboard,
a CRT display, a printer, a
floppy disk, and perhaps one or
more laboratory instruments. If it is
a microcomputer, it may communicate
with smaller devices-motors,
solid-state relays, push-button
switches, display lights, etc.-within
a larger machine or instrument.
When used as a replacement for
discrete logic devices in a complex
digital circuit, a microcomputer may
communicate with other TTL-integrated
circuit chips such as latches,
flip-flops, and three-state buffers.
When communicating with external
I/O devices,2 microcomputers
can operate in two general modes,
polled and interrupt. Polling is the
periodic interrogation of each I/O
device sharing a communications
link to a microcomputer in order to
determine if it requires servicing. A
microcomputer sends a poll that has
the effect of asking the selected
device, "Do you have anything to
transmit?," "Are you ready to receive
data? ," and similar questions.
When a microcomputer services a
polled device, it simply exchanges
digital information with the device
in a manner that is prescribed by
software in a subprogram or subroutine
called a software driver.
Read More: Microcomputer Iterrupts.pdf [45.2KB]
The Vectored Interrupt
THIS MONTH, we continue
the discussion of computer
interrupts, with emphasis upon
vector interrupt hardware and
software associated with the 8080A
microprocessor chip. The three signals
used in vector interrupt circuits
include INT (input pin 14 on the
8080A chip), INTE (output pin
16), and INTA, not available on the
8080A chip but derived externally
with additional logic.
A positive clock pulse from an
interrupting device supplies a logic
1 state at the INT, or interrupt
request, input that generates an
interrupt request, which the CPU
recognizes either at the end of the
current instruction being executed
or while the CPU is in the halt
state. The INTE, or interrupt enable,
output pin indicates the logic
state of the interrupt enable flipflop
present within the 8080A chip.
This internal flip-flop can be set
(enabled) or cleared (disabled) with
the aid of 8080A microcomputer
instructions :
363 DI Disable interrupt flip-flop
373 EI Enable interrupt flip-flop
When cleared, the interrupt enable
flip-flop inhibits interrupts from
being accepted by the CPU. The
flip-flop is automatically cleared
when an interrupt is accepted; it is
also cleared by the RESET input
signal applied at pin 12 of the
8080A chip.
Read More: The Vectored Iterrupt.pdf [45.2KB]
Interfacing a 10-bit DAC
IN THIS COLUMN we shall discuss how to interface an Analog Devices AD7522 monolithic CMOS
lO-bit multiplying digital-to-analog converter (DAC), a 28-pin chip which is a recent example of a new
generation of inexpensive DACs that can be interfaced directly to 8-bit microcomputers.
A digital-to-analog converter is an electronic device that converts digital signals into analog signals. A typical converter
consists of an arrangement of weighted resistors, each controlled by a single bit of input data that develops varying
output analog voltages or currents in accordance with the digital input code.
Read More: Interfacing A 10 Bit DAC.pdf [165KB]
Interfacing Analog-to-Digital Converters
AN analog-to-digital converter is an electronic device that converts analog
signals to digital signals. Typical commercial converters are based upon the principles of successive
approximation, dual-slope integration, staircase-ramp conversion, or voltage-to-frequency
conversion. I The most common use for an ADC is to convert the output from an analog transducer
or analog instrument into digital form suitable for direct observation on a digital display
or as input into a computer. All digital panel meters and digital rriultimeters contain built-in analog-
to-digital converters. Modern ADCs provide standard TTL outputs which may be coded in
binary, binary coded decimal (BCD), or perhaps other less frequently used codes.
Read More: Interfacing Digital To Analog Converters.pdf [69.7KB]