Lesson 9: Bus and Input-Output
From "AMERICAN LABORATORY"
by David G. Larsen, Peter R. Rony, and Jonathan A. Titus
Microcomputer I/O Devices
IN THE DISCUSSION of the
structure of a microcomputer,
we described the various data
paths in a microcomputer, including
data input, data output, external
device addressing, in and out
function pulses, and interrupt signals.
These are the vital lines of
communication between the microcomputer
and the "outside world,"
i.e., those signal lines that are necessary
to interface the microprocessing
unit (MPU) to the input/
output (I/O) devices that you
would like to control.
Some useful definitions pertaining
to I/O devices are given in the
table. The traditional view of an I/O
device is that it is somewhat large
or complex. Card readers, magnetic
tape units, cathode-ray tube displays,
and Teletypes certainly fit
such a description. However, a single
integrated circuit chip, such as a
latch, shift register, counter, or
small memory, can also be considered
an I/O device to a computer.
Read More: Microcomputer IO Devices.pdf [33.0KB]
Microcomputer Output Instructions
IN A PREVIOUS COLUMN, we
d.iscussed. different types of
SImple input/output devices
and provided a listing of general
principles of interfacing that apply
to a wide variety of computers.
This month, we would like to explain
how computer instructions,
i.e., software, cause an I/O device
to operate.
The I/O device that we shall
choose for our discussion is the
optically isolated solid-state ac relay.
These relays can control any ac
power device within the output
current ratings of the relay. Typical
solid-state relays are shown in Figures
la-b. Such relays permit a
single TTL output signal of logic 0
or logic I to control up to lOA of
220-v ac power, as is possible with
the Hamlin model 7522 relay
shown at the top middle part of
Figure I a. Internally, each relay
contains a light-emitting diode, a
light-sensitive transistor, a power
triac, and a transparent dielectric
optical path that isolates the digital
and power circuitry and can itself
withstand a voltage difference of at
least 1000 v.
Read More: Microcomputer Output Instruction.pdf [99.9KB]
Accumulator I/O vs Memory I/O
W H EN DATA ARE transmitted
between a microcomputer
and an input/
output device, three actions must
occur sim ultaneously:
1. The microcomputer must
select the specific input/output device
that will either receive or transmit
eight bits of data.
2. The microcomputer must indicate
to the specific input/output
device when the bidirectional data
bus is available for data transmission.
3. The data must be transmitted
between the microcomputer and
the input/output device in ·a very
short period of time, typically of
the order of microseconds.
In previous columns, we have
discussed accumulator I/O, in
which data are exchanged between
the accumulator and an external
I/O device. A significant disadvantage
is associated with such an
interfacing technique in that only a
single origin or destination for data
exists. In addition to the accumulator,
a typical microprocessor chip,
such as the Intel 8080, has a variety
of internal general-purpose registers
that can exchange information with
memory. These registers include the
B, C, D, H, and L registers, each of
which is an 8-bit register. From a
programming standpoint, it would
be very useful to be able to exchange
data between any of these
registers and any external I/O device.
This is the subject of this
month's column.
Read More: Accumulator IO Vs Memory.pdf [48.7KB]