output simultaneously. When the computer uses a serial
configuration, all bits of information are input or output
one at a time.
I/O INSTRUCTIONS All computers have I/O
instructions. Command instructions are executed by
the IOC under the control of the CPUs main program.
They provide control over IOC single-and dual-channel
A chain consists of IOC control words,
command words, output data words, and specified
locations for external status words and data words
returned (input) from the channel.
I/O OPERATIONS Input/output operations are
initiated by the CPU. Computers with an IOC begin I/O
control functions only after an initiate I/O or equivalent
instruction is executed by the CPU. Computer
instructions inform the external equipment which type
of operations to perform with function codes. They also
specify memory areas for input and output information.
OPERATING MODES I/O operations include
both digital and linear ICs. The linear IC circuits are
the first and last type of circuitry the information
interfaces with when entering and leaving the computer.
Registers in I/O operations provide the interfacing
between the CPU, I/O, and memory. They enable and
route control and data information between the CPU,
I/O, and memory using the internal bus system. The
data registers are used to hold or buffer data during
interchanges between the very fast CPU and the slower
The status registers hold
information for the CPU that indicates the operating
condition and current activities of the external
I/O FUNCTIONS The input and output
functions performed by an I/O processor are defined
and enabled through the interpretation and execution of
input/output and/or input/output controller (I/O(C))
commands obtained from main memory.
DIRECT CPU INTERFACE With direct
communication, also called accumulator-based I/O, the
peripheral devices are tied directly into the CPU
communication bus (control bus, data bus, and so forth).
In a simple I/O scheme, the CPU handles all I/O
transactions by executing one or more instructions for
each word of information transferred.
DIRECT MEMORY ACCESS (DMA) DMA
allows blocks of information to be transferred directly
in and out of memory and from and to an external device
without any CPU intervention.
transferred at a speed compatible with that of the
external device. A DMA controller is usually placed
between the external device and the computers bus.
I/O INTERFACING Computers may have a
small number of channels or ports with multiple
equipments connected to each channel; or they may,
particularly in larger computers, have a number of I/O
channels with limited numbers or types of external
equipments on each channel or port.
I/O INTERFACING STANDARDS There are
two major types of computer/external equipment
communication formats: serial and parallel. The
communication formats are governed by the standard
that is identified by the interface. As a general rule, the
standards can be divided into four categories:
mechanical, electrical, functional, and procedural.
I/O INTERFACING COMPONENTS The
computers I/O processor must ensure that the voltage
levels between the computer and the external
equipments are compatible. The primary circuitry that
accomplishes this is located on an I/O pcb or
modules/pcbs that make up an IOA. Some of the
primary I/O interfacing hardware include universal
receiver-transmitters, line drivers, and line receivers.
Within a digital computer, the data is transferred
internally using a parallel format. All the bits of a byte
or memory word are exchanged simultaneously
between registers, buses, and other computer logic. For
the data to be communicated over a serial channel, it
must be converted from parallel to a serial bit stream.
The USART is designed to function as a peripheral
device to the microprocessor. The actual conversion
from serial to parallel or parallel to serial is peformed
by the USART and is transparent to the microprocessor.
The standard USART chip is comprised of logic
circuits, which are connected by an internal data bus.
LINE DRIVERS/RECEIVERS The line
drivers/receivers are designed to send and receive
signals over short or long distances using serial or
Large voltages or currents are
generated from small voltage or current using TTL or
MOS circuitry. The two types most commonly used are
single-ended and differential.
I/O INTERFACE FORMATS There is a variety
of serial and parallel I/O channel formats. Your
computers technical manual will provide the standards
to be used with the cabinet and cable connectors. They
will match the standards that govern the requirements
for parallel and serial interfacing.