Figure 7-17.—Independent request control.
. Procedural —The procedural characteristics
define how signals are to be exchanged and the
environment necessaryto input and output data.
No matter the format, I/O interfacing components
are generally used by most computers regardless of the
computer type.
I/O INTERFACING COMPONENTS
The computer’s I/O processor, regardless of the
type of computer and regardless of the type of format
(serial or parallel) 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/pcb’s that make
up an IOA. Some of the primary I/O interfacing
hardware includes universal receiver transmitters, line
drivers, and line receivers.
Universal Receiver-Transmitters
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. Universal receiver-transmitters come in
three types: universal asynchronous receiver-
transmitters (UARTs), universal synchronous receiver-
transmitters (USRTs), and universal synchronous/
asynchronous receiver-transmitters (USARTs). A
UART, USRT, or USART may be built into the
computer or added as part of an I/O pcb or serial
interface board. Modern UARTs, USRTs, or USARTs
may consist of a single IC chip.
We take a look at a USART as an example of this
type of logic assembly. The USART is designed to
function as a peripheral device to the microprocessor.
The microprocessor transmits byte-oriented data (data
and command/control words) to the USART and
receives byte-oriented data (data and status words) from
the USART. The actual conversion from serial to
parallel or parallel to serial is performed by the USART
and is transparent to the microprocessor. The standard
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