a small interface card that plugs into the host computers
motherboard. This interface offers transfer rates of up
to 1 MB and can handle hard drives with a maximum
capacity of 300 MB.
Enhanced Integrated Drive Electronics (EIDE)
The Enhanced Integrated Drive Electronics (EIDE)
was developed from the IDE standard. New features
available with EIDE include Plug-n-Play compatibility,
increased maximum drive capacity, faster data
transfers, and the ability to use a CD-ROM or tape drive
with an the interface.
The IDE interface can address a hard drive with a
maximum of 504MB. EIDE increases the maximum
size of a hard drive by using an enhanced BIOS. The
enhanced BIOS uses a different geometry when
communicating with a program than it does when
communicating with the hard drive. For example, the
BIOS will tell a program that a hard drive with 2,000
cylinders and 16 heads is a drive with 1,000 cylinders
with 32 heads. The BIOS controls the address
translation to keep track of where the data is physically
located on the hard drive.
The EIDE interface uses a Programmed
Input/output (PIO) mode to transfer data from the drive.
There are five PIO modes that can be set to control data
transfers. PIO Mode 0 is the slowest with a cycle time
of 600 nanoseconds. Pio Mode 4 has a cycle time of
120 nanoseconds, which is 16.6 megabytes per second.
Most high-end hard drives will support Mode 3 or Mode
4 operations. Using the enhanced BIOS, the hard disk
responds to the Identify Drive command with
information concerning the PIO and DMA modes the
drive can support. The BIOS will automatically set the
PIO mode to match the capability of the drive. If a drive
is set to a higher mode than it is capable of supporting,
data corruption will occur.
I/O SERIAL DATA OPERATIONS
Serial data operations exchange information via a
single path, line, or wire. The channel/port itself is
made up of several wires, but only one is used to transfer
the binary data. Bidirectional channels may use two
wires for data, one for each direction or a single tristate
bidirectional line. The remaining wires are used for
device addressing and to provide the protocol (channel
control) for information exchange. The data is in the
form of an asynchronous or synchronous bit stream.
The bit stream is made up of a sequential series of data
and/or control pulses in one of these two mutually
exclusive formats. Serial data operations can use a
minimum of 4 conductors and up to 37 conductors to
perform serial data operations. Serial operations
generally exchange information between data
communications equipment (DCE) and data
terminal equipment (DTE). The DCE configured
device is considered the controller for the interface.
The DTE is either the computer or a channel
controller. There are variations in the channel pin
connections that depend on the device mode of
operation (DCE or DTE).
Asynchronous Data Exchanges
Asynchronous data is also known as character
framed data; only one character at a time is sent. Each
character is composed of either 7 or 8 bits (depending
upon the coding scheme used), and is identified by a
start and stop bit. At the minimum, each character is
preceded by a start bit and followed by one stop bit.
Asynchronous data transmission protocol allows for a
maximum of the following in sequence: one start bit,
eight data bits, a parity bit, and one stop bit for each
character to be exchanged.
The purpose of a start bit is to notify the modem
that a character is being sent (or received). The bits that
make up the character immediately follow the start bit.
After all these bits have been transmitted, a stop bit is
inserted to indicate the end of the character. Start and
stop bits can immediately follow one another or there
can be a period of idle time following the stop bit,
depending upon the hardware device in use. During an
idle condition, in which no characters are sent, a
continuous MARK signal (equivalent to a logic 1) is
transmitted for one bit time.
transmission is normally used when transmission rates
are between 600 to 2000 bps. The particular format
used varies between computers and may be hardware or
software controlled depending on the type of interface
logic and devices used.
Synchronous Data Exchanges
When more speed is required for sending
information, synchronous data exchanges fulfill the
Synchronous data is also known as
message framed data. The bit stream is divided into
blocks of sequential bits grouped into individual
messages, without the need for start and stop bits.
Again, each character is composed of either 7 or 8 bits.
There are two methods for controlling the exchange of
messages. External control and timing signals may be