Entry
What is IDE Interface?
How does IDE Interface work?
What is EIDE?
Mar 14th, 2008 08:39
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Usually, these devices connect to the computer through an Integrated
Drive Electronics (IDE) interface. Essentially, an IDE interface is a
standard way for a storage device to connect to a computer. IDE is
actually not the true technical name for the interface standard. The
original name, AT Attachment (ATA), signified that the interface was
initially developed for the IBM AT computer. In this article, you will
learn about the evolution of IDE/ATA, what the pinouts are and exactly
what "slave" and "master" mean in IDE.
IDE Evolution
IDE was created as a way to standardize the use of hard drives in
computers. The basic concept behind IDE is that the hard drive and the
controller should be combined. The controller is a small circuit board
with chips that provide guidance as to exactly how the hard drive
stores and accesses data. Most controllers also include some memory
that acts as a buffer to enhance hard drive performance. Before IDE,
controllers and hard drives were separate and often proprietary. In
other words, a controller from one manufacturer might not work with a
hard drive from another manufacturer. The distance between the
controller and the hard drive could result in poor signal quality and
affect performance. Obviously, this caused much frustration for
computer users.
The birth of the IDE interface led to combining a controller like this
one with a hard drive. IBM introduced the AT computer in 1984 with a
couple of key innovations.
The slots in the computer for adding cards used a new version of the
Industry Standard Architecture (ISA) bus. The new bus was capable of
transmitting information 16 bits at a time, compared to 8 bits on the
original ISA bus. IBM also offered a hard drive for the AT that used a
new combined drive/controller. A ribbon cable from the
drive/controller combination ran to an ISA card to connect to the
computer, giving birth to the AT Attachment (ATA) interface. In 1986,
Compaq introduced IDE drives in their Deskpro 386. This
drive/controller combination was based on the ATA standard developed
by IBM. Before long, other vendors began offering IDE drives. IDE
became the term that covered the entire range of integrated
drive/controller devices. Since almost all IDE drives are ATA-based,
the two terms are used interchangeably.
Controllers, Drives, Host Adapters
Most motherboards come with an IDE interface. This interface is often
referred to as an IDE controller, which is incorrect. The interface is
actually a host adapter, meaning that it provides a way to connect a
complete device to the computer (host). The actual controller is on a
circuit board attached to the hard drive. That's the reason it's
called Integrated Drive Electronics in the first place!
While the IDE interface was originally developed for connecting hard
drives, it has evolved into the universal interface for connecting
internal floppy drives, CD-ROM drives and even some tape backup
drives. Although it is very popular for internal drives, IDE is rarely
used for attaching an external device.
There are several variations of ATA, each one adding to the previous
standard and maintaining backward compatibility.
The standards include:
ATA-1 - The original specification that Compaq included in the Deskpro
386. It instituted the use of a master/slave configuration. ATA-1 was
based on a subset of the standard ISA 96-pin connector that uses
either 40 or 44 pin connectors and cables. In the 44-pin version, the
extra four pins are used to supply power to a drive that doesn't have
a separate power connector. Additionally, ATA-1 provides signal timing
for direct memory access (DMA) and programmed input/output (PIO)
functions. DMA means that the drive sends information directly to
memory, while PIO means that the computer's central processing unit
(CPU) manages the information transfer. ATA-1 is more commonly known
as IDE.
ATA-2 - DMA was fully implemented beginning with the ATA-2 version.
Standard DMA transfer rates increased from 4.16 megabytes per second
(MBps) in ATA-1 to as many as 16.67 MBps. ATA-2 provides power
management, PCMCIA card support and removable device support. ATA-2 is
often called EIDE (Enhanced IDE), Fast ATA or Fast ATA-2. The total
hard drive size supported increased to 137.4 gigabytes. ATA-2 provided
standard translation methods for Cylinder Head Sector (CHS) for hard
drives up to 8.4 gigabytes in size. CHS is how the system determines
where the data is located on a hard drive. The reason for the big
discrepancy between total hard drive size and CHS hard drive support
is because of the bit sizes used by the basic input/output system
(BIOS) for CHS. CHS has a fixed length for each part of the address.
Look at this chart:
Cylinder
10-bit
1024
Head
8-bit
256
Sector
6-bit
63*
You will note that the number of sectors is 63 instead of 64. This is
because a sector cannot begin with zero. Each sector holds 512 bytes.
If you multiply 1,024 x 256 x 63 x 512, you will get 8,455,716,864
bytes or approximately 8.4 gigabytes. Newer BIOS versions increased
the bit size for CHS, providing support for the full 137.4 gigabytes.
ATA-3 - With the addition of Self-Monitoring Analysis and Reporting
Technology (SMART), IDE drives were made more reliable. ATA-3 also
adds password protection to access drives, providing a valuable
security feature.
ATA-4 - Probably the two biggest additions to the standard in this
version are Ultra DMA support and the integration of the AT Attachment
Program Interface (ATAPI) standard. ATAPI provides a common interface
for CD-ROM drives, tape backup drives and other removable storage
devices. Before ATA-4, ATAPI was a completely separate standard. With
the inclusion of ATAPI, ATA-4 immediately improved the removable media
support of ATA. Ultra DMA increased the DMA transfer rate from ATA-2's
16.67 MBps to 33.33 MBps. In addition to the existing cable that uses
40 pins and 40 conductors (wires), this version introduces a cable
that has 80 conductors. The other 40 conductors are ground wires
interspersed between the standard 40 conductors to improve signal
quality. ATA-4 is also known as Ultra DMA, Ultra ATA and Ultra ATA/33.
ATA-5 - The major update in ATA-5 is auto detection of which cable is
used: the 40-conductor or 80-conductor version. Ultra DMA is increased
to 66.67 MB/sec with the use of the 80-conductor cable. ATA-5 is also
called Ultra ATA/66.
Cable Key
IDE devices use a ribbon cable to connect to each other. Ribbon cables
have all of the wires laid flat next to each other instead of bunched
or wrapped together in a bundle. IDE ribbon cables have either 40 or
80 wires. There is a connector at each end of the cable and another
one about two-thirds of the distance from the motherboard connector.
This cable cannot exceed 18 inches (46 cm) in total length (12 inches
from first to second connector, and 6 inches from second to third) to
maintain signal integrity. The three connectors are typically
different colors and attach to specific items:
The blue connector attaches to the motherboard.
The black connector attaches to the primary (master) drive.
The grey connector attaches to the secondary (slave) drive.
Along one side of the cable is a stripe. This stripe tells you that
the wire on that side is attached to Pin 1 of each connector. Wire 20
is not connected to anything. In fact, there is no pin at that
position. This position is used to ensure that the cable is attached
to the drive in the correct position. Another way that manufacturers
make sure the cable is not reversed is by using a cable key. The cable
key is a small, plastic square on top of the connector on the ribbon
cable that fits into a notch on the connector of the device. This
allows the cable to attach in only one position.
The connector on an IDE cable
Pin Description Pin Description
1 Reset 23 -IOW
2 Ground 24 Ground
3 Data Bit 7 25 -IOR
4 Data Bit 8 26 Ground
5 Data Bit 6 27 I/O Channel Ready
6 Data Bit 9 28 SPSYNC: Cable Select
7 Data Bit 5 29 -DACK 3
8 Data Bit 10 30 Ground
9 Data Bit 4 31 RQ 14
10 Data Bit 11 32 -IOCS 16
11 Data Bit 3 33 Address Bit 1
12 Data Bit 12 34 -PDIAG
13 Data Bit 2 35 Address Bit 0
14 Data Bit 13 36 Address Bit 2
15 Data Bit 1 37 -CS1FX
16 Data Bit 14 38 -CS3FX
17 Data Bit 0 39 -DA/SP
18 Data Bit 15 40 Ground
19 Ground 41 +5 Volts (Logic) (Optional)
20 Cable Key (pin missing) 42 +5 Volts (Motor) (Optional)
21 DRQ 3 43 Ground (Optional)
22 Ground 44 -Type (Optional)
Note that the last four pins are only used by devices that require
power through the ribbon cable. Typically, such devices are hard
drives that are too small (for example, 2.5 inches) to need a separate
power supply.
Masters and Slaves
A single IDE interface can support two devices. Most motherboards come
with dual IDE interfaces (primary and secondary) for up to four IDE
devices. Because the controller is integrated with the drive, there is
no overall controller to decide which device is currently
communicating with the computer. This is not a problem as long as each
device is on a separate interface, but adding support for a second
drive on the same cable took some ingenuity. To allow for two drives
on the same cable, IDE uses a special configuration called master and
slave. This configuration allows one drive's controller to tell the
other drive when it can transfer data to or from the computer. What
happens is the slave drive makes a request to the master drive, which
checks to see if it is currently communicating with the computer. If
the master drive is idle, it tells the slave drive to go ahead. If the
master drive is communicating with the computer, it tells the slave
drive to wait and then informs it when it can go ahead.
The computer determines if there is a second (slave) drive attached
through the use of Pin 39 on the connector. Pin 39 carries a special
signal, called Drive Active/Slave Present (DASP), that checks to see
if a slave drive is present.
Although it will work in either position, it is recommended that the
master drive is attached to the connector at the very end of the IDE
ribbon cable. Then, a jumper on the back of the drive next to the IDE
connector must be set in the correct position to identify the drive as
the master drive. The slave drive must have either the master jumper
removed or a special slave jumper set, depending on the drive. Also,
the slave drive is attached to the connector near the middle of the
IDE ribbon cable. Each drive's controller board looks at the jumper
setting to determine whether it is a slave or a master. This tells
them how to perform. Every drive is capable of being either slave or
master when you receive it from the manufacturer. If only one drive is
installed, it should always be the master drive.
Many drives feature an option called Cable Select (CS). With the
correct type of IDE ribbon cable, these drives can be auto configured
as master or slave. CS works like this: A jumper on each drive is set
to the CS option. The cable itself is just like a normal IDE cable
except for one difference -- Pin 28 only connects to the master drive
connector. When your computer is powered up, the IDE interface sends a
signal along the wire for Pin 28. Only the drive attached to the
master connector receives the signal. That drive then configures
itself as the master drive. Since the other drive received no signal,
it defaults to slave mode.
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