IDE & SATA Hard Drives

IDE (ATA)

The Integrated Drive Electronics (IDE) interface — also known as AT Attachment (ATA) — got its name because most of the controller circuitry for the drive motors is placed on the circuit board.  This is in contrast to earlier PC hard drive designs (like the Shugart HDD interface).

Therefore the PC's IDE interface card can be connected to any model of drive that follows IDE data and control specifications, no matter how it works physically.  This became a great advantage when capacity grew larger: many IDE drives have different numbers of physical heads, cylinders and sectors from those reported to the PC's IDE interface card.  In addition, the IDE design was easily adapted to connect CD, DVD and tape drives.

Disks are marked magnetically with low-level formatting codes, which are used to measure rotation speed and platter deformation and define sector sizes.  This should not be confused with high-level formatting, which is the marking process used by an OS (like DOS, Windows or Linux) to organise how files are stored.  Low-level formatting is usually carried out at manufacture and is rarely repeated.

Cabling & jumpers

The basic IDE design uses 16-bit parallel data transfer through a 40-pin ribbon cable.  The ribbon cable can be up to 18" long and attached to an expansion card or directly to IDE interface circuitry on the motherboard.

IDE cables allow two drives to be attached.  To differentiate between them, one is designated master (MA or DS) and the other slave (SL)— note that this does not mean that one drive can control the other, merely that one is first and the other second.

IDE drives usually come configured as master.  To configure the drive as slave you will need to move or remove a jumper at the back or the underside of the drive.  The precise change will vary from drive model to drive model, but the manufacturer usually places a diagram on the drive or marks the jumper block settings clearly.

A third option, cable select (CS), forces the drive to master if pin 28 on the ribbon cable is grounded; if 28 is left disconnected, the drive is slave.  Thus a cable with pin 28 grounded on only one connector block is used for CS-ed drives.  Special cables with dual-wire-28 allow this status to be switched if necessary.

Data ribbons are usually keyed to ensure that they only fit in one way round.  If the key is missing, a visual clue to where the key would have been is provided by two grooves.

The red, pink or dotted red side of the ribbon matches cable pin 1 on the connector.  Note that some manu-facturers block off pin 20, which is not used, as a secondary key.

Cables can have keying the wrong way round.  Such cables should be destroyed to avoid confusion.

Partitions are allocated drive letters by MS-DOS as follows: 1st partition on master is C:, 1st partition on slave is D:.  After this, letters are allocated to all further partitions on the master, then all further partitions on the slave.  MS-DOS will recognise a maximum of 24 partitions.

Enhanced IDE (EIDE)

EIDE (also called ATA-2) extended the IDE design to allow a second cable and thus a further two drives.  The cables are referred to as primary IDE and secondary IDE.  (These may be indicated on PC interface connectors as IDE0 / IDE1 or IDE1 / IDE2.)

Another addition with EIDE was the ATA Packet Interface (ATAPI) for CD drives.  Previously, early CD drives required a custom-built interface card.

New DMA high-speed transfer standards were introduced, leading to UltraDMA (UDMA) 66, UDMA 100 and UDMA 133.  These faster transfer speeds required a new 80-pin cable.  The cable looked similar to the 40-pin but with a finer ribbon cable where every other wire is grounded.  Connecting a UDMA 100 drive with a 40-pin cable will limit its speed.

2.5" HDDs

Laptop hard drives are electrically compatible with standard 3.5" drives but not mechanically compatible.  The connector pins are spaced 1 mm apart (as opposed to 0.1" or 2.5 mm apart) and the power connector is integrated into the ribbon cable.

Adaptors can be used to mount 2.5" drives in 3.5" bays.

Serial ATA (SATA)

The parallel design of traditional IDE hard drives meant that drives reached a maximum speed of 133 MBps with UltraDMA 133 mode (also known as ATA-7).

The solution to this speed barrier was Serial ATA (SATA).  It uses much lower signal voltage levels, which allow faster signalling.  It also reduces the electrical problems associated with parallel transmission by using balanced, full-duplex serial transmission.  Although serial is much slower than parallel (typically a twentieth of the speed) the lower signalling voltage means that the increased transmission rate makes up for this.

Otherwise, SATA is backward-compatible with parallel ATA.  Indeed, any traditional IDE device — including CD/DVD and tape drives — can connect to SATA with a cheap adaptor.

Cabling

SATA cables use thinner cables with keyed connectors, and each device has an individual data cable.  This gives better airflow in the system unit case, ensures that cables are in the right way round, and removes the need to define master and slave drives.

Most motherboards released in the last few years have direct support for SATA and the new standard will eventually replace parallel IDE completely.

The data signal cable contains 7 wires and the end connector is designed to connect ground wires first, potentially allowing hot-plugging.  Both ends of the cable are identical so the cable may be connected the wrong way round, and the serial design allows cables of up to 1 metre.

The 15-pin power cable provides 3.3 V, 5 V and 12 V power.  Each wire can carry 1.5 A of current and three wires are used for each voltage.  Some manufacturers support this power connector and others still use traditional power connectors; some drives have both.  Adaptors are available.

PCI SATA cards may be added to non-SATA motherboards.  Most of these cards offer simple RAID implementation.

The future

SATA version 1 drives are rated to transfer data at 150 MBps but SATA version 2 drives run at 300 MBps; 600MBps is due in 2007.  (However, these speed ratings are ideal: in reality, 150 MBps drives transfer less than 50 MBps on average.)

Text by John King, illustrations pinched from Scott Mueller's Upgrading & Repairing PCs (15th Edition)