Learning Resources

Drive Connector and Cables


The desktop computer hard-drive connector is pictured here. It has 4 conductors, with the standard pinout as follows:

Pin # Color Function
1   Yellow +12 V
2   Black Ground
3   Black Ground
4   Red +5 V

Sometimes, especially in older computers, the colors differ. The pins are 0.200 in (5.08 mm) apart (center to center). The connector housing has chamfered corners on one side to prevent the user from plugging it in incorrectly. The connector that provides power (e.g., on a power supply) has female pins and a male housing; the connector that receives power (e.g., on a peripheral) has male pins and a female housing.

The connector is standard on all PATA disk drives and low-end SCSI disk drives; however, newer SATA disk drives will employ a more advanced interconnection with 15 contacts. These new, advanced connection systems are being developed by Molex and other connector companies, often working together to develop interconnection standards.

Despite its widespread adoption, the connector has problems as a 30-year-old product. It is cumbersome and difficult to remove because it is held in place by friction instead of a latch, and some poorly constructed connectors may have one or more pins become unattached from the connector when plugged in.

Insertion may also be difficult due to the tendency for the loosely inserted pins to come askew, preventing easy insertion into the female connector.

Additionally, over a long period of time the receiving socket can spread, making the connection imperfect and subject to arcing. It is strongly advised that, whenever working with these connectors, you check for any sign of blackening or browning on the white plastic shell: This is often a sign that arcing is happening and the connector needs replacing. In extreme cases (for example in vintage pinball machines which often use this style of connector) the whole connector can melt due to the heat from arcing.

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Hard disk drives have basically two connectors, one for power and other for exchanging data with the computer. This second connector is better known as “interface”. The most common hard disk drive interface for end-users is called ATA (Advanced Technology Attachment), while SATA (Serial ATA) interface was created to replace ATA and is becoming more popular these days. After the released of SATA, ATA interface started being also called PATA (Parallel ATA). Another famous interface is called SCSI (Small Computer Systems Interface), but it is targeted to servers and rarely seen on PCs targeted to end-users.

The master/slave jumper on ATA hard drives can be configured in three different ways:

  • Master: this means that this drive will be the only one attached to the cable that connects the hard drive to the computer or will be the first drive in a two-drive configuration.
  • Slave: this means that this drive will be the second drive attached to the cable that connects the hard drive to the computer.
  • CS (Cable Select): this means that you will use a “special” cable (called CS cable) that the configuration of whether a drive will be master or slave will be made by the position of the hard drive on the cable and not by a jumper configuration on the drive.


HDDs are accessed over one of a number of bus types, including as of 2011 parallel ATA (PATA, also called IDE or EIDE; described before the introduction of SATA as ATA), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses with which they cannot communicate natively, such as IEEE 1394, USB and SCSI.

Modern hard drives present a consistent interface to the rest of the computer, no matter what data encoding scheme is used internally. Typically a DSP in the electronics inside the hard drive takes the raw analog voltages from the read head and uses PRML and Reed–Solomon error correction90] to decode the sector boundaries and sector data, then sends that data out the standard interface. That DSP also watches the error rate detected by error detection and correction, and performs bad sector remapping, data collection for Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks.

Modern interfaces connect a hard disk drive to a host bus interface adapter (today typically integrated into the "south bridge") with one data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit.

  • Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, was standard on servers, workstations, Commodore Amiga, and Apple Macintosh computers through the mid-1990s, by which time most models had been transitioned to IDE (and later, SATA) family disks. The range limitations of the data cable allows for external SCSI devices.
  • Integrated Drive Electronics (IDE), later standardized under the name AT Attachment (ATA, with the alias P-ATA or PATA (Parallel ATA) retroactively added upon introduction of SATA) moved the HDD controller from the interface card to the disk drive. This helped to standardize the host/contoller interface, reduce the programming complexity in the host device driver, and reduced system cost and complexity. The 40-pin IDE/ATA connection transfers 16 bits of data at a time on the data cable. The data cable was originally 40-conductor, but later higher speed requirements for data transfer to and from the hard drive led to an "ultra DMA" mode, known as UDMA. Progressively swifter versions of this standard ultimately added the requirement for an 80-conductor variant of the same cable, where half of the conductors provides grounding necessary for enhanced high-speed signal quality by reducing cross talk.
  • EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA eliminates the need for the CPU to copy byte per byte, therefore allowing it to process other tasks while the data transfer occurs.
  • Fibre Channel (FC) is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, and it is the cornerstone of storage area networks (SANs). Recently other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly, drives usually use copper twisted-pair cables for Fibre Channel, not fibre optics. The latter are traditionally reserved for larger devices, such as servers or disk array controllers.
  • Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses a mechanically identical data and power connector to standard 3.5-inch SATA1/SATA2 HDDs, and many server-oriented SAS RAID controllers are also capable of addressing SATA hard drives. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.
  • Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. A similar differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI.