In computer hardware, a port serves as an interface between the computer and other computers or peripheral devices. Computer ports have many uses, to connect a monitor, webcam, speakers, or other peripheral devices. On the physical layer, a computer port is a specialized outlet on a piece of equipment to which a plug or cable connects.
Electronically, the several conductors where the port and cable contacts connect, provide a method to transfer signals between devices.
In this day’s most desktop and notebook computers use only USB, VGA, Ethernet, IEEE 1394, DVI and TRS physical ports. Serial, parallel, PS/2 and SCSI are used more by industrial and professional computers.
Port connectors may be male or female, but female connectors are much more common. Bent pins are easier to replace on a cable than on a connector attached to a computer, so it was common to use female connectors for the fixed side of an interface.
Computer ports in common use cover a wide variety of shapes such as round (PS/2, etc.), rectangular (FireWire, etc.), square (Telephone plug), trapezoidal (D-Sub — the old printer port was a DB-25), etc. There is some standardization to physical properties and function. For instance, most computers have a keyboard port (currently a Universal Serial Bus USB-like outlet referred to as USB Port), into which the keyboard is connected.
Electrical Signal Transfer
Electronically, hardware ports can almost always be divided into two groups based on the signal transfer:
- Serial ports send and receive one bit at a time via a single wire pair (Ground and +/-).
- Parallel ports send multiple bits at the same time over several sets of wires.
After ports are connected, they typically require handshaking, where transfer type, transfer rate, and other necessary information is shared before data are sent.
Hot-swappable ports can be connected while equipment is running. Almost all ports on personal computers are hot-swappable.
Plug-and-play ports are designed so that the connected devices automatically start handshaking as soon as the hot-swapping is done. USB ports and FireWire ports are plug-and-play.
Auto-detect or auto-detection ports are usually plug-and-play, but they offer another type of convenience. An auto-detect port may automatically determine what kind of device has been attached, but it also determines what purpose the port itself should have.
As of 2006, manufacturers have nearly standardized colors associated with ports on personal computers, although there are no guarantees. The following is a short list:
- Orange, Purple or Grey: Keyboard PS/2
- Green: Mouse PS/2
- Blue or Magenta: Parallel printer DB-25
- Amber: Serial DB-25 or DB-9
- Pastel Pink: Microphone 1/8″ stereo (TRS) minijack
- Pastel Green: Speaker 1/8″ stereo (TRS) minijack
Types of Port
Digital Visual Interface (DVI)
Digital Visual Interface (DVI) is a video display interface developed by the Digital Display Working Group (DDWG). The digital interface is used to connect a video source, such as a display controller to a display device, such as a computer monitor. It was developed with the intention of creating an industry standard for the transfer of digital video content.
The interface is designed to transmit uncompressed digital video and can be configured to support multiple modes such as DVI-D (digital only), DVI-A (analog only), or DVI-I (digital and analog).
DisplayPort is a digital display interface developed by the Video Electronics Standards Association (VESA). The interface is primarily used to connect a video source to a display device such as a computer monitor, though it can also be used to carry audio, USB, and other forms of data.
The VESA specification is royalty-free. VESA designed it to replace VGA, DVI, and FPD-Link. DisplayPort is backward compatible with VGA and DVI through the use of adapter dongles.
DisplayPort is the first display interface to rely on packetized data transmission, a form of digital communication found in technologies including Ethernet, USB, and PCI Express. It allows both internal and external display connections and, unlike legacy standards where differential pairs are fixed to transmitting a clock signal with each output, the DisplayPort protocol is based on small data packets known as micro packets, which can embed the clock signal within the data stream, allowing higher resolutions with fewer pins. The use of data packets also allows DisplayPort to be extensible, meaning additional features can be added over time without significant changes to the physical interface itself.
Standardized in 2004, eSATA (e standing for external) provides a variant of SATA meant for external connectivity. It uses a more robust connector, longer shielded cables, and stricter (but backward-compatible) electrical standards. The protocol and logical signaling (link/transport layers and above) are identical to internal SATA. The differences are:
- Minimum transmit amplitude increased: Range is 500–600 mV instead of 400–600 mV.
- Minimum receive amplitude decreased: Range is 240–600 mV instead of 325–600 mV.
- Maximum cable length increased to 2 metres (6.6 ft) (USB and FireWire allow longer distances.)
- The eSATA cable and connector is similar to the SATA 1.0a cable and connector, with these exceptions:
- The eSATA connector is mechanically different to prevent unshielded internal cables from being used externally. The eSATA connector discards the “L”-shaped key and changes the position and size of the guides.
- The eSATA insertion depth is deeper: 6.6 mm instead of 5 mm. The contact positions are also changed.
- The eSATA cable has an extra shield to reduce EMI to FCC and CE requirements. Internal cables do not need the extra shield to satisfy EMI requirements because they are inside a shielded case.
- The eSATA connector uses metal springs for shield contact and mechanical retention.
- The eSATA connector has a design-life of 5,000 matings; the ordinary SATA connector is only specified for 50.
IEEE 1394 Interface (FireWire)
IEEE 1394 is an interface standard for a serial bus for high-speed communications and isochronous real-time data transfer. It was developed in the late 1980s and early 1990s by Apple, who called it FireWire. The 1394 interface is comparable to USB though USB has more market share. Apple first included FireWire in some of its 1999 Macintosh models, and most Apple Macintosh computers manufactured in the years 2000 – 2013 have included FireWire ports. However, as of 2014, FireWire is being replaced by the Thunderbolt interface on most new Macs. The 1394 interface is also known by the brand i.LINK (Sony), and Lynx (Texas Instruments). IEEE 1394 replaced parallel SCSI in many applications, because of lower implementation costs and a simplified, more adaptable cabling system. The 1394 standard also defines a backplane interface, though this is not as widely used.
IEEE 1394 is the High-Definition Audio-Video Network Alliance (HANA) standard connection interface for A/V (audio/visual) component communication and control. FireWire is also available in wireless, fiber optic, and coaxial versions using the isochronous protocols.
The PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987. The PS/2 mouse connector generally replaced the older DE-9 RS-232 “serial mouse” connector, while the PS/2 keyboard connector replaced the larger 5-pin/180° DIN connector used in the IBM PC/AT design. The PS/2 designs on keyboard and mouse interfaces are electrically similar and employ the same communication protocol. However, a given system’s keyboard and mouse port may not be interchangeable since the two devices use a different set of commands.
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time (in contrast to a parallel port). Throughout most of the history of personal computers, data was transferred through serial ports to devices such as modems, terminals and various peripherals.
While such interfaces as Ethernet, FireWire, and USB all send data as a serial stream, the term “serial port” usually identifies hardware more or less compliant to the RS-232 standard, intended to interface with a modem or with a similar communication device.
Modern computers without serial ports may require serial-to-USB converters to allow compatibility with RS 232 serial devices. Serial ports are still used in applications such as industrial automation systems, scientific instruments, point of sale systems and some industrial and consumer products. Server computers may use a serial port as a control console for diagnostics. Network equipment (such as routers and switches) often use serial console for configuration. Serial ports are still used in these areas as they are simple, cheap and their console functions are highly standardized and widespread. A serial port requires very little supporting software from the host system.
USB (Universal Serial Bus)
Universal Serial Bus (USB) is an industry standard developed in the mid-1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication, and power supply between computers and electronic devices.
USB was designed to standardize the connection of computer peripherals (including keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters) to personal computers, both to communicate and to supply electric power. It has become commonplace on other devices, such as smartphones, PDAs and video game consoles. USB has effectively replaced a variety of earlier interfaces, such as serial and parallel ports, as well as separate power chargers for portable devices.
VGA (Video Graphics Array) Connector
A Video Graphics Array (VGA) connector is a three-row 15-pin DE-15 connector. The 15-pin VGA connector is found on many video cards, computer monitors, and high definition television sets. On laptop computers or other small devices, a mini-VGA port is sometimes used in place of the full-sized VGA connector.
DE-15 is also conventionally called RGB connector, D-sub 15, mini sub D15, mini D15, DB-15, HDB-15, HD-15 or HD15 (High Density, to distinguish it from the older and less flexible DE-9 connector used on some older VGA cards, which has the same shell size but only two rows of pins).
VGA connectors and cables carry analog component RGBHV (red, green, blue, horizontal sync, vertical sync) video signals, and VESA Display Data Channel (VESA DDC) data. In the original version of DE-15 pinout, one pin was keyed by plugging the female connector hole; this prevented non-VGA 15 pin cables from being plugged into a VGA socket. Four pins carried Monitor ID bits which were rarely used; VESA DDC redefined some of these pins and replaced the key pin with +5 V DC power supply.
The VGA interface is not engineered to be hotpluggable (so that the user can connect or disconnect the output device while the host is running), although in practice this can be done and usually does not cause damage to the hardware or other problems.
Small Computer System Interface (SCSI)
Small Computer System Interface (SCSI, /ˈskʌzi/ skuz-ee) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives, although not all controllers can handle all devices. The SCSI standard defines command sets for specific peripheral device types; the presence of “unknown” as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements.
SCSI is an intelligent, peripheral, buffered, peer to peer interface. It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner. Up to 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host. SCSI uses handshake signals between devices, SCSI-1, SCSI-2 have the option of parity error checking. Starting with SCSI-U160 (part of SCSI-3) all commands and data are error checked by a CRC32 checksum.
The SCSI protocol defines communication from host to host, host to a peripheral device, peripheral device to a peripheral device. However most peripheral devices are exclusively SCSI targets, incapable of acting as SCSI initiators—unable to initiate SCSI transactions themselves. Therefore peripheral-to-peripheral communications are uncommon, but possible in most SCSI applications. The Symbios Logic 53C810 chip is an example of a PCI host interface that can act as a SCSI target.
HDMI (High-Definition Multimedia Interface)
HDMI (High-Definition Multimedia Interface) is a proprietary audio/video interface for transferring uncompressed video data and compressed or uncompressed digital audio data from an HDMI-compliant source device, such as a display controller, to a compatible computer monitor, video projector, digital television, or digital audio device. HDMI is a digital replacement for existing analog video standards.
HDMI implements the EIA/CEA-861 standards, which define video formats and waveforms, transport of compressed, uncompressed, and LPCM audio, auxiliary data, and implementations of the VESA EDID. CEA-861 signals carried by HDMI are electrically compatible with the CEA-861 signals used by the digital visual interface (DVI). No signal conversion is necessary, nor is there a loss of video quality when a DVI-to-HDMI adapter is used. The CEC (Consumer Electronics Control) capability allows HDMI devices to control each other when necessary and allows the user to operate multiple devices with one remote control handset.
Several versions of HDMI have been developed and deployed since initial release of the technology but all use the same cable and connector. Other than improved audio and video capacity, performance, resolution and color spaces, newer versions have optional advanced features such as 3D, Ethernet data connection, and CEC (Consumer Electronics Control) extensions.
Phone Connector/Audio Connector
In electronics, a phone connector is a common family of connector typically used for analog signals, primarily audio. It is cylindrical in shape, typically with two, three or four contacts. Three-contact versions are known as TRS connectors, where T stands for “tip”, R stands for “ring” and S stands for “sleeve”. Similarly, two- and four-contact versions are called TS and TRRS connectors respectively.
The phone connector was invented for use in telephone switchboards in the 19th century and is still widely used. In its original configuration, the outside diameter of the “sleeve” conductor is 1⁄4 inch (exactly 6.35 mm). The “mini” connector has a diameter of 3.5 mm (approx. 1⁄8 inch) and the “sub-mini” connector has a diameter of 2.5 mm (approx. 3⁄32 inch).