A+ (2006-07) / Lessons

Connectors

I/O overview

PCs feature a variety of input and output (I/O) connections.  Each physical port is the public connection point to an interface circuit that deals with data signals travelling in and out.  The diagram below shows the I/O ports typically fitted to a modern ATX motherboard.

From left to right, the connectors are:

When ordering cables and replacement connectors the PC technician needs to know the correct names for each and the differences between similar connectors and cables.  This is not as easy as it sounds, because ports are often known by a variety of names.

Audio

The primary connector for PC audio is the stereo 3.5mm jack.  Its three sections (left, right and ground respectively) carry a low-voltage analogue signal.

Some soundcards also feature additional S/P-DIF (Sony/ Philips Digital Interconnect Format) outputs for digital signals.  These use standard RCA phono or optical fibre TOSlink connectors to connect to CD or Mini-Disc players.

Video

Analogue

VGA

Early PC video cards — such as the monochrome Hercules & MDA cards, colour CGA & EGA cards, and very early VGA cards — used 9-pin D connectors.

When IBM introduced its PS/2 computer in the late 1980s it used an improved VGA connector based around a 15-pin high-density D connector (abbreviated as HD-DB15).  This type of connector quickly became the standard video connection and is referred to as a VGA connector.

Composite video & UHF

Composite video (CV) combines the picture signals and synchronisation pulses into a single signal, transmitted down a coaxial cable.  CV connections use an RCA phono socket and occasionally BNC connectors on very old equipment.

UHF connections are designed for older televisions and video recorders.  The video signals are shifted onto a carrier wave that enabled them to be fed down a TV aerial cable.  The picture quality is usually poor.

S-Video

S-Video (also called S-VHS) uses 4-pin mini-DIN connectors to carry picture signals.

Component video / RGB

When signals are multiplexed (mixed) or fed through wires that are in close proximity, some interference and degradation can occur.  Component video uses separate, screened, red, green and blue signal wires to connect to the display.  This gives a very high quality image.

Component video usually uses coaxial cable and RCA phono plugs or BNC connectors.

Digital

Modern video cards often feature a digital connector for flat-screen displays.

DVI

The early Digital Flat Panel (DFP) interface used an HPCN20 connector but was soon superseded by the modern Digital Visual Interface (DVI) interface and its custom DVI connector.  DVI-D connection is digital-only but DVI-I uses an extra four pins to carry an analogue signal.

HDMI

HDMI (High-Definition Multimedia Interface) is a fully-digital connection that can carry standard or high-definition video signals.  The speed of transmission means that cable length is usually limited to around 5 metres.

HDMI can carry encrypted HDCP (High-Definition Content Protection) data, ensuring that video signals are decoded only at the display screen.  This makes it much harder to "rip" video to another format.

Data

Parallel port

The standard printer interface for many years was based on a design by the Centronics printer company.  This carried an 8-bit data signal and used a strobe signal to ratify transmission.  Various handshaking wires allowed the computer to check the printer status.

This Standard Parallel Port (SPP) has been used since the inception of the PC and has not yet been fully replaced by USB.

SPP printer cables feature a male 25-pin D connector (DB25M) at the PC end and a male 36-pin IEE488 — commonly called a Centronics connector — at the printer end.  To confuse matters, the DB25 connector is sometimes misnamed a Centronics connector.

Parallel cables suffer from problems with crosstalk (interference from electrical signals on nearby wires) and capacitive attenuation.  Therefore they are typically limited to around 3m (10 feet) length.

SPP can output 150 KBps but its input capabilities are limited.  Therefore, better bi-directional designs became popular, like Enhanced Parallel Port (EPP) and Enhanced Capabilities Port (ECP).  These use the same DB25 connectors as SPP but must use bidirectional (i.e. not traditional SPP) cables.  EPP & ECP standards are governed by the IEEE-1284 committee.

Historically PC line printer ports are given the device names LPT1, LPT2, etc.

Serial port

Synchronous serial transmission uses a common clock at transmitter and receiver to synchronise signal timing.

Asynchronous transmission uses separate clocks at transmitter and receiver.  These clocks may slip out of synch but must be close enough to remain in step for short periods of time.  Data is sent in short bursts (usually 8 bits).

Sometimes the rest state between bursts could be misinterpreted as 0s, so a start bit (value 1) is used to warn that valid data follows.  For similar reasons, the burst is concluded by one or more stop bits (value 0).

Serial transmission is slower than parallel transmission but can carry for longer distances.  Transmission speed is measured in bits per second (bps) but are also quoted in signals per second (baud).

Most of the motherboard's circuits use parallel transmission buses and these need to be converted to asynchronous serial format at interfaces.  The chip used for this purpose is called a Universal Asynchronous Receiver/ Transmitter (UART).  Popular UARTs used in PCs include the 8250 and 16550 chips.

RS-232 COM ports

The RS-232 asynchronous serial standard (also called V.24) has been used for decades and was designed to allow a computer (DTE, data terminal equipment) to control a modem (DCE, data communications equipment).

Instead of using traditional 1 = 5V, 0 = 0V signals, RS-232 uses 1 = –12V and 0 = +12V, which allows cables to easily reach 15m (50 feet).  The left-hand diagram shows standard RS-232 cable wiring.

For serial connections, very old PCs use male 25-pin D connectors (DB25M); most use male 9-pin D connectors (DB9M).  Because MS-DOS calls these RS-232 serial communication devices COM1, COM2, etc., they are known as COM ports.

Where PCs are physically close, they can be linked by crossover cables known as null modem cables.  The right-hand diagram shows the wiring for a null modem cable.

 

A small inline crossover adaptor can convert normal serial cables into to null-modem cables.

USB

Since its introduction in 1995 the Universal Serial Bus has become the predominant connection system for PCs, laptops, many peripherals and a range of home entertainment systems.  By 2004 there were over one billion USB devices in the world.

The heart of the USB bus is the host controller (root hub) inside the PC.  It connects to peripherals directly, or via hubs or by daisy-chaining.  Up to 127 peripheral devices (including hubs) can be connected to a single host; modern PCs usually contain several host controllers.

USB devices are designed to be hot swappable: when a device is attached the host enumerates it (i.e. identifies it with a unique number) and loads the appropriate device drivers.  Note that some equipment requires the driver to be installed before connection.

There are four main styles of connector:

USB cables can reach for a maximum length of 5 metres (approx. 15 feet) and the design allows it to carry up to 500mA of electrical current to power devices.

Versions 1.0 and 1.1 of the USB design (USB1.0, USB1.1) gave a maximum data transfer rate of 12 Mbps (although slower devices such as keyboards communicate at a special lower speed of 1.5 Mbps).  Using multiple high data bandwidth devices (such as webcams) can 'clog' the connection, causing problems.

Version 2 (USB2 or "Enhanced USB") increased the data speed to 480 Mbps.  These high-speed devices must also support the lower 12 Mbps rate if plugged into a USB1 bus.

FireWire (IEEE-1394)

FireWire is a similar system to USB and was also introduced in 1995.  However, despite its technical superiority, the licensing costs stopped it becoming as popular as USB and it is now primarily used for video cameras.

Officially called IEEE-1394, it was called FireWire by Apple and i.Link by Sony, and it is designed as a replacement for external SCSI interfaces.

Up to 63 devices can be attached to the FireWire bus and it can be used to link computers directly (unlike USB, which requires a bridging device).  Although it supports hot swapping it can supply up to 45watts of power to devices — leading to some reported problems with arcing and damage to equipment.  This means that many manufacturers recommend switching equipment off before plugging or unplugging.

FireWire 400 (IEEE-1394a) runs at 100, 200 or 400 Mbps.  Cables are limited to 4.5 metres (14feet) length.

FireWire 800 (IEEE-1394b) runs at 800 Mbps and also supports optical cables of up to 100 metres long.

Game port

The game port on PCs has been largely superseded by USB but older machines still have this female 15-pin D connector (DB15) port.  It is used primarily for joysticks but can also be used for MIDI connections to synthesizers and drum machines.

Infrared ports

The IrDA standard allows devices to communicate with PCs, laptops and PDAs using a beam of pulsed infrared light, much like that generated by a remote control.  Speed varies between 115 Kbps and 4 Mbps and the connection distance usually varies between 1 and 2 metres.