Intro to Computer Systems

Chapter 2: System Basics

Legacy Connections

A "legacy" interface is one that is a hangover from a previous generation of computing, which has since been replaced by a newer equivalent. In some cases, however, the legacy interfaces persist in the marketplace for a number of reasons:

Many legacy interfaces are electrically (and mechanically) simple, and this has promoted their longevity: they are not only cheap and easy to implement, but also very easy to build peripherals for.

This computer motherboard has legacy keyboard and mouse (purple and green) and parallel (red) ports.
This computer motherboard has legacy keyboard and mouse (purple and green) and parallel (red) ports.

The three main legacy interfaces that still exist today are those for keyboards and mice, parallel and serial communications.

Keyboard and Mouse Connections

Keyboards and mice are very undemanding from an interface point of view: they don't require much data to be transmitted (low bandwidth), and they don't require a connection with low latency. There are two legacy connections for keyboards; the AT and PS/2 standards.

An AT keyboard connector.   A PS/2 keyboard (or mouse) connector.   A USB connector.
Connectors for an AT keyboard (left), PS/2 keyboard or mouse (centre), and a USB device (right).

the AT standard, in common use from the IBM PC AT in 1984, up until the early 1990s, uses a large 5-pin circular connector known as "DIN-5".

There was an even older keyboard connector, the "XT standard", used on the first IBM PC computers. It uses the same connector as the AT keyboards, but is electrically incompatible. "XT standard" keyboards have long since disappeared.

During the transition from XT to AT systems in the mid-80s, keyboards often had a switch that let them work with both types of computer system.

All these legacy interfaces are cold swappable, and particularly susceptible to electrical damage from hot-swapping. Before the advent of ADB (on Macintosh systems) and the PS/2 mouse port (on IBM PC systems), mice were connected using a standard serial interface, described in detail below.

These interfaces have since been replaced by the USB (Universal Serial Bus) standard.

Parallel Communication

The legacy parallel port was most often used to connect computers to a printer. Parallel ports could also be used to transfer data between computers relatively quickly for the time. At its development peak, parallel ports could deliver data at speeds of up to 4 megabits (million bits) per second.

Being a parallel interface, many (25) wires were needed, resulting in chunky connectors and thick cables that could not span more than a couple of metres.

A parallel printer cable. The wider connector, known as a Centronics plug, connects to the printer.
A parallel printer cable. The wider connector, known as a Centronics plug, connects to the printer.

One big advantage of the parallel port was that it is very easy to build electronics for; many enthusiasts were attracted to this ease of development, and created many home-built computer peripherals.

The pinout for a DB25 (25-pin "D" connector) parallel port.

The diagram above is that of a DB25 parallel port, commonly seen on most older PCs (and some modern business and specialty PCs). D0 to D7 are the eight data lines, each with a corresponding ground pin. The strobe signal defines the timing of the parallel signal, and other pins such as ACK (acknowledge) and BUSY (device is busy) are for flow control - making sure data is properly understood, and sent at the right time.

Parallel ports and cables with proper grounding can be hot-swapped relatively safely; however there is no guarantee in the standard that this is so, or that the software running on the computer system will tolerate it gracefully.

Serial Communication

A serial cable.
A serial cable.

The legacy serial port protocol (known as RS-232) has been around since the 1960s. Originally, it was used to connect display terminals to mainframe computers. In the context of desktop computing, it is most commonly used to connect a modem to a computer.

The standard calls for a 25-pin connector similar to that of a parallel port; however many computer systems (and virtually all modern systems) use a more compact 9-pin port.

The standard as used on IBM PCs is rated for speeds of up to 115 kilobits per second. Older Apple Macintosh machines had an enhanced serial port which allowed for faster data transfer (230kbit/sec). This interface was fast enough for a basic networking scheme known as LocalTalk.

The pinout for a DB9 (9-pin "D" connector) serial port.

Above is a diagram of 9-pin serial port. The only three lines that are actually necessary for data transmission are Transmit, Receive, and Ground. The other pins are for flow control, which may or may not be used, depending on the protocol of the communication scheme being used.

Like parallel ports, serial ports and cables with proper grounding can be hot-swapped in relative safety. The same caveats apply with regard to standardisation and software compliance.