PC Ports

A modern Personal Computer (PC) has Universal Serial Bus (USB) connections, although older serial ports and parallel ports, as fitted to older peripheral devices, are also provided on many machines.

PC Serial Ports

A serial port, unlike a parallel port, conveys each bit in turn along a common circuit. Older interfaces of this type are slow but use convenient low-cost connectors and lightweight cables. The ports fitted to all PCs are based on the RS-232 standard, with data sent in asynchronous form, consisting of a start bit, followed by eight data bits and a stop bit. These short bursts minimise timing problems, avoiding the need for a separate clock circuit. Hence the data can be eventually sent over a two-wire circuit, such as a telephone line connected via a modem.

To ensure data is sent reliably over a link, some form of handshaking is normally used. Software handshaking, operated by special control codes sent between the devices, is effective, but slows down the interface. Hardware handshaking, as employed, for example, between a modem and a computer, is faster but requires additional wires in the cable.

Connection Systems

A standard serial port can provide a point-to-point connection, such as linking a printer to a computer, as shown below:-

To use several devices you must fit a manually-operated serial port switch box or serial port expander, the latter using software to fool the computer into seeing only one device at a time. The box or port expander is connected as shown here:-

Connectors and Cables

The serial ports on most types of PC are based on the RS-232 standard, which is covered elsewhere in these guides. The connection is provided by a 25-way D plug (DB25) or 9-way D plug (DB9), as illustrated below, although the 9-way connector is more common on recent machines.

To connect a device such as a modem to a PC you’ll need a serial cable, which is usually fitted with a DB25 plug at one end and a suitable socket at the other. This kind of cable, which has swapped pin connections, shouldn’t be confused with a parallel cable, which is wired pin-for-pin, although both cables may come with the same connectors,

In trying to make a connection, you may discover that your computer has a DB25 plug but your device has a DB9 socket. To fix this complication you’ll need an adaptor that incorporates a DB9 plug and a DB25 socket . Fortunately, the connections are fully standardised, so this kind of adaptor shouldn’t give you any problems.

Connecting a PS/2 Device

You can also connect a PS/2 keyboard or mouse to a serial port. For this you’ll need an adaptor with a 6-way mini-DIN socket and a DB9 socket . Unfortunately, not all adaptors are suitable for a mouse, which must be connected to either the COM1 or COM2 port, not the COM3 or COM4 port.

PC Keyboard Ports

These special serial ports are also based on the RS-232 standard. Unfortunately, you can’t connect a modern USB keyboard to such a port without some kind of adaptor, should such a device exist.


This form of keyboard connection is provided on older AT-based machines and is designed for a matching PC/AT keyboard. The port appears as a 5-way 180° DIN socket, as shown below:-

PS/2 Connector

This kind of connection is provided on later machines and is designed for a matching PS/2 keyboard. The port appears as a 6-way mini-DIN socket, as shown below:-

To connect a PS/2 keyboard to an AT machine you’ll need an adaptor that incorporates a 6-way mini-DIN socket and a 5-way DIN plug. Similarly, to connect an AT keyboard to a PS/2 machine, you must use an adaptor with a 5-way DIN socket and a 6-way mini-DIN plug. Unfortunately, adaptors of this type aren’t guaranteed to work with all types of keyboard.

PC Mouse Port

In older machines, the mouse is often connected to a standard serial port .Sometimes however, there’s a designated DB9 plug provided for the mouse, as shown below:-

This example is similar to a standard serial port connection, but lacks a CTS circuit on pin 8.

Later computers often have a special PS/2 mouse socket, which has the same connector as a standard PS/2 keyboard port, but with slightly different electrical characteristics.

Early types of mouse are mechanical, using optical encoder wheels coupled by rollers to a moving ball. A modern optical mouse shines a light onto the surface under the mouse, takes around 1,500 pictures per second and then uses digital processing to compute movement and direction.

The resolution of a mouse or alternative controller, indicates its sensitivity to movement, typically between 200 and 400 dots per inch (dpi). The actual response can be influenced by the performance of your computer, which means the behaviour can change if used with another machine.

  PC Parallel Ports

A parallel port, also known as a Centronics port or the IEEE 1284 high-speed parallel interface, can be used to connect a printer or any another suitable device. The interface uses eight data wires, as well as control lines, and is reasonably fast. Most modern ports are capable of bidirectional operation and can convey data over a distance of 15 metres or more.

The PC identifies each parallel port as a logical port, labelled as LPT followed by a number, such as LPT1 or LPT2. In modern machines the port can be configured to work one of the following modes:-

Cables and Connectors

Computers usually have a 25 way D socket (DB25) whilst a printer often has a 36-way Amphenol socket (C36). Hence a standard printer cable, also known as an A-C cable, has a DB25 plug at one end and a C36 plug at the other.

Sometimes, you’ll encounter a printer with a DB25 socket for the parallel connection, in which case you’ll need an A-A cable with a DB25 plug at each end. This must be wired as a parallel cable and not as a serial cable. You should also check that the printer is really a parallel device: if it has a DB25 plug it probably employs a serial connection.

Pin Connections

The pin connections for a C36 connector are as follows:

1Not StrobeInLogic 0 pulse indicates data valid
10Not AcknowledgeOutLogic 0 pulse indicates printer ready
11BusyOutLogic 1 if printer is busy
12PEOutLogic 1 if printer is out of paper *
13SelectOutLogic 1 if printer is on-line *
14Not Autofeed-XTInMay be connected to ground •
18+ 5 voltsInSupply for devices *
19Pin 1 ReturnInFor Not Strobe circuit
20Pin 2 ReturnInFor D0 circuit
21Pin 3 ReturnInFor D1 circuit
22Pin 4 ReturnInFor D2 circuit
23Pin 5 ReturnInFor D3 circuit
24Pin 6 ReturnInFor D4 circuit
25Pin 7 ReturnInFor D5 circuit
26Pin 8 ReturnInFor D6 circuit
27Pin 9 ReturnInFor D7 circuit
28Pin 10 ReturnInFor Not Acknowledge circuit
29Pin 11 ReturnInFor Busy circuit
30Pin 31 ReturnInFor Not Prime circuit
31Not PrimeInLogic 0 pulse to reset (Not Init) *
32Not FaultOutLogic 0 for fault (Not Error) *
35+ 5 voltsOutVia 3.3 kΩ resistor (for some devices)
36Not Slct-InInLogic 1 if DC1 & DC3 enable printer *

* Not always essential and not wired on some devices

 If set to logic 0 at startup a CR should accompany every LF

The Busy and Strobe control lines ensure smooth communication between the PC and printer.

©Ray White 2004.