The first dot-matrix printers offered by IBM were manufactured by Epson. IBM now makes its own printers, but they still use the same commands as those early Epson machines.

Because of the size of the Epson-compatible market, most printer manufacturers make dot matrix printers that use the same commands and character set.

From the perspective of the user, the printer is a one-way communications device; it simply receives data and delivers a printed version of that data. The printer is, however, a two way communicator, for it also sends information back to the computer. This information consists of various housekeeping functions that keep the computer informed about the status of the printer:

Signal from printer       Meaning
acknowledge           previous data received, ready for more
busy              off line, or printing previously sent data
error              error, off line, or out of paper
out of paper          out of paper
select             printer is on/off line

Printer/Computer Interface

The printer can receive serially, ie. one bit at a time, or by parallel communication in which data is sent one byte at a time.

The advantage of parallel communication is speed; one byte is transmitted in about the time it takes a serial device to transmit one bit.

The primary advantage of serial printers is that they can utilise printer cables of up to 50 feet, while parallel printer cables are generally limited to about fifteen feet.

I/O Cable

In addition to being called simply "the printer cable", the link between any printer and the printer port is often referred to as either an Input/Output cable or as a communications cable.

Parallel Printer Cable

The cable between the parallel port and the printer has a 25-pin male connector at the adapter end and a 36-pin male connector at the printer end. The printer connector is often referred to as a Centronics plug, after the company that popularised its early use.

Serial Printer Cable

The cable configuration between a serial printer and the computer varies according to the adapter used in the computer, ie. 25-pin female to 25-pin male (Asynchronous communications), or 9-pin female to 25-pin male (Serial adapter or port).

The MODE Command

By default your printer is attached to the first parallel port (LPT1).

If a printer is connected to the serial port then the DOS command mode must be used to redirect the output from the parallel port to the serial port. The format of this command is:

mode lpt1 = com1

You cannot redirect a serial port to a parallel port.

Once you have redirected the output you will need to configure the serial port to match the settings on your printer. For example, the following command configures the serial port as 9600-baud, no parity, 8 data bits per character, with1 stop bit:

mode com1:96,n,8,1,p

The p, a retry setting, directs MS-DOS to continue sending the print job until the printer accepts the job.

To discover how your serial printer is configured you will need to consult your printer manual. The settings are usually set by use of the dip switches.

For printers that support Epson-compatible escape sequences, the default is 80 characters across a line and 6 lines per inch of paper. If your printer prints 132 characters per line or 8 lines per inch, use the mode command to set MS-DOS to the new values.

For example, the following command configures MS-DOS for a printer connected to port LPT1 with 132 characters per line and 8 lines per inch:

mode lpt1:132,8

If you use the mode command with no parameters, you receive information about your LPT, COM and CON (console) ports.

You can receive information about a specific port by using the /status switch. For example, use the following command to receive information about LPT1:

mode lpt1 /status

Types of Printers

There are a number of different types of printer available for use with PCs. These include:

• Dot matrix
• Daisy Wheel
• Ink Jet
• Bubble Jet
• Laser Jet

Dot Matrix

The dot matrix printer forms characters on paper by firing a number of needles through an inked ribbon.

Advantages
The dot matrix printer is generally the cheapest form of printer. It is also fast, generally reliable, and can now offer very good quality output. The dot matrix printer offers many different fonts and can produce both text and graphics output. Landscape and portrait output. Some can support 132 column output.

Disadvantages
It does not produce laser quality output. Many do not support cut sheet feeders. Can be noisy. Output speeds vary from 800 cps in draft mode for more expensive printers, to 30 cps in quality mode for cheaper printers.

Daisy Wheel Printer

The daisy wheel printer operates by hammering a character on a wheel against an inked ribbon.

Advantages
Daisy wheel printers are relatively cheap and produce letter quality output. They support cut sheet feeders. They can be used to produce carbon copies. Disadvantages
Daisy wheel printers only support one font at a time. To change a font the print wheel has to be changed. Only support portrait output. Noisy and slow.

Ink Jets and Bubble Jets

These printers work by squirting tiny drops of liquid ink very accurately at the sensitised area of the paper.

Advantages
There is no hammering of the paper, so these printers are near-silent and consume little power.

Disadvantages
They cannot be used to produce carbon copies, and the running costs can be comparatively high, because the reservoir or sometimes the entire printhead of specially formulated ink has to be replenished.

Laser Printers

Advantages
Laser printers produce very high quality output. They are also very fast. Produce both text (in a number of fonts), and graphics output.

Disadvantages
Laser printers are still relatively expensive to purchase, run and service. Cannot be used for carbon copies.