Serial Dot Matrix Printer: How Text & Graphics Form

A serial dot matrix printer forms text and graphics by striking a vertical column of pins against an ink ribbon, pressing it onto paper to create patterns of tiny dots. The printer’s print head moves horizontally across the page, and a microprocessor controls which pins fire at each position, building characters and images from a grid of dots. This mechanism is why dot matrix output has a distinctive, slightly rough texture and why the printers can produce carbon copies.

Formula

The calculator above estimates the effective print speed of a serial dot matrix printer. The formula is:

Speed (cm/s) = Distance (cm) / Time (s)

Where:

• Distance – the length of paper printed, in centimeters.
• Time – the total time taken to print that length, in seconds.

For characters per second (cps), we assume a standard 10 characters per inch (cpi) spacing, which equals about 4 characters per centimeter. So: cps = (Distance × 4) / Time. This is a rough estimate; actual cps varies with font and print quality.

Worked Example

Using the default values from the calculator: Distance = 20 cm, Time = 12 s.

1. Speed = 20 / 12 = 1.67 cm/s (rounded to 2 decimals).
2. Characters per second = (20 × 4) / 12 = 80 / 12 = 6.67 cps (rounded to 1 decimal).

The calculator shows the same results: 1.67 cm/s and 6.7 cps.

How the Print Head Works

The core of a serial dot matrix printer is the print head, which contains a vertical stack of 9, 18, or 24 tiny electromagnet-driven pins. When the printer sends a signal, a pin is pushed forward by a spring, striking the ribbon and paper. The pins are arranged in a single column (or two staggered columns for 24-pin models), and the head moves left to right across the page. At each step, the printer fires a specific combination of pins to form a vertical slice of a character. For example, the letter “A” might be formed by firing pins in a pattern that creates the two diagonal strokes and the crossbar.

The number of pins directly affects resolution. A 9-pin head can form characters with a matrix of about 9×9 dots, which looks coarse. A 24-pin head uses a finer matrix (often 24×18 or more), producing smoother, more legible text. Graphics are printed the same way: the printer interprets an image as a series of horizontal dot rows, and for each row, it fires the pins that correspond to dark pixels.

Text Formation: The Dot Matrix Pattern

Characters are stored in the printer’s read-only memory (ROM) as bitmap patterns. For each character, the ROM contains a grid of bits—each bit tells the printer whether to fire a pin at that location. When you send text to the printer, it looks up the character in its font table, then streams the bit pattern to the print head as it moves.

For a 9-pin printer, a typical character might be 9 dots high and 5 to 7 dots wide. The printer prints the top row of dots for the entire line, then advances the paper slightly and prints the next row. This is why dot matrix printers are sometimes called “line printers” in a loose sense—they print a full line at a time, but the line is built row by row.

Graphics are handled similarly, but instead of character patterns, the printer receives raw bitmaps. The host computer sends a sequence of bytes, each byte representing a vertical column of 8 dots (for a 9-pin printer) or 24 dots (for a 24-pin printer). The printer fires pins according to the bits, building the image one column at a time.

Common Problems and Maintenance

The most frequent issue with dot matrix printers is a broken or bent pin. A damaged pin leaves a white line in the output. You can test for this by printing a solid black block. If a pin is broken, the print head must be replaced—pins are not user-serviceable.

Another common problem is ribbon wear. A faded ribbon produces light, patchy text. Ribbons are consumable and should be replaced when output becomes faint. Ribbon jams can occur if the ribbon is not threaded correctly or if the ribbon cartridge is worn.

Paper jams are less common than with laser printers, but they happen when the tractor feed (the sprocket mechanism that pulls paper) is misaligned. Ensure the paper holes are properly seated on the tractor pins and that the paper path is clear. Also, check the print head gap: if it is too far from the paper, print is faint; too close, and the ribbon may tear.

Choosing a Serial Dot Matrix Printer

When selecting a printer, consider the pin count (9 vs. 24), print speed (measured in characters per second, cps), and interface options. 24-pin printers produce near-letter-quality text and are better for graphics. 9-pin printers are cheaper and faster but coarser.

Key factors:

• Pin count – 24-pin for higher quality, 9-pin for speed and cost.
• Print speed – typically 200–550 cps for draft mode.
• Paper handling – continuous fanfold paper with tractor feed, or single sheets.
• Interface – parallel (Centronics), serial (RS-232), or USB (on newer models).
• Ribbon type – ribbon cartridges are easier to replace than ribbon spools.

For most industrial uses (invoices, shipping labels, multi-part forms), a 24-pin printer with a parallel or USB interface is a safe choice. If you only need basic text on carbon copies, a 9-pin model may suffice.

Costs and Consumables

The initial cost of a dot matrix printer is low compared to laser or inkjet, but ongoing costs include ribbons and paper. Ribbons typically last for 2–5 million characters, depending on quality and print density. A new ribbon costs a small fraction of a printer, so it is a minor expense.

Paper is the bigger variable. Continuous fanfold paper is sold in boxes of 1,000 to 3,000 sheets. It is more expensive per sheet than standard copy paper, but it is necessary for tractor feed. Multi-part carbonless paper is even pricier, but it is the main reason to use a dot matrix printer—no other printer type can print through multiple layers.

Energy consumption is modest—typically 50–100 watts while printing, much less than a laser printer’s warm-up. However, dot matrix printers are noisy, so you may need to factor in soundproofing or place them in a separate room.

Safety and Usage Tips

Dot matrix printers are generally safe, but there are a few precautions:

• Keep fingers clear – the print head moves quickly and can pinch.
• Never open the printer while printing – moving parts can cause injury.
• Use the correct ribbon – a wrong ribbon may jam or damage the head.
• Allow ventilation – the print head gets hot during long runs; do not block air vents.
• Disconnect power before cleaning – use a soft brush or compressed air to remove paper dust.

Also, avoid printing on labels that are not designed for dot matrix—the pins can tear them. If you print on multi-part forms, ensure the paper thickness is within the printer’s range (usually up to 0.5 mm).

Frequently Asked Questions

Can a dot matrix printer print graphics?

Yes, it can print graphics, but the quality is limited by the pin resolution. A 24-pin printer can produce acceptable logos and bar codes, while a 9-pin printer will look jagged. For high-resolution graphics, use a laser or inkjet printer.

Why do dot matrix printers use continuous paper?

Continuous fanfold paper has sprocket holes that engage with the tractor feed, ensuring precise paper advancement. This is essential for multi-part forms and for maintaining alignment over long print runs. Single sheets can be used with a friction feed, but alignment is less reliable.

What does “near letter quality” mean?

It refers to the print quality of a 24-pin printer in its highest resolution mode. The dots are small enough that the text looks almost as smooth as a typewriter or a low-end laser printer, but not quite as crisp. It is a marketing term, not a standard.

How do I know if a pin is broken?

Print a solid black rectangle. If you see a white horizontal line running through it, a pin is likely broken. The line will be at the same vertical position across the entire print. You can also listen for a change in the sound—a broken pin often causes a slightly different ticking noise.

Is a dot matrix printer still useful today?

Yes, for specific tasks: printing multi-part carbonless forms (invoices, delivery notes), continuous labels, and applications where impact printing is required (e.g., to stamp through a card). They are also used in industrial environments where dust or heat would damage inkjet or laser printers.

Related

• Show Hidden Files on Mac OS X: 3 Ways
• Edit Hosts File Without Admin Password?
• Battlefield 2042 Black Screen Fix

in t | Print time | s | 12 | 0
in d | Print length | cm | 20 | 0
out speed | Print speed | cm/s | d / t | 2
out cps | Characters per second | chars/s | (d * 10) / t | 1