Plessey Code facts for kids
The Plessey Code is a special type of barcode. It was created in 1971 by a British company called The Plessey Company plc. This code is one of the very first barcodes ever made!
Think of a barcode as a secret message made of lines. The Plessey Code uses lines of different widths to store information. It's still used sometimes in places like libraries or on price tags in shops. For example, a big supermarket chain named J. Sainsbury used it in the 1970s to keep track of items on their shelves.
One cool thing about the Plessey Code is that it was easy to print using older printers. It could also hold more information than some other barcodes from that time. Over the years, a few similar codes were made, like MSI Barcode, which is a modified version of Plessey.
Contents
How Plessey Barcodes Work
A Plessey barcode uses groups of four lines (or "bars") to represent numbers and letters. These are called hexadecimal digits, which include numbers 0-9 and letters A-F.
Each group of four bars represents one digit. A "0" is shown by a thin bar followed by a wide space. A "1" is shown by a wide bar followed by a thin space.
You can actually read a Plessey barcode from left to right or right to left! This makes it quite flexible.
Every Plessey barcode has a few main parts:
- A special "start code" at the beginning.
- The actual information or "data" you want to store.
- A "check code" to make sure there are no errors.
- A "termination bar" at the end.
- Another "start code" for reading in reverse.
Starting the Code
The barcode begins with a "forward start code." This is a special pattern of "1101" that tells the scanner where the data starts.
Storing Information
The main part of the barcode holds the actual information. This information is made up of hexadecimal values. Each value is turned into a specific pattern of four bars.
For example, the number 0 is encoded as "0000", while the number 1 is "1000". The letter F is "1111".
| Value | Encoding | Value | Encoding | Value | Encoding | Value | Encoding |
|---|---|---|---|---|---|---|---|
| 0 | 0000 | 4 | 0010 | 8 | 0001 | C | 0011 |
| 1 | 1000 | 5 | 1010 | 9 | 1001 | D | 1011 |
| 2 | 0100 | 6 | 0110 | A | 0101 | E | 0111 |
| 3 | 1100 | 7 | 1110 | B | 1101 | F | 1111 |
Checking for Errors
After the main information, there's a "check code." This part helps the scanner find out if there are any mistakes when reading the barcode. It uses a special math trick to make sure the data is correct. This check code is 8 bits long, which means it uses two characters.
Ending the Code
Right after the check code, there's a "termination bar." This is a single, full-width bar that marks the very end of the main barcode data.
Reading Backwards
Finally, there's a "reverse start code." This code, along with the forward start code, helps the barcode scanner know which way it's reading the barcode. It's like a special signal that tells the scanner if it's going from left to right or right to left.
Barcode Sizes
The bars and spaces in a Plessey barcode have very specific sizes. These sizes help the scanner read the code correctly. For example, a thin bar has a certain width, and a wide bar has another. The spaces between them also have exact measurements.
Before and after each barcode, there needs to be a clear empty space. This space is usually about the width of four of the smallest bars.
