Plessey Code is a 1D linear barcode

Barcode

A barcode is an optical machine-readable representation of data, which shows data about the object to which it attaches. Originally barcodes represented data by varying the widths and spacings of parallel lines, and may be referred to as linear or 1 dimensional . Later they evolved into rectangles,...

 symbology based on pulse width modulation, developed in 1971 by The Plessey Company plc

Plessey

The Plessey Company plc was a British-based international electronics, defence and telecommunications company. It originated in 1917, growing and diversifying into electronics. It expanded after the second world war by acquisition of companies and formed overseas companies...

, a British-based company. It is one of the first barcode symbology, and is still used in some rare libraries and for shelf tags in retail stores, in part as a solution to their internal requirement for stock control. The system was first used in the early 1970s by J.Sainsbury to identify all of its products on supermarket shelves for its product restocking system.

The chief advantages are the relative ease of printing using the dot-matrix printers popular at the time of the code's introduction, and its somewhat higher density than the more common 2 of 5 and 3 of 9 codes. It has latter led several variations as Anker Code by ADS Company, Telxon, and MSI

MSI Barcode

MSI is a barcode symbology developed by the MSI Data Corporation, based on the original Plessey Code symbology. It is a continuous symbology that is not self-checking...

 (also known as Plessey modified). It is difficult to have the specifications for them nowadays and thus hard to tell the differences between them (except for MSI), because is was mainly available as paper document and in discontinued since then.

Encoding

The barcode encodes hexadecimal

Hexadecimal

In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...

 digits (i.e., the hex digits 0-F) as 4 bars (bits). The LSB

Least significant bit

In computing, the least significant bit is the bit position in a binary integer giving the units value, that is, determining whether the number is even or odd. The lsb is sometimes referred to as the right-most bit, due to the convention in positional notation of writing less significant digits...

 is on the first bar on the left, and the MSB

Most significant bit

In computing, the most significant bit is the bit position in a binary number having the greatest value...

 is the last bar on the right. A "0" bit is represented as a narrow bar, followed by a wide space. The "1" is represented by a wide bar, followed by a narrow space.

The barcode can be read in either direction (left to right or right to left).

The barcode comprises:

• the forward start code
• the label / data digits
• the check code, for error detection
• the termination bar
• the reverse start code

Forward start code

The forward start code is "1101". It defines the beginning of the encoded text.

Label

The character of the text to encode are hexadecimal values encoded as reversed BCD

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

Check code

The check code for error detection and correction is as CRC

Cyclic redundancy check

A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...

, using polynomial

Polynomial

In mathematics, a polynomial is an expression of finite length constructed from variables and constants, using only the operations of addition, subtraction, multiplication, and non-negative integer exponents...

 division

Mathematics of CRC

The cyclic redundancy check is based on division in the ring of polynomials over the finite field GF , that is, the set of polynomials where each coefficient is either zero or one, and arithmetic operations wrap around .Any string of bits can be interpreted as the coefficients of a message...

. Is uses 2 characters, or 8 bits. The generator polynomial is with n=8, in binary "111101001".

Reverse start code

The reverse start code, with the forward start code, are used to detect the direction of reading : from left to right or right to left. It's encoded as a reversed "0011". Reversed because the "0" is represented by a wide space and simple bar, and "1" as a simple space and wide bar.

Dimensions

The dimensions of the bars and spaces are defined by the following rules :

, with b < 0,229mm

, with c > 0,127mm

Bit dimension in mm :

		| "1"				| "0"
bit/inch	pitch 'p'	width	min	nom	max	width	min	nom	max
40	0.635	bar 'a'	0.305	0.343	0.381	bar 'c'	0.114	0.127	0.152
		space 'b'	0.254	0.292	0.305	space 'd'	0.471	0.508	0.533
32	0.787	bar 'a'	0.394	0.432	0.470	bar 'c'	0.127	0.152	0.173
		space 'b'	0.318	0.356	0.394	space 'd'	0.584	0.635	0.685
25 (std.)	1.020	bar 'a'	0.533	0.584	0.635	bar 'c'	0.127	0.178	0.229
		space 'b'	0.381	0.432	0.483	space 'd'	0.787	0.838	0.889

Values are slightly different for Anker Code

Before and after each barcode, there should be margins of 4 bits.

External links

• Plessey Code description by Barcode 1
• Plessey Code description(fr)
• Anker Code partial specification