Plessey Code

Plessey Code is a 1D linear barcode symbology based on Pulse Wide Modulation, developed in 1971 by The Plessey Company plc, 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 (also know 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 an unlimited number of hexadecimal values, the characters 0-9 and A-F, as reversed BCD as 4 bars (bits). The LSB is on the first bit on the left, and the MSB is the last bit on the right. The "0" is represented as a simple bar, followed by a wide space. The "1" is represented by a wide bar, followed by a simple space.x

It can be read in both directions (from left to right and right to left) thanks the the start and end code.

The barcode is composed by :

a forward start code
a label, the text to encode
a check code, for error correction
a termination bar
a 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, using polynomial division. Is uses 2 characters, or 8 bits. The generator polynomial is $g(x)=x^{8}+x^{7}+x^{6}+x^{5}+x^{3}+1$ with n=8, in binary "111101001".

termination

The termination bar follows the CRC. It's a wide bar, or a full pitch.

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 :

${\frac {a}{b}}<1$ , with b < 0,229mm

${\frac {c}{d}}>0,33$ , 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
40	0.635	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
32	0.787	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
25 (std.)	1.020	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