Plessey Code Specification Page

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Plessey Code was developed by the Plessey Company in England with formal specifications first dated March 1971. Plessey Code has been extensively used in libraries. A variation of Plessey Code and the associated scanning equipment was provided by Plessey to the ADS Company and this variation is known as Anker Code. Anker Code was used in European point of sale systems prior to the advent of EAN. The basic encoding principle in Plessey Code was used by MSE Data Corporation to construct its MSI barcode, sometimes known as modified Plessey Code. The primary application for MSI Code is marking of retail shelves and subsequent scanning with portable devices to accomplish inventory re-ordering. More information about MSI code is available here.

Basic Code Rule

• Wide bar following by narrow space is a "1" bit.
• Narrow bar followed by wide space is a "O" bit.

Features

• Character set defined by reading a fixed number of bits per character. Example 4 bit hexadecimal set or 6 bit 64 character set.
• Not self-checking. Usually used with a cyclic redundancy check code or with one or more "check digits".
• Continuous code.
• 12 Units per character (hexadecimal set).
• 3 units per bit.

Start Code

Data Block

0	0000
1	1000
2	0100
3	1100
4	0010
5	1010
6	0110
7	1110
8	0001
9	1001

A	0101
B	1101
C	0011
D	1011
E	0111
F	1111

Check Code

Termination Block

In the case of labels which are read from either left to right or right to left, the check code is followed by a single thick bar.

Reverse Start Code

Margins

Bit Definition and Dimensions

a/b  	<  	 1     	Subject to 'b' < 0.009 ins. or 0.229 mm

c/d  	 >  	 0.33  	Subject to 'c' > 0.005 ins. or 0.127 mm

Table 1: Bit Definition and Dimensions (in mm)

		Binary '1'				Binary '0'
bits/inch	Nom. Pitch 'p'	Width of "1"	Min.	Nom.	Max.	Width of "0"	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 (Standard)	1.02	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

Spots and Voids

Voids (i.e. white spots in black areas) should be less than 0.051 mm across in any direction. In addition, any such spot should be farther than 0.051 mm (0.002 ins) from the nominal edge of an adjacent bar and not less than 0.203 mm (0.008 ins) from the next void.

Reflectance Measurements

Measurements refer to diffuse reflectance in the near-infra-red spectrum between 850 and 1025 nm (nanometers).

The primary white standard will be "Bristol Board". The reflectance of this is taken as 100%. Absence of reflected light in the band is taken as 0%.

Bar Reflectance

PLESSEY CODE

Cyclic Check Code (CRC)

Techniques to overcome these problems range from simple parity and check digit techniques, to the very sophisticated error detection and correction codes. All these techniques have one factor in common - they introduce redundant information into the message, and the error detection "power" of the technique bears a relation to the amount of redundant information introduced.

The Plessey Company have adopted a cyclic (or polynomial) check code technique which is applied to the reading of barcode labels and transmission of data. This technique is adjudged to be a fair compromise between the extra redundancy - 8 bits per message - and the error detecting power- roughly one undetected error per hundred million 6 digit transactions.

Between system modules, messages are transmitted as a string of 4 bit characters, starting with the most significant character. Within each character, the least significant bit comes first.

E.G.	1049	is	1000 0000 0010 1001
	767	is	1110 0110 1110

Computer Generation of Check Code

d (x) = x^19 + x^12 + x^4

and the logical remainder can be found logically adding the remainders of each individual term.

i.e. r (x) = r19 (x) + r12 (x) + f4 (x)

Thus all that is necessary to calculate the logical remainders for each co-efficient for the particular polynomial being used, and as successive bits come in, they logically add in the remainder or not (depending on whether it is a non-zero co-efficient). The properties of logical addition are 1 + 1 = 0 + 0 = 0

1 + 0 = 0 + 1 = 1 which is the same as an exclusive or function.

The algorithm becomes :

Steps 	1	Set r = 0, counter   - 20
	2	Input next data bit
	3	If data = 0, go to step 6
	4	Fetch r from table
	5	Exclusive or r with R. Store in r
	6	Decrease n by i. If not zero go to Step 2
	7	Remainder in r

r19	dec	134	10000110
r12	dec	186	10111010
		exor	00111100
R4	dec	49	00110001
		exor	00001101

	= x^3 + x^2 + 1 as previously

Principle of Cyclic Check Generation

x^n.d(x)		q(x) + r(x)
______	=	           ___
g(x)		           g(x)

Example

Thus is g(x) = x^8 + x^7 + x^6 + x^5 + x^3 + 1

		then n = 8

For encoding a number		18080
The binary equivalent	=	10000001000000010000

				d(x) = x^19 +x^12 + x^4 


x^8.d(x)		=	x^27 + x^20 + x^12
______			________________________
  g(x)			x^8 + x^7 + x^6 + x^5 + x^3 + 1

= x^19 + x^18 + x^15 + x^11 + x^10 + x^6 + x^2 + 1 + x^3 + x^2 + 1
                               _________________________
				            x^8 + x^7 + x^6 + x^5 + x^3 + 1


f(x) = x^3 + x^2 + 1 = 00001101

Hence the complete code as one would find on a label is :

1	 d(x)	1	r(x)	1

1000000100000001000000001101

The Plessey Code never had a formal code specification, and documentation has been out of print for many years. Mr. Paul Bergé was kind enough to supply the above information to BarCode1.

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