Plessey’s barcode pattern is so simple that, with casual familiarity with the code, you can actually read the bar codes with your eyes.
Plessey basic code rule is:
And each digit number is represented by four bit.
If you only calculate the bar (horizontal black line), the wide bar means ‘1’ and the narrow bar means ‘0’.
So, the digit ‘1’:
Table 1.1. Plessey Barcode Format
|start character||Any number of labels/characters/digits||Checksum / check digit / code||A stop / termination character / block / pitch bar||the reverse start code|
|4it =1101||XXX...(eg, 012345...)||8bit(eg. 0110 0111)||1bit=X(eg, 1)||4bit=0011|
The forward start code is "1101". It defines the beginning of the encoded text.
The characters of the text to encode are hexadecimal values encoded as reversed BCD.
Table 1.2. Plessey Bits Encoding Rule
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) = x8 + x7 + x6 + x5 + x3 + 1 with n=8, in binary "111101001".
The termination bar follows the CRC. It's a full pitch bar.
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.