Intermec 700 Series User Manual page 445

Plessey
MSI Code (Variant of Plessey)
700 Series Color Mobile Computer User's Manual
Plessey finds its origin in the pulse width modulated (PWM) code devel-
oped in England. It is widely used for shelf markings in grocery stores.
Pulse width modulated codes represent each bit of information by a bar
and space pair. A zero bit consists of a narrow bar followed by a wide
space, while a one bit consists of a wide bar followed by a narrow space. It
is mainly a numeric symbology (0-9) with six extra characters available for
assigning any symbol or letter desired.
Plessey codes are not self-checking and employ a variety of check charac-
ters. Plessey employs a polynomial-based Cyclic Redundancy Check
(CRC). For start and stop characters, Plessey employs a 1101 and pre-
viously used a 0101.
This symbology is very limited about what information can be encoded. It
is not considered for new applications.
In addition to Plessey characteristics, the MSI Code employs a Modulus
10 Check. For start and stop checks, MSI employs a single bit pair of 1 as
a start symbol and a single bit pair of 0 as a stop symbol. MSI reverses the
1-2-4-8 BCD pattern for bit pair weighting to 8-6-2-1.
Appendix B — Bar Codes
423