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This article describes a two-wire transmission system that limits frequency
spectrum and radiated noise while still providing self-clocking, balanced error
Ideally, a serial digital transmission code will have the following properties: 1)
low frequency spectrum 2) high signal-to-noise ratio 3) self-clocking
characteristics (many transitions) 4) balanced number of logic one and zeros for
AC coupling and optimum detection 5) error detection properties
Many bit coding techniques have some of these properties but not all. For
example, non-return to zero (NRZ) has a low frequency spectrum and high
signal-to-noise ratio but does not provide self-clocking, balance or error
detection. Biphase codes have poor (high) frequency spectrum but exhibit all the
other desirable features. In some applications, such as communication between
electronic units on automotive vehicles, the spectrum should be kept low.
A two-wire system (such as a twisted pair system) is disclosed herein that
provides all of the desired transmission characteristics cited above. The two
wires are encoded in a binary two level format in accordance with the following:
To maintain balance which provides for an equal number of zeros and ones
on each line, the parameter "C" is defined as the accumulative sum of previous
values for each line. The algorithm would be in accordance with the following
rules: 1) Each binary "1" would alternate for line 1/line 2 between 1/0 and 0/1. 2)
Each binary "0" would similarly alternate between 1/1 and 0/0. 3) The first bit
can be arbitrary. 4) The first bit with a binary value different than the binary
value of the first bit is determined to minimize "C". An example sequence is
illustrated in Fig. 1. The system has the following characteristics: 1) There is a