Bit coding

Working on avoiding faults means working on noise emissions and susceptibility or immunity. Of great significance to radiated emissions is bit coding. Intelligently designed bit coding helps to reduce emissions significantly. Nonetheless, this requirement must often be harmonized with a required transport capacity.

NRZ bit coding

NRZ bit coding (NRZ: Non Return to Zero) was chosen for CAN. This means that the binary signals to be transmitted are mapped directly: a logic “1” to a high level, a logic “0” to a low level. Characteristic of NRZ coding is that consecutive bits of the same polarity exhibit no level changes.

Missing synchronization

This is how NRZ coding enables very high data rates yet keeps emissions within limits. However, NRZ coding is not self-clocking; that is, it does not have any synchronization properties. If no level change occurs over a longer period of time, the receiver loses synchronization. That is why the use of NRZ coding requires an explicit synchronization mechanism, which however reduces transmission efficiency.

Bit stuffing for
synchronization

With CAN what is known as the bit stuffing method is used as the synchronization mechanism: after five homogeneous bits the sender inserts a complementary bit in the bit stream (Manchester coding, for example, does without such a mechanism, since it is self-clocking).