Different CAN Networks Have Different Physical Layer Requirements

CAN, like all major networking protocols, requires a physical layer device in order to communicate. This physical layer comes from the ISO/OSI seven layer stack model. The physical layer is responsible for current and voltage control for the bus, dealing with current and voltage transients, and signalling bus (line) faults and possibly correcting them.

The Bosch CAN specification does not dictate physical layer specifications for anyone implementing a CAN network. This is both a blessing and a curse to the designer. Over the course of the last decade, two major physical layer designs have come to the fore and become the basic physical layer designs used in most CAN applications. They both communicate using a differential voltage on a pair of wires and are commonly referred to as a high-speed and a low-speed physical layer.

The low-speed architecture has the ability to change to a single-wire operating (referenced off ground) when one of the two wires is faulted through a short or open. Although both architectures use a voltage difference on a pair of wires, the termination methods for each are different and incompatible in production systems.

Since there are no requirements on physical layer in the CAN specification, other standards organizations help designers create compatible CAN devices. The International Organization for Standardization (ISO) creates standards to ensure inter-operability of components at the physical layer and recommends design practices. ISO standards are generally followed for industrial applications.