Triggered communication protocol

The introduction of CAN and CANOpen into the industrial controls market presented several challenges, balancing timeliness and latency vs. high average performance.  This patent describes a triggered protocol layer that addresses some of these cases.

CAN and CANOpen seemed like a good fit for the industrial automation market, after its success in automotive applications.  CAN provides a conceptually simple 1-wire bus, easy to implement with low-end processors.  CANOpen provides an “object dictionary” containing a large set of predefined messages.    Specialized interface chips can provide hardware-assist at each node for filtering messages of interest.

There are performance and implementation issues that are important in industrial control applications that were not addressed by the standards.

Often CAN systems are built in a “continuous broadcast” mode, in which each device on the bus publishes its message periodically whether the information is new or not.

Because the bus messages are organized into a priority hierarchy, some method must be implemented to ensure that every device on the bus has a chance to publish its data.  This is normally only a problem when the bus is very busy.  Critical control applications must be able to encounter that case and still operate reasonably.

This patent is one of several that provide improvements on top of CAN/CANOpen to CAN bus performance in industrial control applications.