CAN Node Health

All networks including CAN are vulnerable to faults such as breaks in the bus wiring or loss of power by some of the nodes. CANopen defines two methods for the master node (the XMP in our case) to periodically check the presence of nodes on the network-node guarding and heart beating.

Using these services the XMP can monitor the health of the communications to each of the nodes. The current health of each node is reported in the live field of the MEICANNodeStatus structure.

It is mandatory for a node to either support the node guarding or heart beating protocols, or to support both. The heartbeat protocol has recently been introduced to CANOpen (in June 1999), and will probably NOT be supported on many nodes, but its adoption is recommended for all new nodes. The XMP's implementation will operate with either protocol and will automatically detect the protocol that each node supports and then use the most appropriate protocol for the CAN network. The healthType field of the MEICanNodeInfo structure reports the health checking protocol being used with each node.

Node Guarding protocol

The Node Guarding protocol has the master sending an RTR message to all nodes on the network and checks to see whether a response is received from each of the nodes.

Heart Beating protocol

In the Heart Beating protocol, each node periodically broadcasts a heartbeat message. The period between transmitting the heartbeat messages is half the health period. If the XMP does not receive a message within a specific time window, it generates a heartbeat error for that node.

The advantage of the Heart Beating protocol over the Node Guarding protocol is that the number of messages is reduced in half, thereby freeing up bandwidth for other messages.

Health Period

The healthPeriod field of the MEICanConfig structure allows the user to specify the Node Guard and Heartbeat times for the health protocols according to the following table. The same period is used for all nodes.

For most applications it is recommended that the healthPeriod should be set to ten times the cyclic period.