Civil Engineering Reference
In-Depth Information
Fig. 4.3
CANopen NMT slave state
in 6 bits of the 1-byte data field the NMT state in which the device is situated at the
moment (see Fig.
4.4
). The frequency of the heartbeat could be configured in the
producer-Heartbeatheartbeat-time
(1017
h
) parameter. If a CANopen device shall
receive a heartbeat from another device, it has to be configured in the parameter
set
consumer-heartbeat-time
(1016
h
). Of course, the consumer time always has to
be bigger than the corresponding producer time; otherwise, the device is always
considered to be “lost”.
If the consumer heartbeat time expires without reception of the corresponding
heartbeat message, it will lead to an event (heartbeat event) in the heartbeat ap-
plication of the device. The action is taken because this event is device or profile
specific.
The state transitions are normally commanded by the CANopen device with
NMT master functionality. For this purpose, the NMT master sends the CAN data
frame with the highest prior identifier (CAN ID = 0). In this 2-byte message, the first
byte contains the command (command specifier) and the second byte contains the
node ID of the device that shall perform the state transition. In case of an error, the
devices are able to change their states autonomously. The value 0 is interpreted as
broadcast command, meaning all nodes have to perform this command. The execu-
tive state transition could be configured in the
error-behaviour
(1029
h
) parameter.
For applications where no master/slave NMT is allowed due to safety issues,
flying master solutions, as specified in CiA 302, are available. The protocols, speci-
fied in this standard, allow that if a device with NMT master functionality has a
failure another device activates its NMT master functionality. In fact, it is possible
that more than one device with “sleeping” NMT masters are located in the network.
If the NMT master with the highest priority returns into a functional state, after a
temporary failure, it could reclaim the NMT with the help of the specified protocols.
