Parameterization and commissioning
EL6751
96
Version: 3.5
Node Monitoring
Heartbeat and guarding mechanisms are available to monitor failures in the CANopen network. These are of
particular importance for CANopen, since modules do not regularly speak in the event-driven mode of
operation. In the case of "guarding", the devices are cyclically interrogated about their status by means of a
data request telegram (remote frame), whereas with "heartbeat" the nodes transmit their status on their own
initiative.
Guarding: Node Guarding and Life Guarding
Node Guarding is used to monitor the non-central peripheral modules, while they themselves can use Life
Guarding to detect the failure of the guarding master. Guarding involves the master sending remote frames
(remote transmit requests) to the guarding identifier of the slaves that are to be monitored. These reply with
the guarding message. This contains the slave’s status code and a toggle bit that has to change after every
message. If either the status or the toggle bit do not agree with that expected by the NMT master, or if there
is no answer at all, the master assumes that there is a slave fault.
Guarding procedure
Fig. 112: Schematic diagram: "Guarding procedure"
Protocol
The toggle bit (t) transmitted in the first guarding telegram has the value
0
. After this, the bit must change
(toggle) in every guarding telegram so that the loss of a telegram can be detected. The node uses the
remaining seven bits to transmit its network status (s):
s
Status
4 = 0x04
Stopped (previously: Prepared)
5 = 0x05
Operational
127 = 0x7F
Pre-Operational
Sample
The guarding message for node 27 (0x1B) must be requested by a remote frame having identifier 0x71B
(1819
dec
). If the node is
Operational
, the first data byte of the answer message alternates between 0x05 and
0x85, whereas in the
Pre-Operational
state it alternates between 0x7F and 0xFF.
