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User manual SCAIME: NU-soft-eNod4B-E-0716_236704-C.doc
If corresponding heartbeat is not received within this time, then eNod4 sends an emergency
telegram and switches to pre-operational state.
5.6.2 Node guarding
Node guarding protocol is another way to check the nodes state. But unlike Heartbeat protocol, it needs requests
from a NMT master. In this case, the NMT master sends periodically a remote transmit request (remote frame) to the
node with COB-ID 700
H
+ ID eNod4. eNod4 has to respond by sending a single-byte data frame with its coded state.
This frame is similar to Heartbeat frame but there is an important difference. Most significant bit of the state byte is a
toggle-bit. The value of this bit must alternate between two consecutive responses from the NMT slave. The value of
the toggle-bit of the first response after the Guarding Protocol becomes active is 0.
It is only reset to 0 when a ‘reset communications’ or a ‘reset node’ command is received. If two consecutive
responses have the same value of the toggle-bit, then the new response should be handled as if it was not received by
the NMT master.
Two parameters of the object dictionary are necessary to set and define node guarding protocol: the ‘guard time’ and
the ‘life time factor’:
Guard time: this parameter expressed in milliseconds indicates the period with which the node is
being polled by the NMT master. This value can be different from one node to another.
Life time factor: when node guarding protocol is active, node life time is given by multiplication of the
guard time and the life time factor.
Node guarding activation is effective when guard time has been set (and if Heartbeat protocol is not used) and after
reception of the first remote transmit request. If life time factor is also configured and if no remote transmit request is
handled within the node life time, eNod4 sends an emergency telegram then switches to pre-operational state. The
life guarding error is acknowledged when the state is changed by a NMT command and after reception of a new
remote transmit request.
Switching to the stopped NMT state because of a node guarding error may cause eNod4 to be set into a configurable
safety mode where parts of its functioning are inhibited
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