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Network Management (NMT)


CANOPEN describes the communication state of a network node in a state diagram. By sending specific CAN messages (NMT messages), the network management master can control the communication state of the other nodes (the network management slaves) of a CANOPEN network, i.e. it can change the state of all nodes or of an individual node by a single command.

NMT messages are transmitted with the highest priority message identifier (CAN-ID 0). The data field consists of only two bytes: the required target state is coded in the first data byte, the second data byte specifies the number of the node whose communication state is to be altered. All nodes of a network are jointly addressed with the virtual node-ID 0; in this way, for example, all nodes can be set to "Operational" state at the same time for the sake of a simultaneous start of operation.

Images/NMT-States.png

In order to enable even a partial reset of a certain node, this state is subdivided into three sub-states: "Reset-Application", "Reset-Communication" and "Initializing". After an HW-Reset or Power-On, a node is in the "Initializing" state. After completion of the basic node initialization (e.g. host controller, CAN controller, application software, etc.), the node transmits the so-called "boot-up message" and switches itself to the "Pre-operational" state. In the sub-state "Reset-Application", the parameters of the manufacturer-specific and of the standard device profile are reset to their Power-On values (corresponds to the last values saved). Then the node changes to the sub-state "Reset-Communication". In the sub-state "Reset-Communication", the parameters of the communication profile are reset to the Power-On values. Then the node state switches to "Initializing".

The main difference between the various NMT states is that not all types of CANOPEN communication are actively used in each state. The following table shows which communication a node may perform when it is in a particular NMT state.

InitializingPre-operationalOperationalStopped
Boot-up*
SDO**
Emergency**
SYNC/TIME**
Heartbeat/Nodeguard***
PDO*

Pre-operational

This state is primarily used for the configuration of CANOPEN devices. Therefore exchange of process data (via PDOs) is not possible in this state. The entries of the device object dictionaries can be accessed via "service data objects" (SDOs). By transmitting an SDO message, the object dictionary of a certain device can be modified, e.g. with a configuration tool. In addition to communication via SDO messages, emergency, synchronization, time stamp and of course NMT control messages can also be transmitted or received in the Pre-operational state. By transmitting a "Start-Remote-Node", a node switches to the "Operational" state.