Civil Engineering Reference
In-Depth Information
Fig. 4.2
Field device model
4.1.3
Network Management
The NMT is based on the master/slave principle. Only one active NMT master
exists per CANopen network. The master controls all NMT-FSAs (referred to as
NMT slave state machine) of the NMT slave devices. This also includes the FSA
of its own CANopen NMT slave. To avoid misunderstanding, even the CANopen
manager, which contains the NMT master, has to have an NMT slave state machine
and an object dictionary. The so-called CANopen masters that do not have their own
object dictionary are by definition not a CANopen device!
The NMT slave state machine supports four states (see Fig.
4.3
): the volatile state
“initialization” as well as the “pre-operational”, “operational” and “stopped” states.
The state transition from initialization to pre-operational takes place automatically.
When the device reaches the pre-operational state, it starts its boot-up protocol, i.e.
it sends its boot-up message, a single CAN data frame with a CAN identifier built-
up out of the 4-bit function code (most significant CAN-ID bits) 1110
b
as well as
the 7-bit node ID (least significant CAN-ID bits). The boot-up message has a 1-byte
data field that contains a zero. It is used to introduce the device after power-on or
reset to the other members in the network. The same data frame is used in pre-oper-
ational and operational states to send the periodical
heartbeat protocol
. It contains
