编辑dbc文件的时候，各内容的创建
CANdb++ Data Model
Control Units (ECUs: Electronic Control Units) (3)
Info (4)
Environment Variables (5)
Network Nodes (7)
Info (7)
Messages (9)
Info (10)
Signals and Message Signals (10)
Network：一个网络中往往有很多个控制单元ECU，ECU之间通过网络节点(NN)连接，并且在同一个CAN总线上交换信息。
Network
In a network there are usually multiple control units (ECUs) that are connected to one another by their network nodes (NN) and exchange information over the same CAN bus.
控制单元（电子控制单元ECUs）：控制单元在网络中的角是分配处理单元，并通过网络节点与别的控制单元之间的信息交换。
控制单元可能包含多样的sub-function（功能节点）。
variable怎么记环境变量在CANdb++的数据模型中被分配到控制单元。
多个网络节点可以被分配到一个控制单元，这会使能网关的建模。
当控制节点在处理过程中时候，其名字以及备注不能修改，因为CANdb数据库不支持控制单元作为一个独立的目标。
当一个数据库打开，一个控制单元就会被每一个节点install（安装，设置）。“还没理解明白install是怎么回事”
注：任何一个网络节点被定义的时候，CANdb就会自动定义一个同名的ECU，以及在在新的网络节点和ECU之间install a link。
当创建了一个网络节点，会为一个DBC数据库的一个网络建立一个link。
由于一个DBC数据库总是会包含既存的网络的数据，所以新的网络不能被install
Control Units (ECUs: Electronic Control Units)
Control units are the distributed processing units in the network.
Information is exchanged with other control units via network nodes,which represent the control unit's interface to the CAN bus.
Control units can also contain multiple sub-functions (functional nodes).
Environment variables are assigned to the control unit in the CANdb++ data model.
Several network nodes can be assigned to a control unit, which enables the modeling of gateways.
The name and comment of a control unit cannot be changed while processing, because a CANdb database doesn't support control units as independent objects.
When a database is opened a control unit is installed for every node.
Info
∙Whenever a network node is defined, CANdb automatically defines a ECU with the same name, and a link is installed between the new network node and the
ECU.
∙When a network node is designed (installed), this establishes a link to the one network of a DBC database.
Since a DBC database always contains the data of exactly one network, a new network cannot be installed
Environment Variables
Input and output variables of network nodes, such as switch position, sensor signals and actuator signals.
In the CANdb++ data model environment variables are assigned to the control unit.
Environment variables are defined by the following system parameters:∙Symbolic name of the environment variable
∙Value type (data type) of the environment variable
Integer – Signed 32 Bit Integer
String – ASCII String
Float – 64 Bit Float
Data – Byte field of a specified length (the length is set in the Length attribute).
∙Access rights
Defines the rights that Electronic Control Units (ECU's) have when they access
environment variables.
Unrestricted: All ECU's have read and write access.
The assigned ECU's can read the environment variable.
Read:
(The environment variable represents a sensor.)
The assigned ECU's can write to the environment variable.
Write:
(The environment variable represents an actuator.)
Read/Write:The assigned ECU's can read from and write to environment variables.

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