turck BL20-GW-CANOPEN, Руководство пользователя

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D301087 1211 - BL20 CANopen
3-3
CANopen
3.1.2 Communication
The lower layers of CANopen are defined according to the ISO-OSI model in the ISO 11898 standard.
Communication between the individual nodes is made by transmitting "Telegrams".
4 different types of telegram message are defined for CANopen:
„
Network management messages
„
Service data objects SDO
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Process data objects PDO
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Predefined messages
Network Management Messages
Network management messages are used in the network to control the nodes and their operating
states. This type of message makes it possible, for instance, to configure the data transmission
mechanism of a node.
The Network Management objects include Boot-up message, Heartbeat protocol and NMT message.
Boot-up message, Heartbeat and Node Guarding are implemented as single CAN frames with 1-byte
data field.
The NMT message is mapped to a single CAN frame with a data length of 2 byte. Its identifier is 0. The
first byte contains the command specifier and the second contains the Node-ID of the device that must
perform the command (in the case of Node-ID 0 all nodes have to perform the command). The NMT
message transmitted by the NMT master forces the nodes to transit to another NMT state. The
CANopen state machine specifies the states Initialization, Pre-Operational, Operational and Stopped.
After power-on, each CANopen device is in the state Initialization and automatically transits to the state
Pre-operational. In this state, transmission of SDOs is allowed. If the NMT master has set one or more
nodes into the state Operational, they are allowed to transmit and to receive PDOs. In the state Stopped
no communication is allowed except that of NMT objects.
The state Initialization is divided into three sub-states in order to enable a complete or partial reset of a
node. In the sub-state Reset Application the parameters of the manufacturer-specific profile area and
the standardized device profile area are set to their power-on values. In the sub-state Reset
Communication the parameters of the communication profile area are set to their power-on values.
The third sub-state is initializing, which a node enters automatically after power-on. Power-on values
are the last stored parameters.
The Heartbeat protocol is for error control purposes and signals the presence of a node and its state.
The Heartbeat message is a periodic message of the node to one or several other nodes. It indicates that
the sending node is still working properly.
A device sends the Boot-up message to indicate to the NMT master that it has reached the state Pre-
operational. This occurs whenever the device initially boots-up but also after a power-out during
operation. The Boot-up message has the same identifier as the Heartbeat object, however, its data
content is zero.
Service Data Objects (SDOs)
A Service Data Object (SDO) reads from entries or writes to entries of the Object Dictionary.
The SDO transport protocol allows transmitting objects of any size. The first byte of the first segment
contains the necessary flow control information including a toggle bit to overcome the problem of
doubly received CAN frames. The next three bytes of the first segment contain index and sub-index of
the Object Dictionary entry to be read or written. The last four bytes of the first segment are available
for user data. The second and the following segments (using the very same CAN identifier) contain the
control byte and up to seven bytes of user data. The receiver confirms each segment or a block of
segments, so that a peer-to-peer communication (client/server) takes place.