Manual VIPA System 100V
Chapter 2 Decentral block periphery CAN-Bus CANopen
HB100E - SM-CAN - Rev. 15/02
2-21
PDO - Process Data Object
For the process data exchange there are 2
p
rocess
d
ata
o
bjects (PDO)
available. Every PDO exists of max. 8 data bytes. PDOs are transferred
unacknowledged since the CAN protocol guarantees the transfer.
For input data 1 transmit PDO is at disposal and for output data 1 receive
PDO.
In many fieldbus systems every time the whole process image is
transferred - more or less cyclically. CANopen is not reduced to this
communication principle, for CAN offers other possibilities due to the multi
master bus access control.
With CANopen, the process data is divided in segments of max. 8 byte.
These segments are called process data objects (PDOs). The PDOs
correspond each to one CAN message and are assigned via those specific
CAN identifier and defined in their priority.
The PDOs are named from the view of the bus coupler: Receive-PDOs
(RxPDOs) are received by the coupler and contain output data.
Transmit-PDOs (TxPDOs) are sent by the coupler and contain input data.
For the process data exchange there is each 1 RxPDO and 1 TxPDO
available. The occupancy of these PDOs with input res. output data occurs
automatically.
The PDO-Mapping is fixed and must not be altered.
The most important communication parameter of a PDO is the CAN
identifier (also called Communication Object Identifier, COB-ID). It identifies
the data and specifies their priority at the bus access.
For every CAN data telegram there may be only one sending node
(producer), but due to CAN, where the messages are sent in a
broadcasting procedure, a telegram may be received from that many nodes
you want (consumer). Thus one node may place its input information at the
disposal of several bus participants at the same time - even without the
passing on by a logical bus master.
In the System 100V for the send and receive PDOs default identifiers in
dependence on the node address are provided.
The COB identifiers for the receive and send PDO transfers, fixed after the
boot-up, are listed below.
The transmission type is set in the object directory (Indices 0x1400-0x1404
and 0x1800-0x1804, Subindex 0x02) fix to asynchronous and event
triggered (=0xFF). Via the EVENT-Timer (Value * 1ms) the PDOs may be
transferred cyclically.
Send:
0x180 + module-ID: PDO1S2M Digital
(acc. DS-301)
Receive:
0x200 + Module-ID: PDO1M2S Digital
(acc. DS-301)
PDO
PDO Mapping
PDO Identifier
COB-ID