3
CANopen access procedure
Festo – GDCP-CMMP-M3/-M0-C-CO-EN – 1510b – English
25
3.3
PDO Message
With
Process Data Objects (PDOs), data can be transmitted in an event-driven manner or cyclically. The
PDO thereby transmits one or more previously established parameters. Other than with an SDO, there
is no acknowledgement when a PDO is transmitted. After PDO activation, all recipients must therefore
be able to process any arriving PDOs at any time. This normally means a significant software effort in
the host computer. This disadvantage is offset by the advantage that the host computer does not need
to cyclically request parameters transmitted by a PDO, which leads to a strong reduction in CAN bus
capacity utilisation.
EXAMPLE
The host computer would like to know when the motor controller has completed a positioning from
A to B.
When SDOs are used, it must frequently, such as every millisecond, request the statusword object,
which uses up bus capacity.
When a PDO is used, the motor controller is parametrised at the start of the application in such a way
that, with every change in the statusword object, a PDO containing the statusword object is
deposited.
Instead of constantly requesting, the host computer thus automatically receives a corresponding
message as soon as the event occurs.
A distinction is made between the following types of PDOs:
Type
Path
Comment
Transmit PDO
Motor controller
Host
Motor controller sends PDO when a
certain event occurs.
Receive PDO
Host
Motor controller
Motor controller evaluates PDO when a
certain event occurs.
Tab. 3.4
PDO types
The motor controller has four transmit and four receive PDOs.
Almost all objects of the object directory can be entered (mapped) into the PDOs; that is, the PDO con
tains all data, e.g. the actual speed, the actual position, or the like. The motor controller must first be
told which data have to be transmitted, since the PDO only contains reference data and no information
about the type of parameter. In the example below, the actual position is transmitted in the data
bytes0 … 3 of the PDO and the actual speed in the bytes 4 … 7.
