5. Access Procedure
30
Festo P.BE-CMMP-CO-SW-EN 0708NH
5.3
PDO Message
With
P
rocess
D
ata
O
bjects (PDOs), data can be transmitted in an event-driven manner.
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 query
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 constantly, such as every millisecond, query
the
statusword
object, with which it uses up the bus capacity.
When a PDO is used, the motor controller is parameterised 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 querying, 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:
Transmit PDO (T-PDO)
Controller
Host
Motor controller sends PDO when a certain
event occurs
Receive PDO
(R-PDO)
Host
Controller
Motor controller evaluates PDO when a certain
event occurs
The motor controller has four transmit and four receive PDOs.
In the PDOs, almost all objects of the object directory can be entered (mapped). This
means, the PDO contains the actual speed, position value, etc. as data. The motor
controller must first be informed of the data 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 bytes 0 ... 3 of the PDO and the actual
speed in the bytes 4 ... 7.
Number of data bytes (here 8)
Start velocity feedback value
(D4 … D7)
181
h
Len
D0 D1 D2 D3 D4 D5 D6 D7
Identifier
Start position feedback value
(D0 … D3)
In this way, almost any desired data telegrams can be defined. The following chapters
describe the settings necessary for this.
Summary of Contents for CMMP Series
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