Festo SEC-AC-305-CO, Руководство пользователя

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4. Access methods/SDO access
4−10
Festo P.BE−SEC−AC−CO−E N en 0802a
4.3 PDO−Message
P
rocess
D
ata
O
bjects (PDOs) are suitable to transmit data
event−controlled, whereat the PDO contains one or more
predefined parameters. In contrast to SDOs no hand−shake is
used. So the receiver has to be able to handle an arriving
PDO at any time. In most cases this requires a great deal of
software in the host computer. This disadvantage is in con
trast to the advantage that the host computer does not need
cyclically inquiry of the objects embedded in a PDO, which
mean strong reduction of bus load.
EXAMPLE
The host computer wants to know when the servo controller
has reached the target position at a positioning from A to B.
If SDOs are used the host constantly has to poll the object
statusword , e.g. every millisecond, thus loading the bus ca
pacity more or less depending on the request cycle time.
If PDOs are used the servo controller is parametrized at the
start of an application in such a way that a PDO is send on
each modification of the
statusword . So the host computer
need not to poll all the time but a message is sent to it auto
matically if an event occurs.