WAGO I/O-SYSTEM 750 Manual page 151

WAGO-I/O-SYSTEM 750
750-354/000-001 EtherCAT® Fieldbus Coupler, ID Switch
PDO (Process Data Objects)
Process Data Objects (PDO) are used to quickly and efficiently exchange real-
time data, such as input and output data, set and actual values.
No objects are addressed in the EtherCAT
process data are sent directly from the previously mapped parameters.
SDO (Service Data Objects)
The Service Data Objects (SDO) are used to access reading and writing to the
Object Dictionary and to transmit device parameters. Based on the non-cyclical
transmission of parameters, such as only once during run-up, the SDO's are low
priority.
Command Objects
Command objects can be used when actions or commands must be
implemented that cannot, however, be realized via a single SDO upload or SDO
download service, e.g. because request and response data are necessary, or
because the execution takes too long temporally, such that an SDO timeout
would occur.
Using a command object divides the command into at least two SDO services.
The command is started by writing the command object on subindex 1 using the
SDO download.
The answer is retrieved by reading subindex 3 using the SDO upload.
If the answer is not available during the reading of subindex 3, the first byte of the
answer data can provide information about the progress.
Subindex 1 is not writable until the command has been ended and the answer
has been read, so that the accesses can no longer be mutually overwritten by
different applications.
Subindex 2 is only defined for compatibility reasons using the appropriate
CANopen
The data type of the object description for command objects is 0x25.
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DS301 definition.
Fieldbus Communication 151
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telegram. The contents of the
Manual
Version 1.2.1