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A transmit PDO (TPDO) transmits objects from the SLK to the PLC, e.g., read MDT
data or the actual link state. The SLK sends a TPDO when the content of an object
assigned to the TPDO changes. This assignment is known as PDO mapping. If the
content of an object mapped in TPDO1 changes, e.g., data read from the MDT, the
SLK sends the TPDO1 with the MDT data to the master.
A receive PDO (PDO) transmits objects from the PLC to the SLK, e.g., setting the
commanded link state. This means CANopen masters that have no implemented
SDO communication can also work with the ID 40 system. However, the aspects
described in Chapter 9.9 "PDO communication" have to be observed.
The default mapping for all PDOs in described in Chapter 9.9.1 "PDO mapping". PDO
mapping can be changed by command-oriented data exchange.
Detailed PDO transmission conditions are explained in Chapters 9.9.2 "PDO
communication parameters" and 9.9.3 "PDO transmission types".
9.5 MDT data transmission with transfer buffers
The ID 40/SLK-CAN provides four transfer buffers for flexible data exchange. The
payload areas in the transfer buffers can be used both for direct data exchange with
the MDT as well as for parameterized data exchange. The content of an entire MDT
can be saved in each buffer.
• Multiple payload blocks of any size can be transferred directly from and to the MDT
through a command.
• Parameterized data exchange (see Chapter 6.4 "Parameterized data exchange with
MDT") is supported by all four buffers independently of one another. Buffered
prefetch and pretransmit are available.
• The transfer buffers can be activated and deactivated at will to select specific
MDTs. These have to be selected before the MDT arrives.
• The contents of up to four MDTs can be buffered.
• Status information on all transfer buffers can be transmitted to the bus master
through event-oriented data exchange.
The functionality of each transfer buffer forms four objects, the buffer status object,
the buffer mode object, the buffer mapping object and the transfer buffer itself.
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