DeviceNet Master Protocol API

DeviceNet-Master Dual-Port Memory • 27
3 Dual-Port Memory
All data in the dual-port memory is structured in blocks. According to their functions, these blocks use
different data transfer mechanisms. For example, data transfer through mailboxes uses a
synchronized handshake mechanism between host system and netX firmware. The same is true for IO
data images, when a buffered handshake mode is configured. Other blocks, like the status block, are
read by the host application and use no synchronization mechanism.
Types of blocks in the dual-port memory are outlined below:
• Mailbox
transfer non-cyclic messages or packages with a header for routing information
• Data Area
holds the process image for cyclic I/O data or user defined data structures
• Control Block
is used to signal application related state to the netX firmware
• Status Block
holds information regarding the current network state
• Change of State
collection of flags that initiate execution of certain commands or signal a change of state
3.1 Cyclic Data (Input/Output Data)
The input block holds the process data image received from the network whereas the output block
holds data sent to the network
For the controlled / buffered mode, the protocol stack updates the process data in the internal input
buffer for each valid bus cycle. Each IO block uses handshake bits for access synchronization. Input
and output data block handshake operates independently from each other. When the application
toggles the input handshake bit, the protocol stack copies the data from the internal buffer into the
input data image of the dual-port memory. Now the application can copy data from the dual-port
memory and then give control back to the protocol stack by toggling the appropriate input handshake
bit. When the application / driver toggles the output handshake bit, the protocol stack copies the data
from the output data image of the dual-port memory into the internal buffer. From there the data is
transferred to the network. The protocol stack toggles the handshake bits back, indicating to the
application that the transfer is finished and a new data exchange cycle may start. This mode
guarantees data consistency over both input and output area.
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Edition 3 – API:DeviceNet-Master#EN– 2009/01