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P-NET is specified and implemented according to the Open Systems Interconnection Reference Model, on layers 1, 2, 3, 4, and 7, as shown in the diagram in fig. 11.
Normally, a Fieldbus is only implemented on layers 1,2 and 7, but since P-NET features the multi-net structure, the protocol also implements layers 3 and 4.

Layer 1 is concerned with transmitting raw bits over the bus. It specifies the cable, how a “1” and a “0” is represented on the bus, what the voltage level is, etc.
Layer 2 takes care of the multi-master token, packs the data to be sent into a frame, including source and destination addresses, and performs error detection.
Layer 3 is the P-NET “post office”, which receives and sends the frames according to the destination address. A message may be required to be sent out of another P-NET port, or into the P-NET service, or back to the requesting application, or return a message indicating an unknown address. It also performs the address conversion necessary to ensure a response finds its way back.
Layer 4 handles two different tasks.The first provides the P-NET service, which reads or writes data to internal memory via the SOFTWIRE list, or reroutes a request, if the SOFTWIRE list indicates that the variable is located in another node.The second task holds details about the number ofrequests which have been sent out but are waiting for a reply. When the reply arrives, it is sent back to the calling application task.
Layer 7 is used by application programs to access variables in other nodes. This is done by sending a command block containing references to the SOFTWIRE list, which is where detailed information such as node address, internal address etc.is specified. The SOFTWIRE list is also used for internal variables.

Index Previous chapter Next chapter
P-NET in General Access to P-NET from PC's
The History of P-NET Software
Application Areas Ease of P-NET Implementation
Principles of P-NET P-NET Architecture
Multi-net Structures Virtual Token Passing
Advantages of the P-NET Protocol P-NET Compared to...
Intelligent P-NET Modules International P-NET User Organization
"Layer 8": P-NET Channel Structure
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