P-NET Compared to Dedicated Fieldbus Chip Solutions

Bus systems using special chips (e.g. Profibus FMS), typically receive the complete frame first. Then the chip sends an acknowledge to the master and an interrupt to the host CPU.

Slave processing is then started, and when all data is ready, it is transferred to the chip. Now the Master has to make a second request to obtain the desired result. This is illustrated in the diagram in fig. 13.

P-NET slaves handle the processing of data and the reception and transmission of frames, in parallel. The processing of the request begins in the slave, as soon as the first data bytes arrive. In this way, the standard P-NET data rate of 76,800 bit/s, is not a limiting factor in performance.

Profibus chip principles

P-NET without special chip

Figure 13: P-NET communication compared to Profibus chip principle.

Every P-NET slave module must answer a request within 390 microseconds (“immediate response”). This eliminates the need for multiple requests for a single variable, or even continuous polling until a result is ready. The immediate response eliminates the need for buffers in the slave to contain a queue of requests or polling from different masters.

The immediate response, coupled with parallel operation and fast token passing, results in a performance similar to other bus systems with a much higher data rate (e.g. 500 kbit/s).

One of the drawbacks of increasing the data rate , is that it leads to a significant reduction in the Fieldbus cable length allowed. For example, at 76.8 kbit/s the bus length can be in the region of 1.2 Km, but at 500 kbit/s the bus length would need to be reduced to 200 m.

The consequence of this is, that for a comparable full size system, 5 extra repeaters would have to be considered for the higher rate system.