Principles of P-NET
The electrical specification of P-NET is based on the RS485 standard using a shielded twisted pair cable. This allows a cable length of up to 1200 m without repeaters. Data is sent as an asynchronous transmission in NRZ code.
P-NET interfaces are galvanically isolated, and up to 125 devices per bus segment can be connected, due to a special clamp circuit, and again without the use of repeaters.
P-NET is a very efficient Fieldbus protocol, in that it can handle up to 300 confirmed data transactions per second, from 300 independent addresses.
Data can be transferred in the form of fully processed values (floating point), such as temperature, pressure, current, voltage etc., or as blocks of 32 independent binary signals, indicating valve states, switch positions etc.
This results in a performance of up to 9,600 binary signals per second being accessed from anywhere within the complete system.
This high rate of fully acknowledged data transmissions can be achieved, because P-NET slaves handle the processing of data and the reception or transmission of frames, in parallel. The processing of a request by the slave is initiated as soon as the first data bytes arrive.
This is in contrast to dedicated chip solutions, where the entire frame arrives before processing begins. In this way, the standard P-NET data rate of 76,800 bit/s, is not a limiting factor in performance.
The performance can be compared with systems using data rates up to 500,000 bit/s.
P-NET is a multi-master bus, which can accept up to 32 masters per bus segment. All communication is based on the principle, where a Master sends a request, and the addressed Slave returns an immediate response. Requests can be of a read or write type.
Data transferred on the bus can be of a simple or complex type, to satisfy the requirements of measurement and control. Simple types include boolean, byte, char, word, integer, long integer, real, long real and timer. Complex types include array, string, record and buffer.
The data format is a part of the P-NET standard.
The right to access the bus, is transferred from one P-NET master to another, by means of a token. P-NET uses a method called “virtual token passing”, which does not require messages to be sent over the bus.
When a master has finished bus access, the token is automatically passed on to the next master, by a cyclic mechanism based on time. The method used in P-NET differs from that used in other multi-master systems.
Other busses such as Profibus for example, use real message telegrams for transferring the token. This results in an increase in master processing time, and reduces the capacity of the bus.
The virtual token passing principle also accepts that a master might not even be present. In this situation, all devices, including other masters, will continue performing normally. See page 16 “Virtual Token Passing” for a detailed description.