Connection Types

For Change of State connections (transfer in case of a state change), a DeviceNet™ subscriber only transmits the data if they have changed since the last transmission. In case of slow processes, this can lead to longer send pauses. Due to this, all devices are equipped with an adjustable heartbeat timer. If this heartbeat timer is elapsed and the data did not change in the mean time, they are transmitted nevertheless. The heartbeat timer is triggered with every new transmit process. That means, the subscriber transmits the data in case of a state change, but definitely when the heartbeat interval is elapsed. This way, the receiver is informed that the sender works correctly and did not fail. Hence, transmitting a request to the sender of the data is not required.

Cyclic connections are mainly used if a fixed time slot pattern is set by default. If, for example, a temperature sensor is used in a slow closed-loop control circuit with a sampling rate of 500ms, it is recommended to update the temperature value cyclically in intervals of 500ms. Shorter update intervals would not provide additional information to the loop controller but cause unnecessary high load on the bus instead.

Change of State connections and Cyclic connections are Acknowledged Exchanges per default, i.e. the consumer (receiver) of the data transmits an acknowledgement to the producer (transmitter) as a receipt confirmation. These acknowledgements can be suppressed using the Acknowledge Handler Object (refer to the object model) in order to avoid additional telegram traffic for systems which are anyway heavily loaded due to fast update times.

Multi Cast connections are also managed using the Acknowledge Handler Object. The slave data are not only read by the master but also by other subscribers, e.g. by operating stations which monitor the telegram traffic and process and acknowledge the data they require for display, alarm and archive purposes.

In case of Polling connections, the master transmits a poll command to the slave. If the master contains data intended for the slave, these data are also transferred. This can be performed using one single telegram or with help of the fragmentation services. Data volumes with a size greater than 8 bytes are divided into subsets and then transferred one after the other. The slave recombines the data packages received from the master. If the slave contains data that are directed to the polling master, it transmits these data to the master. The response telegram of the slave alternatively or additionally contains status information. If the slave does not respond to a polling request of the master, a timeout occurs. Especially in case of slow processes, a disadvantage of Polling connections is that they cause unnecessary high load on the bus since often the data do not change between two procedures.

Bit Strobe connections are used to transfer small data volumes between a master and one or several slaves. The Bit Strobe telegram transmitted by the master contains 64 bits of user data. Each bit is assigned to one subscriber or bus address. Therefore, Bit Strobe telegrams have a broadcast functionality. As all addressed slaves receive the telegram at the same time, Bit Strobe is often used for the synchronization of input or output data. A slave can provide the assigned bit directly at an output or interpret the bit as a trigger condition in order to freeze input values and to transfer these values to the master with the next polling telegram. The data sent back by the slave are limited to a size of 8 bytes. Thus, Bit Strobe causes lower bus load than the Polling method.