240 • LON

Network communication

5.5 Network communication

LON is a decentral bus system, i. e. the individual components can communicate via the bus without requiring a master. In this manner, the information paths from the sensors via the host to the actuators are drastically shortened, and the rest of the system is no longer loaded with unwanted data traffic.

The exchange of messages between two nodes in one network is performed by defining of so-called network variables (NV's).

Network variables are data interfaces for communication via the network backed with a physical type. There are network variables for instance for current, voltage, power, temperature, pressure, date, time, presence, ...etc.

The LON bus coupler writes the input and output data into defined network variables, which in turn can be linked with network variables of other nodes.

The maximum number of input and output channels is determined by the maximum number of network variables (62).

With LON, only agreed status changes, elapsing timers and/or limit value overruns are signalled. Not every signal is subject to a cyclical screening. As a consequence, the function of the actuators, sensors and controllers is event controlled and a network variable only transmitted when its value has changed.

This value is received by all nodes in which this network variable together with a network input and network output link has been defined.

This means that the programmer need not care about node addressing, data buffers, message transmission services and other details when using network variables.

Just one example to clarify the function of a data exchange between network nodes via a network variable.

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Network communication

5.5.1 Data exchange via network variables

An example (source [2]):

Let us assume that node 1 (sensor) is a temperature sensor. A network output variable ‘temperature’ (NVO_temperature) is defined and assigned the current temperature value measured.

Fig. 5-1: Example for a network output variable

For another node 2 (actuator) intended to control a heat radiator, a network input variable ‘temperature’ (NVI_temperature) is defined which is assigned the current temperature value from the sensor node via the network.

Fig. 5-2: Example for a network input variable

The definition of these NV's is made independent from one another.

Binding now links these two network nodes together. The current value is automatically transferred by node 1 to node 2 via the network.

Fig. 5-3: Example for the data exchange of the network variables

The temperature sensor signals a temperature change, and the logically assigned controls correspondingly switches the heat radiator on or off.

5.5.2 Standard types of network variables

The so-called standard types of network variables (SNVT's) are invariably predefined types of variables for network variables. Almost any size occurring in automation tasks have been standardised. The ensure an automatic understanding of network nodes originating from different programmers.

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Network communication

5.5.3 Protocol

Individual network nodes communicate via one common communication protocol, the so-called LonTalk protocol.

The LonTalkprotocol is based on the OSI reference model (ISO 7498).

It is part of the Neuron chip as a complete communication protocol and makes services available for the transmission of data to other nodes. For this reason, there is no need to program the sequence of these services. All the programmer has to do is select among them and to concentrate on the programming of measuring and control algorithms.

Following each task, the LON technology offers four different transmission services:

•Request/Response(confirmation of receipt only once the message has been processed)

•Acknowledged (always simultaneously with the confirmation of receipt, the safest method of data transmission)

•Unacknowledged Repeated

•Unacknowledged Service

The Acknowledged transmission service is typical for the LonTalk protocol. To exclude transmission errors, the messages can be sent together with the acknowledgement request. It is also possible to repeat their transmission.

With the Unacknowledged transmission service, the receivers do not supply a confirmation of receipt.

5.5.4 Channel access method

The channel access method, also termed access or control method, represents one variant of the CSMA method (Carrier Sense Multiple Access ~ free channel access with data traffic detection on the bus) of the LonTalk protocol. The channel access is performed without the assignment of an access authentication, and only if the channel is free.

LON offers the additional possibility to assign message priorities which can be useful for alarms or critical events. A special authentication process guarantees an optionally increased access reliability, for which reason LON can also be used in special safety relevant applications, such as, for instance in fire alarm and burglar alarm systems.

The channel access method of the LonTalk protocol allows the communication through various media with a good data throughput both with a low and with a high channel load. This also applies to large networks.

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