Issue
Too much LON communications traffic can cause LON communications errors.
Environment
MNL-50, MNL-100, MNL-150, MNL-200, MNL-110, MNL-130, MNL-VxRVx controllers
Cause
Sending output LON SNVT values too frequently or more often than reasonably needed.
Resolution
In a particular application the engineer needs to consider the amount of data being transmitted on the LON network due to LON network bandwidth limitations. On an FTT-10 LON network, the bandwidth is limited to approximately 72 packets (messages) per second. Because the LON network is operating using CDMA (Collision Detect, Multiple Access) communications techniques, data collisions will reduce the practical data limit to approximately 1/2 of the maximum. Typically the LON network designer will limit the packet transmission rate of 30 to 35 packets per second.
There are a series of NCI (configuration) objects that control the transmission and reception of these data values on the LON network.
- The nciSndHrtBt determines the maximum amount of time that is allowed before all bound NVOs are updated, that is, the data is marked to be "re-sent" to all bound nodes. This value is typically set to 120 seconds, causing all bound NVO SNVT values to be transmitted at least once every two minutes.
- The nciRcvHrtBt sets the fallback timer value for each NVI object. If a value is not received via a binding in this time period, the NVI object will revert to its application defined default value. It should be noted that this value does not directly affect the LON bandwidth but it does affect how the controller handles the loss of the bound data communications. This value is typically set to about 2.5 times the nciSntHrtBt to allow for occasional lost communications packets and delays due to data collisions occurring on the network.
- The nciMinOutTm controls how often a bound NVO value will be transmitted when the value is changing within the controller. Each specific SNVT type has a specific data resolution and any time the data value changes by one count, the value will be sent. For example a temperature value bound through a NVO as a SNVT_temp_p type has a resolution of 0.01 degrees Celsius. If bound this temperature NVO will be transmitted each time the value changes by 0.01 degrees C. The setting of the nciMinOutTm throttles this bound NVO value transmission by forcing the controller to ignore any changes to that NVO value for a specific time interval each time it is transmitted. The nciMinOutTm value defaults to NA causing the bound NVO value to be transmitted each time it changes. Typically the nciMinOutTm is set to a value in the 1 to 5 second range causing the controller to throttle the transmission of the changing NVO values.
