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[KB684]BACnet – Segmentation feature

Tags: Segmentation

Data acquisition
11 years ago
By BO
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Applies to:

All PcVue version
Summary:

 This article describes how the segmentation of BACnet messages works and how to manage it with Pcvue.
Details:

The segmentation is the possibility to split a big frame into several smaller frames. This segmentation is only performed on confirmed-request messages and complex-ACK messages that is to say ReadProperty services.

ReadProperty services are divided in two services:

  • ReadProperty: This service allows the possibility to read only one property by frame and by device. Every device normally supports this service but is really not efficient.
  • ReadPropertyMultiple: This service allows the possibility to read several properties by frame and by device. The service is more efficient but takes in account others parameters such as “Max APDU Length” or “Segmentation Supported”. These parameters are provided in the PICS of the device or could be read during the discovery of the network.

The Max APDU Length is the max size of a frame that a device could exchange. Below you can see the example of a SAIA device which can accept a frame of 1476 byte size.

APDU

Keep in mind that PcVue can exchange frames of 1476 byte size too.
So if PcVue sends a request to much bigger for a device, PcVue will split the frame in several smaller frames.
Then, the “Segmentation Supported” parameter informs you if a device support or not the segmentation.

Segmentation Supported

Now if the device doesn’t support the segmentation, the purpose is to reduce the size of the frame.
PcVue is able to optimize the size of the frame to fit with the capability of the device.
In case of interoperability problem, you can set the size in the advanced properties of the device.

Size

 

Created on: 15 Jan 2014 Last update: 13 May 2024