BACnet - A Data Communication Protocol for Building Automation and Control Networks

Abstract

Chris Howard, Schneider Electric.

800 Federal Street, Andover, Massachusetts.

This request for interpretation refers to the requirements presented in ANSI/ASHRAE Standard 135-2016, Clauses 5.2.1.2, 9.7.1 and 19.4, regarding the requirements for devices to know the Maximum Conveyable APDU to remote networks

Clause 19.4 outlines a procedure for a 'node' to determine the Maximum Conveyable APDU to a remote network and requires the node to cache the determined value.

References in the Standard that refer to Clause 19.4 are found in Clause 5.2 (Segmentation) and 9.7.1 (Routing of BACnet Messages from MS/TP).

There are some inconsistencies in Clause 5.2.1.2 because Clause 5.2.1.2 (b) implies that a device must know the maximum APDU size conveyable to a remote network to choose an appropriate APDU size that will reach the recipient