BACnet is a communication protocol for Building Automation and Control networks that has been developed by ASHRAE Standards Project Committee (SPC) 135P. The committee has been working on the standard since the middle of 1987 and published a draft version for public review in August, 1991. As a result of the public review (which generated 507 formal comments), numerous substantive changes have been made to the proposed standard. Included are changes in object type definitions, application layer services, and the network layer, MS/TP and Point-to-Point protocols. Before describing these changes and why they were made, a brief overview of BACnet would be helpful.