Architecture and Protocols

Learning objectives

• >Describe the field, controller, supervisory and front-end tiers of a BAS
• >Explain BACnet objects, properties and core services
• >Compare BACnet/IP and MS/TP, and explain the 16-level priority array
• >Explain Modbus registers and coils, and recognize legacy protocols
• >Separate protocol openness from programming openness

Sessions

Three 2-hour weeknight sessions plus a weekend block. Check each one off as you finish it.

1. SESSION 1· 2 h Three-tier architecture (podcast or video plus notes). Supervisory layer vs controller layer
2. SESSION 2· 2 h BACnet Institute intro course: objects, properties, services
3. SESSION 3· 2 h BACnet/IP vs MS/TP, the priority array, and COV. Modbus basics. Note the legacy protocols (LON, N2)
4. WEEKEND· 3 h Exercise: network riser for an office building (IP trunk to AHUs, MS/TP to 40 VAVs, Modbus to the chiller and boiler, pump VFDs). Read the Karg BACnet paper for concepts

Answer in your notebook first, then reveal the answer and grade yourself honestly.

1. Q1What BACnet service does a device use to discover others, and what is the reply?

   [ reveal answer ]

   Who-Is. The reply is I-Am.

   ✓ I got it ✗ Missed it

2. Q2What physical layer does MS/TP use, and why do large MS/TP trunks get slow?

   [ reveal answer ]

   RS-485 with token passing. Low baud rates, many devices, and heavy polling or COV traffic slow the token.

   ✓ I got it ✗ Missed it

3. Q3How many levels are in the BACnet priority array, which is highest, and what happens when every level is null?

   [ reveal answer ]

   16 levels, 1 is highest (8 is typically manual operator), and the relinquish default applies when every level is null.

   ✓ I got it ✗ Missed it

4. Q4What is the difference between a Modbus holding register and a coil?

   [ reveal answer ]

   A holding register is a 16-bit read/write value. A coil is a single read/write bit.

   ✓ I got it ✗ Missed it

5. Q5A controller is a native BACnet device. Why does that not mean you can edit its program with any tool?

   [ reveal answer ]

   BACnet only exposes points and services. Editing the control logic usually requires the manufacturer's programming tool.

   ✓ I got it ✗ Missed it