Which of the following is not a type of control signal for a building automation system?

• A. Protocol
• B. Analog
• C. Digital
• D. Discrete

Answer: (A)

In a building automation system, the control information that drives equipment comes in specific physical forms, while protocol refers to how data is exchanged. The actual control signals are either analog or digital/discrete. Analog signals vary continuously, like a 0–10 V (or 4–20 mA) signal that can position a valve or modulate a damper smoothly. Digital or discrete signals, by contrast, are binary on/off states used for devices such as pumps or relays.

Protocol, however, is not a signal form; it’s the set of rules that governs how devices communicate with each other—how data is packaged, addressed, timed, and checked for errors. It dictates the communication method between controllers and field devices (for example, BACnet or Modbus) but does not define the electrical or logical signal level that the device interprets. This is why protocol isn’t a type of control signal.