What is a transmission signal?

• A. A rapid change in voltage used for signaling.
• B. The data used to power a device.
• C. A fixed binary pattern transmitted over a channel.
• D. The data sent from one device to another.

Answer: (D)

In instrumentation, a transmission signal is the information content that travels from one device to another over a channel. It represents the data or instructions being exchanged, such as a measurement, a control command, or status information. In a 4–20 mA loop, the current carries the analog value that represents the process variable, and in systems like HART, additional digital data can ride on that same channel. The essential point is that the signal is about what is being communicated—the data sent from one device to another—rather than the power that may also be used to run devices or the specific waveform or pattern of that signal. While a rapid voltage change or a fixed binary pattern can describe certain kinds of signals, they’re not the defining idea of a transmission signal here, which is the information conveyed from transmitter to receiver.