An unipolar input module can accept an input signal that varies in the:

• A. Positive direction only
• B. Negative direction only
• C. Both directions
• D. Amplitude direction only

Answer: (A)

Unipolar input modules are designed to measure signals that stay on one polarity. In PLC systems, this means the signal ranges from zero up to a positive maximum and does not go negative. Because the module uses zero as its reference, any variation must occur in the positive direction relative to ground. If the signal could swing negative, a bipolar input module would be required. Typical unipolar ranges are 0 to +10 V or 0 to +24 V, or 0 to +20 mA for current loops. The other ideas—negative-only, both directions, or any notion of “amplitude direction”—don’t fit how unipolar inputs are defined, since they rely on a single, nonnegative polarity.