Do the CEI-420's discrete inputs have voltage input limitations? If so, what are they, and where are these voltages in the internal circuit?

The CEI-420's discrete input voltage range is +/- 100 volts. The input voltage referred to in the CEI-x20 User's Manual is the voltage at the I/O connector pin, relative to ground. The input threshold is also relative to the input signal at the I/O connector.

For additional information, review the diagram (located under the "Attachments" section) showing the discrete input circuit.