The Spectrum modules are universal input modules, not modules dedicated to RTD inputs only. Universal modules are almost always isolated and do not share a common return or (-) terminal.
The need for isolation becomes apparent with thermocouples or mV signals that can be grounded on the field end, which creates common mode problems with non-isolated analog inputs.
An RTD measurement is based on pumping a fixed, known current source through a field resistor and reading the voltage drop and compensating for wire resistance.
The (reasonable) assumption is that an RTD is isolated from the process so that other electrical (ground) currents do not interfere with the AI card's constant current source. The assumption holds true for all commercial RTD's which are packaged in SS sheaths with MgO insulation electrically isolating the RTD element from the sheath. Since the RTD element itself is isolated from the process, the RTD input board does not have to have the level of isolation that a universal input card does.
I could not find the word 'Isolate' in the AB 1794-IR8 manual so it isn't clear how much isolation is provided with that card.
The Spectrum 1794sc-IRT81 module is a universal temperature input module, it can handle either thermocouple or RTD inputs. Thermocouple inputs desperately need isolation. The RTD's channels get isolation because it's there for the thermocouples, even though it isn't critical for RTDs.
The Spectrum 1794sc-IF8U module has isolation because it is a universal input card handling not only thermocouple or RTD, but 0/4-20mA, current or mV sources.
