An RTD measures temperature by changing its electrical resistance, so reading one accurately means measuring resistance accurately - and that is harder than it sounds when the sensor sits at the end of a long cable. The resistance of the lead wires adds to the sensor's resistance and looks like extra temperature if you are not careful. An RTD input module is the card built to handle this, and the number of wires it uses - two, three, or four - is all about cancelling that lead resistance. This guide covers the RTD card and its wiring, complementing what an RTD sensor is with the module and connection side.
RTD Input Module in one line: An RTD input module (RTD card) is a PLC or RTU module that measures the resistance of a resistance temperature detector, such as a Pt100, and converts it to a temperature. It supplies a small, precise excitation current, measures the resulting voltage, and uses 2-, 3-, or 4-wire connections to compensate for the resistance of the field lead wires, which would otherwise add error to the reading.
An RTD's resistance rises predictably with temperature - a Pt100 reads about 100 ohms at zero degrees Celsius and climbs from there - so the input module's job is to determine that resistance precisely. It does this by forcing a small, known excitation current through the sensor and measuring the voltage that develops across it; resistance is voltage divided by current. The excitation is kept small on purpose, because too much current would heat the RTD element itself and inflate the reading, an effect called self-heating that a well-designed card deliberately avoids.
The complication is the field wiring. An RTD often sits far from the controller, and the lead wires between them have their own resistance that adds to the sensor's. Because a Pt100 changes only a fraction of an ohm per degree, even a modest lead resistance can look like several degrees of error. A plain two-wire connection has no way to separate lead resistance from sensor resistance, so it measures both together and reads high. Solving that is precisely what the three- and four-wire schemes exist to do, and it is why RTD wiring gets so much attention in the field.
A two-wire connection is the simplest and least accurate: the module cannot tell sensor resistance from lead resistance, so all the lead resistance is counted as temperature. It is acceptable only for short runs or where precision is not critical. A three-wire connection - the most common industrial choice - adds a third wire that lets the module measure the resistance of one lead and, assuming all leads are the same type and length, subtract that amount from the reading. This cancels most of the lead-resistance error with just one extra conductor, which is why three-wire RTDs dominate plant installations.
A four-wire connection is the most accurate. It uses two wires to carry the excitation current and two separate wires to sense the voltage right at the sensor. Because essentially no current flows in the sensing wires, their resistance drops no meaningful voltage, so the module reads only the true resistance of the RTD element itself, fully independent of lead length or wire mismatch. Four-wire is favored for laboratory-grade or custody-critical measurements where the extra conductor and terminal are worth it. The module must be configured to match how the sensor is actually wired - telling a three-wire card that it has a four-wire sensor, or the reverse, defeats the compensation and reintroduces the very error the wiring was meant to remove.
RTDs are the preferred choice where accuracy and stability matter more than the very high temperatures thermocouples reach, so their readings often feed the more precision-sensitive temperature points a SCADA system watches - process temperatures on separation and treating equipment, bearing temperatures, and custody-relevant measurements. Those values, measured on an RTD card and scaled in the controller, become the trends and alarm points a platform like Merobix presents to operators who may be nowhere near the site.
Because remote operators cannot inspect the wiring, correct RTD connection quietly underpins the trust in those numbers. A three- or four-wire installation that properly cancels lead resistance means a temperature on the dashboard reflects the process, not the cable run, even for a sensor hundreds of feet from the RTU. Module-level diagnostics that flag an open or shorted RTD add another layer, letting the SCADA system show a sensor fault rather than a plausible wrong value - so a broken RTD becomes a visible alarm instead of a silent measurement error on an unattended site.
A three-wire connection uses one extra wire to measure a single lead's resistance and subtract it, assuming all leads match, which cancels most lead-resistance error. A four-wire connection separates current-carrying and voltage-sensing wires so the module reads only the true sensor resistance, fully independent of lead length. Four-wire is more accurate; three-wire is the common industrial compromise.
An RTD like a Pt100 changes only a fraction of an ohm per degree, so even a small amount of resistance in the field lead wires adds to the sensor's resistance and looks like extra temperature. On long cable runs this can amount to several degrees of error. Three- and four-wire connections exist specifically to compensate for that lead resistance.
It usually can be done, often by jumpering terminals, but you lose the lead compensation and the reading will be high by the lead resistance. Two-wire is acceptable only for short runs or non-critical points. For accuracy, wire the RTD as three- or four-wire and set the module to match the actual wiring, or the compensation will not work.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.