Wiring a 4-Wire PT100 RTD to the Omron CPM1A-TS101-DA Input

Overview

Connecting a 4-wire PT100 RTD to an analog input module designed for 3-wire termination is a routine field issue. The Omron CPM1A-TS101-DA provides one Pt100/JPt100 input with three terminals labelled PT1A, PT1B, and a second PT1B. When the field cable is a 4-wire PT100 (Kelvin construction), one lead becomes redundant. The correct installation is to leave that fourth conductor physically disconnected and insulated. This article documents the underlying lead-resistance compensation math, the step-by-step wiring procedure, CX-Programmer configuration, a verification method, and the error budget produced when the fourth wire is mis-terminated.

PT100 RTD Fundamentals

A PT100 is a platinum resistance temperature detector with a nominal resistance of 100.00 Ω at 0 °C. The resistance/temperature characteristic is defined by IEC 60751 as a polynomial with a nominal slope of 0.385 Ω/°C in the 0–200 °C industrial range. Selected reference points:

Tolerance classes per IEC 60751:

• Class A: ±(0.15 + 0.002 |t|) °C — at 0 °C this is ±0.15 °C; at 100 °C it is ±0.35 °C.
• Class B: ±(0.30 + 0.005 |t|) °C — at 0 °C this is ±0.30 °C; at 100 °C it is ±0.80 °C.
• 1/3 DIN (Class AA): ±(0.10 + 0.0017 |t|) °C — at 0 °C this is ±0.10 °C.

Wiring Configurations

Three wiring topologies exist for an RTD probe:

• 2-wire: Excitation and sense share the same conductor. Lead resistance adds directly to the element reading. Acceptable only for short runs (≤ 2 m) where lead resistance is small compared to the element change.
• 3-wire: Two leads share the same resistance (assumed matched) and a third lead provides the compensation reference. The lead-resistance contribution is cancelled as long as leads are equal length, equal gauge, and same conductor material.
• 4-wire (Kelvin): Separate excitation and sense pairs. Sense leads carry only the high-impedance voltmeter input current (nA range), so their IR drop is negligible and lead resistance is mathematically eliminated.

Omron CPM1A-TS101-DA Hardware Reference

The CPM1A-TS101-DA is a CPM1A-series expansion module combining one Pt100/JPt100 input channel with one analog output (voltage or current). Refer to the Omron Industrial Automation portal for the operation manual revision shipped with your module. Pt100 input section terminals:

Key module specifications (from the CPM1A series datasheet):

• Input range: Pt100 0–200 °C; JPt100 0–200 °C.
• Resolution: 1/256 of full scale (≈ 0.78 °C/bit) for the RTD input channel.
• Conversion time: ≤ 250 ms per channel.
• Accuracy: ±1.0 % of full scale (subject to lead-resistance compensation).
• Isolation: photocoupler between analog section and PLC bus.
• Terminal screw torque: 0.5 N·m.

The two PT1B terminals are electrically distinct. The module drives a constant current through PT1A and the first PT1B; it then measures the voltage drop across the lead on the second PT1B and doubles it (since both B-leads are assumed to have the same resistance) to recover the true element resistance. This topology is fundamentally 3-wire and cannot accept a 4-wire Kelvin signal.

Lead-Resistance Compensation Theory

For a 3-wire PT100 with lead resistances R1 (on A-lead), R2 (on B-lead 1), R3 (on B-lead 2), the module performs two measurements:

1. A-to-B1: V_AB1 = I × (R_element + R1 + R2).
2. A-to-B2: V_AB2 = I × (R_element + R1 + R3).

With R2 = R3 (the matched-pair assumption) the element resistance is recovered as:

R_element = (V_AB1 / I) − (V_AB2 − V_AB1) / I = V_AB1 / I − R2

Any deviation from R2 = R3 produces a residual error equal to (R2 − R3)/2 at the element-resistance reading. This is the source of the +2 °C error example calculated later for a 10 m run when the 4th wire is tied to PT1B.

A true 4-wire Kelvin measurement would require separate excitation (EXC+, EXC−) and sense (SENSE+, SENSE−) terminals with independent current sources. The CPM1A-TS101-DA does not provide this. For a deeper treatment, see the Texas Instruments application note on 4-wire RTD measurement circuits with low-side reference.

Root Cause: Why the 4th Wire Must Stay Disconnected

The module's compensation algorithm assumes exactly three conductors reach the element: PT1A, PT1B, and the second PT1B. The two B-leads must share the same physical node at the probe end. Three improper terminations of the fourth wire produce errors:

Termination A — Tie to PT1A

Connecting the redundant A-lead to PT1A creates a low-impedance parallel path from the constant-current source back to the element's A-end. The element current splits between the two leads in inverse proportion to their resistances. The module's compensation reference, however, is built on the PT1B-side sensing and still subtracts (R2 + R3)/2 from V_AB1. Because R1 has effectively become R1 || R1' = R1/2, the measured element voltage drops by (I × R1/2) — a fixed offset — rather than scaling with lead length. The reading shifts positive by ≈ R1/2 / 0.385 Ω/°C. For a 10 m run this is ≈ 1.0 °C.

Termination B — Tie to PT1B

Connecting the redundant B-lead to either PT1B places two conductors in parallel between the terminal and the probe end. The effective B-lead resistance drops from R2 to R2 || R2' = R2/2. The compensation algorithm, which doubles the measured lead resistance, now over-corrects by R2/2. Residual error is +R2/2 / 0.385 Ω/°C, ≈ 2.0 °C for 10 m of 24 AWG copper.

Termination C — Tie to AG or Earth

Connecting the redundant wire to analog ground or chassis earth shorts one end of the element to ground through a low impedance. The constant-current source will drive into the short, the ADC saturates, and the module reports out-of-range. This is the worst-case failure mode and must be avoided during commissioning.

Correct Termination — Leave Disconnected and Insulated

The fourth conductor is allowed to float. To prevent antenna pickup and 50/60 Hz induced voltage, terminate it with a heat-shrink sleeve or terminal-block end-sleeve and dress it back into the cable jacket.

Prerequisites

• Omron CPM1A or CPM2A CPU with the TS101-DA module seated on the expansion bus.
• CX-Programmer 9.x or later (CX-One 4.x recommended).
• PT100 Class A or Class B probe with 4-wire tails; ferruled leads.
• Calibrated decade resistance box or PT100 simulator.
• True-RMS digital multimeter for lead-resistance verification.

Step-by-Step Wiring Procedure

1. De-energise the CPM1A system. Lock out the 24 VDC supply feeding the I/O bus.
2. Strip 9 mm of insulation from each PT100 lead. Crimp insulated ferrules.
3. Identify the two leads bonded to each end of the element. With an ohmmeter, probe between every lead pair. A near-zero reading (≤ 0.1 Ω) identifies the same-end pair.
4. Land one lead of the A-pair on terminal PT1A.
5. Land one lead of the B-pair on the first PT1B terminal.
6. Land the other lead of the B-pair on the second PT1B terminal.
7. Insulate the remaining A-lead (the 4th wire) with heat-shrink. Tuck it back into the cable jacket. Do not land it on any terminal.
8. Torque all terminal screws to 0.5 N·m. Confirm no stray strands bridge terminals.
9. Restore 24 VDC and observe the input word in CX-Programmer online mode.

Schematic representation of the wiring:

   PT100 Element (4-wire Kelvin construction)
   +-------- A-end
   |         |
   |       (red lead 1) ---- PT1A  (excitation + sense high)
   |       
   |       (red lead 2) ---- [HEAT-SHRINK INSULATED, NOT CONNECTED]
   |
   +-------- B-end
             |
           (white lead 1) -- PT1B  (sense return 1)
           (white lead 2) -- PT1B  (sense return 2, lead compensation)

CX-Programmer Configuration

The CPM1A-TS101-DA reports the Pt100 input as a 12-bit value (0x0000–0x07FF) in the assigned IR input word (typically IR n). Scaling: 0x0000 = 0 °C, 0x07FF = 200 °C, so each bit ≈ 0.156 °C. CX-Programmer setup:

1. In the PLC I/O Table, verify the TS101-DA is recognised and the word allocation is correct (input word = n, output word = n+1).
2. Open the DM area. Locate DM n (analog I/O range setting). Set bits 8–15 to 0000 for Pt100 0–200 °C, or 0001 for JPt100.
3. DM n+1 selects averaging count. Recommended value: 0003 (8-sample average) for mains-frequency rejection in 50/60 Hz panels.
4. DM n+2 enables the input. Set bit 0 to 1 to enable the Pt100 channel.
5. Read IR n in a contact/PLC monitor window. Convert to engineering units: T(°C) = IR n × 200 / 2047.

Example conversion: IR n = 0x04B0 = 1200 decimal. T = 1200 × 200 / 2047 = 117.3 °C.

Ladder Logic Example: Scaling the PT100 Reading

The following CX-Programmer ladder illustrates the conversion from the raw 12-bit input to a floating-point temperature in 0.1 °C engineering units:

|--[ IR 200 ]--[ MOV(21) #0 D200 ]--|                       ; Clear working register
|--[ IR 200 ]--[ *F(456) #0.0977 D202 ]--|                 ; Scale: 200/2047 = 0.0977 °C/bit
|--[ D202 ]--[ROUND(45) D202 ]--|                          ; Integer result in 0.1 °C
|--[ D202 > 0 ]--[ MOV(21) D202 D204 ]--|                   ; Sanity check lower bound
|--[ D202 < 2000 ]--[ MOV(21) D202 D204 ]--|                ; Sanity check upper bound

For applications requiring 0.01 °C resolution, use double-precision arithmetic and a moving-average filter over 16 samples. Always check the result against the under-range (≈ −20 °C) and over-range (≈ +250 °C) module outputs before scaling.

Verification Procedure

1. Substitute resistance box: Disconnect the PT100 probe and connect a decade box in 4-wire mode across PT1A, PT1B, PT1B, with the fourth box lead left floating.
2. Set 100.00 Ω. Read IR n. Expected raw value = 0x0407 (decimal 1031) ± 1 LSB (≈ ±0.16 °C). Module tolerance plus box tolerance yields expected reading 0.0 ± 0.5 °C for Class B.
3. Set 138.50 Ω. Expected raw value = 0x0589 (decimal 1417). Module reading 100.0 ± 0.8 °C.
4. Set 175.84 Ω. Expected raw value = 0x0703 (decimal 1795). Module reading 200.0 ± 1.2 °C.
5. Reconnect the actual probe. Place probe and a calibrated reference RTD in an isothermal block. Compare readings; deviation should be within sum of Class tolerances.
6. Measure the floating 4th wire with a true-RMS DMM referenced to earth. Acceptable induced voltage: < 50 mV RMS. Above 200 mV indicates routing too close to VFD cables.

Lead Resistance Error Budget

The error scales linearly with run length. For runs longer than 25 m or applications requiring Class A tolerance, upgrade to a 4-wire-compatible module or use a remote 4-wire-to-4–20 mA transmitter at the probe head.

Alternative 4-Wire Compatible Modules

If the application demands the full 4-wire accuracy, consider these Omron modules from the CP1W series:

The CP1W-TS003 and TS004 accept the full 4-wire Kelvin connection. Refer to the Omron Industrial Automation portal for the specific CP1W-TS003/TS004 operation manual before retrofitting.

Cross-vendor equivalents with native 4-wire Pt100 inputs:

• Siemens SM 1231 RTD (6ES7231-5PD32-0XB0) — supports 2-/3-/4-wire RTD.
• Allen-Bradley 5069-IY4 (Logix 5000) — 4-channel universal RTD/thermocouple, 4-wire capable.
• Schneider Electric BMX ART0414 (Modicon M340) — 4-channel RTD, 2-/3-/4-wire.

Edge Cases and Field Diagnostics

Case 1 — Shielded cable with drain wire

If the PT100 cable includes a shield/drain, terminate the drain at the panel end only to the analog ground (AG) terminal. Do not ground at the probe end — this creates a ground loop that injects 50/60 Hz noise into the high-impedance sense lines.

Case 2 — Mixed-gauge leads

If the field installer has used different gauges for the A and B leads, the matched-pair compensation assumption is violated. For example, 18 AWG on PT1A and 24 AWG on PT1B produces a residual error proportional to (R_A − R_B). Replace the cable with matched conductors.

Case 3 — Probe replacement without rewiring

If a 4-wire probe replaces a 3-wire probe, the original cable has only three conductors. Connect both ends of the probe element to two of the three cable conductors as before; the third cable conductor provides lead compensation. The probe's internal 4th lead is bonded internally to one end — confirm by measuring element resistance.

Case 4 — Calibration drift over time

Pt100 Class A tolerance drifts at < 0.05 °C/year if the probe is operated below 400 °C. Above 500 °C, expect drift up to 0.5 °C/year due to platinum oxidation. Recalibrate annually for Class A work; biennially for Class B.

Case 5 — Common-mode voltage on long runs

On runs longer than 50 m, the cable capacitance to ground can store enough charge to develop a common-mode voltage relative to the AG terminal. Add a 100 kΩ resistor from each PT1B terminal to AG at the terminal block to bleed off static. This does not affect the DC measurement because the ADC input impedance is > 10 MΩ.

Safety and Standards Notes

Installation must follow:

• IEC 60751 — Industrial platinum resistance thermometers and platinum temperature sensors.
• IEC 60381-1 — Analogue signals for process control systems.
• NFPA 79 (Electrical Standard for Industrial Machinery) where the panel is part of a machine.
• Local electrical code for conductor sizing, segregation, and overcurrent protection.

Always de-energise the panel before changing sensor wiring. PT100 sensors in intrinsically safe areas require a barrier; consult the relevant ATEX or IECEx certificate before installing in hazardous locations.

Troubleshooting Matrix

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