Connecting PT100 2-Wire to a Siemens C7-626 CPU Input

Connecting a PT100 (2-Wire) to a Siemens C7-626 CPU Analog Input

The Siemens C7-626 / C7-626 DP control system integrates a CPU (functionally equivalent to an S7-314), 24 digital inputs, 16 digital outputs, 8 analog inputs, 4 analog outputs, and an operator panel into a single package. The unit is widely used in machine building, packaging lines, HVAC skids, and small process loops where the integrated panel and PLC footprint reduce cabinet space. When an application calls for a PT100 RTD temperature measurement, integrators discover a hard limitation: the C7-626 onboard analog inputs do not accept resistance or RTD signals. This reference documents the hardware constraint, the available workarounds, and the STEP 7 configuration and scaling steps required to read a 2-wire PT100 reliably through a C7-626.

1. Why the C7-626 Cannot Read a PT100 Directly

According to the C7-626 / C7-626 DP Control Systems manual (Volume 1: Installation, Assembly, Wiring, available on the Siemens Industry Online Support), each of the eight onboard analog input channels is software-configurable in HW Config (STEP 7) for one of three measurement modes:

• ±10 V voltage
• 0 to 20 mA current
• 4 to 20 mA current

The hardware lacks the constant-current excitation source, ratiometric reference, and differential lead-resistance compensation required for resistance or RTD measurement. The measurement-type dropdown in HW Config contains no PT100, PT1000, NI1000, or generic resistance entry. Connecting the two PT100 leads directly to the AI+ and AI− terminals produces an unstable, near-zero reading because the input is a voltage or current front end—not a resistance bridge.

2. PT100 Sensor Characteristics (DIN EN 60751)

A PT100 is a platinum resistance thermometer with a nominal resistance of 100.00 Ω at 0 °C and a positive temperature coefficient of approximately 0.385 Ω/°C (3850 ppm/K). The exact resistance-vs-temperature relationship is standardized in IEC 60751:2022 (formerly DIN EN 60751). The TE Connectivity thin-film PT100 element is the de-facto industry reference part. Use the table below for a quick lookup; the polynomial Callendar-Van Dusen coefficients provide full-range interpolation.

2-Wire PT100 Lead-Resistance Error

A 2-wire PT100 places both lead wires in series with the sensing element. The lead resistance adds directly to the measured value. For 0.5 mm² copper conductors, resistance is approximately 0.035 Ω/m at 20 °C. A 10 m run therefore introduces ~0.35 Ω of lead resistance, equivalent to ~0.9 °C measurement error at 0 °C. This is the principal reason field experience often reports PT100 2-wire accuracy degrades at the higher end of the range: the absolute error grows linearly with measured resistance. For high-accuracy or long-cable applications, specify a 3-wire PT100 (with a transmitter that performs lead compensation) or a 4-wire PT100 with an RTD input module.

3. C7-626 Onboard Analog Input Specifications

The 12-bit signed resolution limits the absolute temperature resolution. In the 4-20 mA mode with a transmitter scaled 0-600 °C, the input word range 0-27648 maps to 0-600 °C, giving ~0.022 °C per LSB—plenty for most process loops.

4. Solution Path Comparison

Three practical paths exist. Each is suitable in a different cost, accuracy, and panel-space envelope.

For the vast majority of C7-626 retrofits, the PT100-to-4-20 mA transmitter is the recommended path: it preserves the 4-20 mA noise immunity, provides on-board linearization, requires no STEP 7 hardware change beyond selecting the 4-20 mA input type, and fits on a 6 mm DIN-rail footprint.

5. Solution 1: PT100-to-4-20 mA Transmitter (Recommended)

5.1 Transmitter Wiring (2-Wire PT100)

The example below uses a Phoenix Contact MINI MCR-2-RTD-UI. The same wiring pattern applies to WAGO 857-815 and Siemens SITRANS TR200 with terminal labels A1/A2 swapped to match the manufacturer's pinout. Always verify against the device data sheet.

Wire the PT100 leads to the transmitter's RTD input terminals. Provide 24 VDC loop power to the transmitter. Route the 4-20 mA output back to a C7-626 analog input (e.g., AI0 = terminals 22, 23 on the C7-626 base unit). Configure the transmitter for the desired temperature span; e.g., 0-200 °C = 4-20 mA.

5.2 Transmitter DIP / DIP-Switch Setup (Typical MINI MCR-2-RTD-UI)

5.3 Verification at the Transmitter

1. Apply a precision decade box or calibrated 100 Ω resistor (simulating 0 °C) to the PT100 input.
2. Measure the output current with a calibrated mA meter.
3. For 0-200 °C scaling, 100 Ω must produce exactly 4.000 mA; 175.86 Ω (200 °C) must produce 20.000 mA.
4. If reading is off, return to the manufacturer's data sheet and re-confirm DIP settings before touching STEP 7.

6. Solution 2: PROFIBUS DP Remote RTD Module

If the C7-626 in the cabinet is the DP variant, an ET 200S or ET 200M distributed I/O head can be added on the PROFIBUS network with an RTD-capable analog input module. This avoids an analog signal run and provides higher accuracy because the RTD measurement is performed at the module using a precision current source.

6.1 Module Options

Configuration is performed in STEP 7 HW Config by adding the appropriate IM 153 head (e.g., 6ES7 153-1AA03-0XB0 for ET 200S or 6ES7 153-2BA02-0XB0 for ET 200M) to the DP master system, dropping the RTD module into the slot, then selecting the channel's measurement type as "Pt100" and the connection as "2-wire". The module returns a temperature-scaled value directly to the process image, eliminating any FC105-style scaling work.

7. Solution 3: PT100-to-Voltage Bridge (Lowest Cost)

For laboratory or low-budget installations, build a passive bridge with a precision current source. A 1.000 mA constant-current source applied across the PT100 produces 100.00 mV at 0 °C and 138.51 mV at 100 °C—directly readable by the C7-626 in ±10 V mode. The drawback is non-linearity: the C7-626 will read mV, not °C, and the PLC must apply the Callendar-Van Dusen polynomial to recover temperature.

7.1 Scaling Formula in STEP 7

// Inputs
//  PIW  304   : raw mV reading, 0-10000 mV = 0-27648 counts
// Outputs
//  MD   100   : temperature in °C x 100 (e.g., 25.34 °C = 2534)

// 1) Convert counts to mV
#mV_raw   := INT_TO_REAL(PIW_304) * 10000.0 / 27648.0;

// 2) Convert mV to resistance (assuming 1.000 mA excitation)
#R_PT100   := #mV_raw / 1.0;             // 1 mA source

// 3) Callendar-Van Dusen inverse (0 to 850 °C range)
//    T = SUM[n=0..4] a_n * (R/100 - 1)^n
#x         := (#R_PT100 / 100.0) - 1.0;
#T_C       := (-245.95 + 2.3320E2 * #x
              - 5.7531E-4 * #x * #x
              - 1.2769E-6 * #x * #x * #x
              + 1.4046E-10 * #x * #x * #x * #x);

// 4) Scale to 0.01 °C resolution
MD100      := REAL_TO_INT(#T_C * 100.0);

8. STEP 7 HW Config Setup for the 4-20 mA Path

Open the SIMATIC Manager project, navigate to HW Config, and double-click the C7-626 rack slot containing the analog input submodule. The properties dialog exposes the channel-by-channel measurement type selector.

8.1 Configuring One C7-626 AI Channel for 4-20 mA

1. In HW Config, double-click the analog input module under the C7-626.
2. Select channel 0 and choose Measurement type: 4DMU (4-wire, current 4-20 mA) or, depending on firmware, the equivalent "Current (4-20 mA)" entry.
3. Set the Integration time to 60 Hz (50 Hz if line frequency is 50 Hz; reduces mains-noise pickup).
4. Enable wire-break detection only if the channel is configured for 4-20 mA and the transmitter supports the diagnostic bit (most do).
5. Click OK. STEP 7 now writes the configuration to the C7-626 during the next download.

9. STEP 7 Scaling with FC105 "SCALE"

The C7-626 places a raw 16-bit value in the process image (PIW). For 4-20 mA input, 4 mA = 0 counts and 20 mA = 27648 counts. FC105 (or its S7-1200/S7-1500 equivalent, NORM_X + SCALE_X) converts the raw value to engineering units.

9.1 FC105 Input/Output Mapping for 0-200 °C

9.2 STL Code Snippet

      CALL  FC   105
        IN    := PIW304           // Raw value
        HI_LIM:= 2.000000e+002    // 200.0 °C
        LO_LIM:= 0.000000e+000    //   0.0 °C
        BIPOLAR:= FALSE
        RET_VAL:= MW220           // 0 = OK
        OUT   := MD200            // Temperature REAL °C

9.3 Structured Text (S7-1200/1500 Equivalent)

// ScaledTemp := NORM_X(IN := AI_Value, IS := 0, IM := 27648)
//               + SCALE_X(MIN := 0.0, MAX := 200.0, VALUE := ...)
#NormRaw   := NORM_X(IN := "AI_4_20mA",   // INT 0..27648
                     IS := 0, IM := 27648);
#Temp_C    := SCALE_X(MIN := 0.0, MAX := 200.0,
                      VALUE := #NormRaw);

10. Wiring & Installation Best Practices

• Use shielded, twisted-pair cable for the PT100 run. Ground the shield at the transmitter end only.
• Route the PT100 cable away from VFD output cables (typically 200 mm minimum parallel separation, 90° crossings).
• Mount the transmitter in a stable-temperature enclosure; many DIN-rail RTD transmitters drift ±0.01 °C/°C of ambient.
• Provide surge protection at the panel entry for outdoor sensors (e.g., Phoenix Contact PT-IQ).
• Loop-power the transmitter from the same 24 VDC that powers the C7-626 digital outputs to avoid ground loops.

11. Verification & Acceptance Test Procedure

1. Power up the transmitter; confirm 4.00 mA with PT100 disconnected (or with 100 Ω shunt = 0 °C).
2. Substitute a calibrated decade resistance box in place of the PT100. Verify at three points: 0 °C (100 Ω), 100 °C (138.51 Ω), and the high end (e.g., 175.86 Ω for 200 °C). Reading must be within ±0.5 °C after FC105 scaling.
3. Connect the actual PT100 and let the system settle for at least 5 thermal time constants (typically 3-5 minutes for a thermowell-mounted probe).
4. Compare the displayed temperature to a separate calibrated reference thermometer (Fluke 1523, WIKA CTH6500, or similar) in a stable bath at one operating point.
5. Cycle the C7-626 power; confirm the scaled value recovers without requiring re-download.
6. Force the channel to over-range (decade box set to 300 Ω for a 0-200 °C span); confirm 4-20 mA transmitter goes to > 21 mA (sensor-break setting) and C7-626 returns 7FFFh overflow.

12. Troubleshooting Matrix

13. Spare-Parts Selection Guide

14. Frequently Asked Questions