S7-300 CPU 314C-2DP: Configuring 3 PT100 + 4-20mA Inputs

1. Overview

The S7-300 compact CPU 314C-2DP (order number 6ES7314-6EH04-0AB0, firmware V3.3.x and later) integrates digital and analog I/O directly on the backplane, which makes it the most cost-effective Siemens controller for refrigeration and freezer skids where four or five sensors must be read locally. A typical refrigeration package uses three PT100 RTDs (suction line, discharge line, evaporator outlet) plus one 4-20 mA pressure transducer (refrigerant high-side or low-side).

The integrated analog front-end of the 314C-2DP exposes 5 analog inputs and 2 analog outputs, but only one of those five channels accepts an RTD directly. The remaining four channels are voltage/current-only. This article documents the minimum additional hardware required, the wiring topologies that apply to PT100/4-wire RTD sensors and to a current-loop pressure transducer, and the STEP 7 (Classic) and TIA Portal parameter assignment that brings every scaled engineering variable into the user program.

2. CPU 314C-2DP Integrated I/O Map

The RTD channel (channel 0 of the analog subsystem, hardware input terminals 2 / 3 / 4 on the AI connector X2) accepts only one 2-wire, 3-wire, or 4-wire PT100. If you wire three physical probes to it, the integrated front-end will read only the first probe; the firmware does not multiplex the single RTD ADC. Plan the rack expansion accordingly.

For full CPU ordering data refer to the official Siemens CPU 31xC manual on the Siemens Industry Online Support portal (entry ID 109479141).

3. I/O Accounting for 3 PT100s + 1 Pressure Sensor

Minimum hardware for the application:

1. CPU 314C-2DP — handles the first PT100 on its integrated RTD input.
2. One SM 331 AI 8xRTD module (preferred order number 6ES7331-7PF11-0AB0) — handles PT100 #2 and PT100 #3 on two of its eight RTD channels.
3. The 4-20 mA pressure transducer is wired to AI CH1 of the CPU (a free 4-20 mA channel that is not used as RTD). Alternatively, channel 0 of the SM 331 can be reconfigured to 0/4-20 mA and used for the pressure sensor, freeing the integrated AI entirely.

4. SM 331 RTD Module Alternatives

The 7PF11 module is the current production replacement. The 7RD00 variant is targeted at laboratory or high-accuracy Pt1000 monitoring, while the 1KF02 cannot read RTDs directly and must be paired with an in-head 4-20 mA transmitter per probe. For a refrigeration application where the engineer usually prefers to keep the front-end simple, an in-head transmitter (Rosemount 0065N, WIKA TC10, IFM TN2) is an alternative path that lets you reuse the cheaper 1KF02 instead of the 7PF11. Refer to the SM 331 analog module manual (entry ID 77376101) for the exact channel assignment, interrupt, and diagnostics block.

5. PT100 Wiring Topologies

PT100 sensors are defined by IEC 60751 with a nominal resistance of 100.00 Ω at 0 °C and a temperature coefficient of 0.00385 Ω/Ω/°C. Lead resistance introduces a fixed positive error in proportion to its length and copper cross-section. The S7-300 family compensates for lead resistance automatically when 3-wire or 4-wire topology is selected in HW Config; 2-wire requires manual offset.

Terminal assignments for the integrated RTD input on the CPU 314C-2DP (front connector X2):

• 2-wire: IC+ and IC− (terminals 2 and 3); jumper terminals 3-4.
• 3-wire: IC+, IC−, sense+ (terminals 2, 3, 4).
• 4-wire: IC+, IC−, sense+, sense− (terminals 2, 3, 4, 5).

For the SM 331 AI 8xRTD (7PF11), each channel uses four terminals (M0+/M0−/M0A+/M0A− through M7A+/M7A−) on the 20-pin front connector. Pin assignment and shielding recommendation are detailed in the module manual entry cited above.

6. Pressure Transducer Wiring (4-20 mA)

A standard refrigeration pressure transmitter (e.g. Danfoss AKS 32R, WIKA A-10, IFM PN7) is a 2-wire loop-powered device. The PLC supplies 24 V DC through the AI channel's current loop, the transmitter regulates the loop current between 4 mA and 20 mA proportional to pressure.

Total loop burden must stay below 600 Ω (sum of transmitter burden, cable loop resistance, and any in-line indicator). Verify with:

R_total [Ω] = R_burden + 2 * R_cable + R_indicator
V_loop [V] > R_total * 0.020 + 12 V (transmitter minimum compliance)

For a 100 m run of 0.34 mm² Cu cable (~58 Ω/km) the loop resistance is approximately 11.6 Ω, leaving ample headroom for the transmitter compliance voltage.

7. Hardware Configuration in STEP 7 V5.x (Classic)

1. Open the SIMATIC Manager project and double-click Hardware.
2. Insert the S7-300 rack 0, then drop the CPU 314C-2DP (6ES7314-6EH04-0AB0) into slot 2.
3. Open the CPU properties (double-click the CPU icon). Confirm the Analog Inputs tab; channel 0 type is Pt100 (3-wire). Set the integrated AI channel that will be used for the pressure transducer to 4-wire Transducer, 4-20 mA.
4. In slot 4 of the same rack, place SM 331 AI 8xRTD (6ES7331-7PF11-0AB0). Channels 0 and 1 are set to Pt100 (3-wire); channels 2 through 7 are set to disabled to save on ADC conversion time and to suppress diagnostic interrupts.
5. For each active RTD channel enable Hardware Interrupt at Limit Exceeded only if the application requires it; the cold-room discharge-line probe is a typical candidate for a high-temperature alarm at 130 °C.
6. Save and compile (Station > Save and Compile).
7. Download the hardware configuration to the CPU.

8. Hardware Configuration in TIA Portal

1. In the project tree, open Devices & networks > Device view.
2. Select the CPU 314C-2DP and switch to Properties > Analog inputs. The integrated RTD input is exposed under the AI 0 (RTD) subtree. Configure Pt100, 3-wire, °C, smoothing 4 cycles.
3. For the SM 331 module, drag the AI 8xRTD/TC from the catalog into slot 4. Set channels 0 and 1 to RTD 3-wire, smoothing = 4, integration time = 20 ms.
4. Right-click each AI module and select IO tags > Add new tag to expose the raw word as Sensor_AI_PIW752, Sensor_AI_PIW0, Sensor_AI_PIW2, and Sensor_AI_PIW754.
5. Download the device configuration. The portal will issue a STOP/START transition; for production cabinets this is normally scheduled in a maintenance window.

The TIA Portal manual set is published on the official Siemens Industry Online Support portal under entry IDs starting with 109751302 (programming and operating manual) and the STEP 7 V5.7 reference on the same support site.

9. Scaling the Raw Word to Engineering Units

The CPU 314C-2DP and SM 331 deliver the analog value as a 16-bit integer (PIW). The integer is scaled by Siemens according to the configured measuring range:

Use FC105 "SCALE" from the STEP 7 Standard Library / TI-S7 Converting Blocks or the TIA Portal equivalent NORM_X and SCALE_X to convert the integer to a floating-point engineering value. In Structured Text (TIA Portal / SCL):

// Pressure transducer: 4 mA = 0.0 bar, 20 mA = 30.0 bar
Sensor_Pressure_Real := NORM_X(MIN := 0,    // 4 mA scaled
                                VALUE := Sensor_Pressure_PIW,
                                MAX := 27648); // 20 mA raw

Sensor_Pressure_Bar := SCALE_X(MIN := 0.0,
                               VALUE := Sensor_Pressure_Real,
                               MAX := 30.0);

For PT100, IEC 60751 provides the Callendar–Van Dusen coefficients A=3.9083e-3, B=−5.775e-7, C=−4.183e-12 (negative range). The 0.1 °C/bit scaling in the CPU 314C-2DP already applies the inversion; the engineer only needs to multiply by 0.1:

// Temperature scaling, climatic range 0.01 °C / LSB
Sensor_Temp_C := INT_TO_REAL(Sensor_Temp_PIW) * 0.01;

If the SI unit °F is required downstream, apply T_F := T_C * 9.0 / 5.0 + 32.0.

10. Smoothing, Cycle Time, and Resolution

For a refrigeration cycle with chiller step response slower than 5 s, configure smoothing = 4 cycles. A freezer defrost probe, which is exposed to rapidly changing air temperature during the defrost phase, should remain at no smoothing.

11. Cold-Junction Compensation Notes

RTD sensors do not require cold-junction compensation (CJC); the PT100 element is a passive resistive sensor whose absolute resistance is measured by the module and converted to temperature in the firmware. CJC is only required for thermocouple (TC) sensors and is not relevant to a refrigeration application built around PT100s.

If a single application uses both PT100 (ice-bank tank) and K-type TC (exhaust), the SM 331 7PF11 module supports mixed TC/RTD configuration per channel. Reference the channel type configuration in the module properties dialog.

12. Commissioning Checklist

1. With the cabinet powered, navigate to CPU > Online > Monitor/Modify. Read the PIW of every configured channel and verify the raw value is in range.
2. Short each RTD terminal pair with a 100 Ω 0.01 % decade resistor (PT100 simulator). The scaled reading should equal ambient ±0.2 °C.
3. Apply a 4 mA and 20 mA current source to the pressure transducer input. Confirm the scaled bar reading matches the transducer datasheet within tolerance.
4. Open the diagnostic buffer (CPU > Diagnostic buffer) and confirm no OB 82 calls related to wire break or overrange.
5. Trigger a deliberate wire break on PT100 #2 by disconnecting one lead at the sensor head. Verify OB 82 fires, the PIW is forced to 0x7FFF (overflow), and the alarm word lights up in the HMI.
6. Restore the connection, perform a warm restart, and verify the PIW returns to the normal process value.

13. Troubleshooting Matrix

14. Notes on Alternate Platforms

Engineers working on smaller refrigeration controllers sometimes evaluate non-Siemens PLC families for cost reasons. The CLICK series from AutomationDirect, for instance, integrates discrete and analog I/O in a brick-style form factor and supports PT100 inputs directly on certain CPU modules. A short application video from the manufacturer demonstrating thermocouple and RTD wiring on the CLICK platform is hosted at AutomationDirect's CLICK PLC temperature sensing video. Where the application does not require PROFIBUS integration, an integrated brick-style controller can compress footprint further. For multi-rack systems or where the S7-300 is already established, the CPU 314C-2DP plus the SM 331 7PF11 expansion module remains the lowest-cost path with the broadest Siemens ecosystem support.

15. Field-Proven Caveats

• The integrated RTD input on the CPU 314C-2DP does not support Pt1000, Ni1000, or thermocouple. If the customer specifies a Pt1000 probe for accuracy reasons, the SM 331 7PF11 must be used from the start.
• The CPU 314C-2DP outputs 24 V at the analog front-end for loop-powered transmitters, but the per-channel current limit is 50 mA. Do not daisy-chain two 4-20 mA transmitters on the same supply terminal; each must have its own supply line.
• The default STEP 7 channel diagnostic addresses of the SM 331 7PF11 (PIW 256+ and PQW 256+) shift with the slot number. Use symbolic I/O (e.g. I_DischargeLineTemp) instead of absolute addresses so that future rack swaps do not break the application code.
• EMC performance is strongly affected by routing the analog shielded cable in the same conduit as the VFD motor cables. Maintain a minimum 200 mm separation, or use a separate shielded cable tray.
• For battery-backed restart after power loss, the diagnostic buffer of the CPU must be evaluated in OB 100 (warm restart). Use SFC 51 to read the diagnostic buffer and broadcast the most recent error to the SCADA layer.