Wiring S7-1200 CM 1234 Analog Inputs to 4-Wire Sensors

1. Overview

This reference describes how to connect 4-wire analog transmitters (CO2 and temperature in the field case described) to the SIMATIC S7-1200 signal module CM 1234 (order number 6ES7 234-4HE30-0XB0) attached to a CPU 1214C DC/DC/Rly (6ES7 214-1HE30-0XB0). It covers the full field wiring path: 4-wire current-loop topology, the requirement to short unused analog input channels, the common-ground rule between sensor supply, sensor return, and PLC reference, and the TIA Portal configuration that turns the raw counts into engineering units.

The most common wiring mistake on first-time installations of the CM 1234 is leaving unused channels floating or, conversely, tying the sensor signal return to chassis ground at multiple points. Both conditions cause diagnostic nuisance faults (channel break, overflow) and unstable readings. The wiring procedure below eliminates those classes of error by enforcing a single equipotential reference and by bridging every unused X+ to its own X-.

2. Hardware Identification and MLFB Decode

Before wiring, confirm the exact module on the DIN rail. The CM 1234 and CM 1231 are commonly confused.

The trailing block of the MLFB encodes the variant:

• 1HE30 on the CPU = 1214C, DC supply, DC inputs, relay outputs, hardware revision 30.
• 4HE30 on the CM = 4AI/2AO combination, hardware revision 30.
• 0XB0 = original release. Later -0XB1, -0XB2 etc. denote revised hardware; firmware is selected separately in TIA Portal device configuration.

The CM 1234 ships in TIA Portal as AI 4 x 12 bit / AO 2 x 12 bit. If the device is missing from the project, install the HSP (Hardware Support Package) for S7-1200 signal modules or upgrade TIA Portal to V15.1 or later.

3. Prerequisites

1. CPU 1214C DC/DC/Rly powered up, firmware V3.0 or later, TIA Portal project compiled without errors.
2. CM 1234 mounted to the right of the CPU (or another S7-1200 module) on the DIN rail and detected in the device configuration. Confirm the green DIAG LED is off before continuing.
3. 24 V DC field power supply sized for the transmitters plus a 20% margin. For two 4-wire transmitters drawing 50 mA each, a 1 A supply is sufficient.
4. Shielded twisted-pair cable (e.g., Belden 8761 or Lapp Ölflex 110CY) for the analog runs. Conduit or cable tray bonded to ground at one end only.
5. Sensor datasheet identifying whether the output is 0-10 V, 0-20 mA, or 4-20 mA. Most CO2 and TE (temperature element) transmitters in the field use 4-20 mA.
6. Slim (2.5 mm) screwdriver for the CM 1234 cage-clamp terminals.

4. CM 1234 Analog Input Pinout and Signal Types

The CM 1234 exposes four differential input channels on the front connector, plus two analog outputs. Each input is a true differential pair; the PLC does not internally short X+ to a shared common at the terminal block. This is why unused channels must be jumpered, and why the sensor return wire must land on X- rather than on the M terminal of the field supply.

5. 4-Wire Current Sensor Connection Procedure

The example below uses channel 0 for the CO2 transmitter and channel 1 for the TE (temperature) transmitter. Channels 2 and 3 are unused and will be bridged in step 6.

5.1 Wire the transmitter power pair

1. Run a 2-conductor shielded cable from the field 24 V supply to the transmitter terminal block labeled +V and GND (or PWR+ / PWR-).
2. Bond the cable shield to the ground bar at the panel entrance only. Do not terminate the shield at the sensor end; pigtail to the transmitter housing ground if the manufacturer specifies a chassis ground stud.
3. Verify 24 V DC at the transmitter terminals with a meter before proceeding.

5.2 Wire the signal pair back to the CM 1234

1. Run a second 2-conductor shielded cable from the transmitter's OUT+ and OUT- (or SIG+ / SIG-) to the CM 1234 front connector.
2. Connect OUT+ to terminal 1 (AI0+) for the CO2 sensor. Connect OUT- to terminal 2 (AI0-).
3. For the temperature transmitter on channel 1, repeat: OUT+ to terminal 3 (AI1+), OUT- to terminal 4 (AI1-).
4. Terminate the signal-cable shield at the panel ground bar only.

5.3 Establish the common reference

The sensor datasheet states whether the signal return is galvanically isolated from the supply return. For 4-wire transmitters, the two pairs are usually isolated, so the only common point between the field supply, the sensor, and the PLC is the panel ground bar. Tie the following to the same ground bar:

• Field 24 V supply M terminal (DC negative).
• Sensor supply GND.
• Sensor signal return (this is the wire you brought back to X-; the PLC X- terminal is the preferred termination point because it is already at the same potential as the backplane).
• CM 1234 M terminal (backplane 24 V common) and the CPU's M terminal.

The single-point ground at the panel prevents the shield currents and the 4-20 mA return current from finding a parallel path through the building steel.

6. Shorting Unused AI Channels

The CM 1234 manual explicitly states that unused voltage and current input channels must be bridged between X+ and X- to prevent noise pickup that the module's diagnostic logic would otherwise flag as out-of-range or wire-break.

1. On the front connector, install a short insulated jumper between terminal 5 (AI2+) and terminal 6 (AI2-).
2. Install a second jumper between terminal 7 (AI3+) and terminal 8 (AI3-).
3. Use the same wire gauge as the signal cable (typically 0.5 mm² / 20 AWG stranded) and keep the jumper length under 20 mm.
4. In TIA Portal, set the unused channel measurement type to Voltage with the range set to ±10 V (the most tolerant default) and disable diagnostics. This prevents the wire-break diagnostic from firing on the jumpered pair.

7. Configuration in TIA Portal

After the wiring is mechanically complete, configure the two active channels in the device view.

1. Open the project in TIA Portal, expand Device configuration > CM 1234 > Analog inputs.
2. Select channel 0. Set:
   • Measurement type: Current
   • Current range: 4-20 mA
   • Smoothing: leave at None for first commissioning; enable later for slower-changing processes.
   • Diagnostics: enable Wire break and Overflow / underflow.
3. Repeat for channel 1 with the same settings if the temperature transmitter is also 4-20 mA.
4. Set channels 2 and 3 to Voltage, ±10 V, diagnostics disabled.
5. Compile the project and download to the CPU.

Scale the raw word (%IW64 for AI0 of the first CM 1234) to engineering units with the standard NORM_X and SCALE_X instructions, or the legacy SCALE FC105 if you are on firmware V3.x:

// 4-20 mA on AI0 (%IW64), 0-2000 ppm CO2
// Raw 0 = 4 mA = 0 ppm, Raw 27648 = 20 mA = 2000 ppm
// Scaled = NORM_X(VALUE := %IW64, MIN := 0, MAX := 27648) * 2000.0
// Cleaner with SCALE_X and bipolar-disable:
"CO2_ppm" := SCALE_X(
    MIN := 0.0,           // 4 mA end
    MAX := 2000.0,        // 20 mA end
    VALUE := NORM_X(
        VALUE := %IW64,
        MIN    := 0,      // 0x0000 = 4 mA on unipolar 4-20 mA
        MAX    := 27648   // 0x6C00 = 20 mA
    )
);

8. Analog Module Diagnostics in TIA Portal

After download, open Online & diagnostics on the CM 1234 in the project tree. The Online and diagnostic tools > Analog module diagnostics page in the S7-1200 manual collection enumerates the diagnostic events the module can raise. The ones that matter for a 4-wire current installation are:

Each event populates a bit in the channel status word (%IW66 for the diagnostics of the first CM 1234 on the CPU) and the module's DIAG LED flashes red. Map the relevant bits to a global tag for HMIs and alarm logs.

9. Verification and Commissioning Checklist

Run through this list with a multimeter and TIA Portal online view before energising the transmitters for the first time.

1. Power down the field 24 V supply and disconnect the transmitters.
2. Measure resistance between AI0+ and AI0- on the unplugged field cable: should be < 50 Ω if a current-loop transmitter is connected, or 0 Ω if the channel is jumpered.
3. Verify no continuity between AI0- and chassis ground at the panel; the loop return must be isolated except at the common ground bar.
4. Reconnect the transmitters, restore power, and watch the TIA Portal online Watch table for %IW64 and %IW66.
5. Force the CO2 sensor to read a known value (e.g., zero-gas calibration gas, or a current calibrator in series with the loop). The scaled CO2_ppm tag should agree within the sensor's stated accuracy.
6. Force a wire-break by disconnecting one signal wire; confirm the diagnostic bit sets and the HMI shows the alarm within one scan cycle (typically 100 ms with default integration time).
7. Restore the wiring, write the project to the PLC's RETAIN card if any startup defaults are required, and back up the project to the plant archive.

10. Troubleshooting Matrix

11. Frequently Asked Questions