Resolving SITRANS P Z 7MF1564 0.5V Offset on S7-200 CPU 224XP

1. Problem Overview

Field reports describe a recurring zero-point anomaly when a SITRANS P Z series pressure transmitter (order code 7MF1564-3DD10-1AA1) is wired directly to the integrated analog input of a Siemens S7-200 CPU 224XP. With applied pressure at the low end of the calibrated range, the analog value at AIW0 (or AIW2) reads approximately 0.5 V rather than the expected 0.0 V, producing a persistent offset in every process variable calculated from that word.

The fault exhibits three diagnostic signatures:

• At zero process pressure, the integrated AI shows a constant 0.45 – 0.55 V instead of 0.0 V.
• The first 0 – 0.5 V (≈ 40 – 50 digital counts at default 0 – 10 V scaling) is a "dead band" – the transmitter cannot pull the input below 0.5 V, but the input tracks the transmitter from roughly 0.6 V upward with linear accuracy.
• When the same transmitter is reconnected to an EM 235 analog expansion module, the same point reads 0.00 V – 0.02 V (i.e. within quantisation noise), confirming the field device itself is healthy.

Substituting a third-party 0 – 10 V transmitter at the CPU 224XP integrated input yields the expected 0.0 V at zero pressure, which rules out PLC-side ADC failure. The fault is therefore interaction-based, not a hard failure of either device in isolation.

2. Affected Hardware and Order Codes

The combination known to trigger the offset is listed below. Verify each article number against the device label and the order documentation before commencing troubleshooting.

Refer to the official product support entries for the SITRANS P Z series 7MF1564 and SITRANS P Z series 7MF1563 to confirm device revision, firmware label, and approved accessories.

3. Root Cause Analysis

Two independent hardware traits combine to produce the offset. Neither is a defect in the normal sense; both are documented in their respective manuals, but the interaction is rarely called out in a single document.

3.1 Integrated analog input loading of the CPU 224XP

The CPU 224XP contains two integrated analog inputs (AIW0 and AIW2). They are implemented as single-ended voltage inputs with an input impedance that is lower than that of a typical signal-conditioning expansion module. The integrated channel presents a finite resistive load to the source. The output stage of the SITRANS P Z voltage variant cannot drive that load all the way to its negative rail: at very low commanded voltages the output stage saturates at a residual level that is determined by the transmitter's minimum output swing under load, the receiver's input impedance, and the reference ground return path.

3.2 Galvanic isolation in the field device

The SITRANS P Z series is provided with galvanic isolation between the process side, the supply, and the output. The CPU 224XP integrated analog input is itself isolated from the logic ground. Two isolation stages in series, combined with the cable shield and screen-earth practice used in the field installation, allow a small common-mode or ground-loop potential to appear at the input. The 0 – 0.5 V reading is consistent with that residual being superimposed on the intended 0 V signal at the analog input pin.

3.3 Why the EM 235 is unaffected

The EM 235 presents a much higher input impedance on its voltage ranges (the EM 235 manual lists ≥ 10 MΩ for the configured voltage range) and uses a true differential input stage with on-board signal conditioning. With that higher impedance the loading effect disappears and the residual ground-potential is rejected by the differential front end, which is why the same transmitter returns 0.00 V on the EM 235 in identical wiring.

3.4 Why the 4 – 20 mA variant works

The 7MF1563 is a current-loop device. Current loops are inherently immune to small series voltage drops and to the input-impedance mismatch that disturbs the 0 – 10 V variant. The 4 – 20 mA current is converted to a voltage at a defined burden resistor inside the receiver, and the burden value is the same regardless of which Siemens input is used (within the receiver's compliance range).

4. Diagnostic Procedure

1. With the transmitter powered and pressurised at 0 % of range (vent to atmosphere for a gauge unit), read the integrated analog input value in STEP 7-Micro/WIN: AIW0 or AIW2 as wired. A reading of 0.45 – 0.55 V (≈ 46 – 56 counts in 0 – 32000 unipolar scaling, or ≈ 74 – 90 counts in 0 – 10 V / 0 – 27648 scaling) confirms the symptom.
2. Disconnect the field wires at the PLC terminal and apply a precision 0.000 V reference from a calibrator directly to the AI input. The integrated input must read 0.000 V ± 1 count. If it does, the PLC ADC is healthy; the offset is field-side.
3. Move the same field cable to a free channel on an EM 235 configured for 0 – 10 V (DIP switch configuration per EM 235 manual). The reading must fall to 0.00 – 0.02 V. If it does, the transmitter and cable are exonerated; the issue is the integrated AI loading and isolation path.
4. Substitute a different 0 – 10 V brand of transmitter on the same CPU 224XP integrated input. Many will read 0.0 V because their output stages are designed to drive the lower CPU input impedance to true zero. This test confirms that the SITRANS P Z voltage output is unusually sensitive to the integrated-AI load.
5. Inspect shield earthing. The SITRANS P Z installation guide specifies single-point earthing of the cable shield at the cabinet end. Multi-point earthing allows ground loops to couple into the isolated output stage and can re-introduce the 0.5 V offset on otherwise-correct installations.

5. Solution 1 – Migrate the Channel to an EM 235

This is the cleanest fix and preserves the existing 0 – 10 V transmitter. The EM 235 is a true differential input, has the high input impedance the SITRANS P Z needs, and is the recommended Siemens answer to the symptom.

5.1 EM 235 DIP switch settings for 0 – 10 V unipolar

The EM 235 is configured by a 6-position DIP switch. For 0 – 10 V unipolar on all four channels use switch SW1 = ON, SW2 = OFF, SW3 = OFF, SW4 = OFF, SW5 = OFF, SW6 = ON. Power-cycle the EM 235 after changing the switches so the new configuration is latched.

5.2 Wiring

Wire the SITRANS P Z 0 – 10 V output to A+ and A- of an EM 235 input pair. Maintain shield continuity and single-point earth at the cabinet gland plate. Polarity: + to A+, signal return to A-.

5.3 STEP 7-Micro/WIN scaling for 0 – 10 V on EM 235

// EM 235, 0-10 V unipolar, full scale 0 - 32000 counts
// AIW address depends on module position; for module 0 (first EM 235): AIW0
// Example: read into a real variable 'rPressureBar'
// 10 V = 32000 counts = 10.0 bar (for 0-10 bar transmitter)
LD     SM0.0
ITD    AIW0, AC0         // integer to double integer
DTR    AC0, AC0          // double integer to real
MOVR   10.0, AC1         // full-scale engineering value (bar)
MOVR   32000.0, AC2      // full-scale counts
/R     AC2, AC0          // scale factor
*R     AC1, AC0          // engineering value
MOVR   AC0, VD100       // store in VD100 as 'rPressureBar'

5.4 Verification

With the process at 0 % of range, AIW0 on the EM 235 should read 0 – 2 counts (≤ 0.001 V). With pressure at 100 % of range, the same word reads 32000 ± 4 counts. Both ends of the scale must pass before the channel is released to production.

6. Solution 2 – Replace the Transmitter with the 4 – 20 mA Variant (7MF1563)

If the channel is essential and no EM 235 is available, or if the loop is to be extended over a long cable run, switch the field device to a 4 – 20 mA variant. The 7MF1563 is the current-output counterpart of the 7MF1564 and uses the same process connection, the same supply voltage, and the same calibration. It is the preferred device for noisy or long-cable installations.

6.1 Wiring to a CPU 224XP integrated analog input

The CPU 224XP integrated analog input is a voltage input only; a 4 – 20 mA loop cannot be terminated into it directly. Insert a 250 Ω precision burden resistor across A+ and A- of the input. The 4 – 20 mA signal develops 1.0 – 5.0 V across the resistor, which is within the integrated input's 0 – 10 V unipolar range.

SITRANS P Z (7MF1563)          CPU 224XP
   OUT+  ----+----------- A+ (AIW0 or AIW2)
              |
              +--- 250 Ω 0.1% burden ---+
              |                          |
   OUT-  ----+----------- A-            |
                                          |
   (24 VDC supply powers the loop separately)

Use a 0.1 % metal-film burden with low temperature coefficient; 50 ppm/°C or better is recommended for class-0.1 measurement performance.

6.2 Scaling

// 4-20 mA through 250 Ω = 1.0-5.0 V at AIW0 (CPU 224XP integrated)
// 0-10 bar transmitter
LD     SM0.0
ITD    AIW0, AC0         // 1.0 V = 3200 counts, 5.0 V = 16000 counts
DTR    AC0, AC0
MOVR   10.0, AC1         // full-scale bar
MOVR   12800.0, AC2      // 16000 - 3200
MOVR   3200.0, AC3       // offset (1.0 V at zero pressure)
-R     AC3, AC0
/R     AC2, AC0
*R     AC1, AC0
MOVR   AC0, VD100

7. Solution 3 – Buffer the Output with an External Conditioner

Where neither an EM 235 nor a transmitter change-out is acceptable, an in-line signal buffer (voltage follower) with sufficiently low output impedance, high input impedance, and the same 0 – 10 V span will isolate the SITRANS P Z from the CPU 224XP input. The buffer must be powered from a clean 24 VDC rail and must itself offer galvanic isolation on the output side, otherwise the original ground-loop symptom will reappear.

Acceptable choices include the Siemens SITRANS I isolation amplifier family or any third-party 0 – 10 V / 0 – 10 V buffer with ≥ 10 MΩ input impedance, ≤ 1 Ω output impedance, ≥ 1 kV isolation, and ≤ 0.1 % linearity. Insert the buffer between the transmitter and the CPU 224XP integrated input. The buffer is not a substitute for the EM 235 for high-accuracy work; it is a workaround and must be documented in the loop drawing as such.

8. Wiring and Shielding Reference

The following diagram summarises the three wiring options. The 0 – 10 V branch is the as-installed configuration that exhibits the 0.5 V offset; the EM 235 branch and the 4 – 20 mA branch are the corrected configurations.

9. Loop Verification Procedure

1. Power-cycle the PLC after any wiring change so the analog module re-initialises.
2. Force the process to 0 % of range (vent to atmosphere for gauge). Read AIW0 in the status chart. Acceptable reading: 0 – 4 counts on a 0 – 32000 scale (≤ 0.001 V on a 0 – 10 V range).
3. Apply a precision 25 %, 50 %, 75 % and 100 % pressure from a dead-weight tester or calibrated hand pump. Read AIW0 at each step. Maximum error: ± 0.1 % of full scale, equivalent to ± 32 counts on a 0 – 32000 scale.
4. Reverse to 0 % and re-check zero. If zero has walked, repeat the calibration procedure and check the shield earthing.
5. Sign the loop test sheet. The loop is released to production only when all five points are within tolerance.

10. Troubleshooting Matrix

11. Key Specifications Reference

Always confirm the latest figures against the active revision of the S7-200 System Manual and the EM 235 module manual. Specifications are subject to change across product revisions and firmware updates of STEP 7-Micro/WIN.

12. Field-Proven Caveats

• The 0.5 V offset is not a transmitter calibration fault and is not corrected by re-zeroing at the device. A SITRANS P Z that is at 0.0 V at its own terminals still presents 0.5 V at a CPU 224XP integrated input because the offset is produced by the interaction, not the device.
• Do not attempt to "trim" the offset away by adding a software bias in the PLC. The bias will track the transmitter linearly only at a single point, and the residual non-linearity will corrupt the upper end of the scale.
• Replacing the CPU 224XP with another CPU 224XP will reproduce the offset. The behaviour is a function of the integrated input stage, not of any individual unit.
• If the loop is to be expanded in future, plan the spare channels on the EM 235 from the start. The integrated AI should be reserved for signals whose source devices are specified by Siemens to drive the integrated input stage without external buffering.
• For hazardous-area installations, verify that the 7MF1563 current variant and the EM 235 are both within the Ex certificate scope of the installation before substituting the device or the module. The certificate of conformity for the SITRANS P Z and the EM 235 Ex rating must be matched to the zone classification of the process line.

13. Related Documentation

• SITRANS P, Z series (7MF1564) – support entry
• SITRANS P, Z series (7MF1563) – support entry