Troubleshooting S7-200 EM 231 Analog Pressure Input Drift on CPU 216-2AF22

Field service engineers maintaining legacy industrial sterilizers, autoclaves, and process skids built around the Siemens SIMATIC S7-200 Micro PLC family routinely encounter pressure-loop anomalies more than fifteen years after commissioning. One recurring failure mode is unstable atmospheric pressure indication, where the displayed value wanders between roughly 10.3 PSIA and 12.4 PSIA at ambient conditions instead of holding a steady 14.7 PSIA. The most frequent root cause in this architecture is not the transducer, not the wiring, and not the CPU program; it is a degraded EM 231 analog input module. This article reconstructs the diagnostic path used on a Kuhlman steam sterilizer (CPU 216-2AF22-0XB0 with four EM 222 DO modules and one EM 231 AI module) and gives a complete field-replacement procedure, verification checklist, and lifecycle-mitigation plan for an obsolete platform.

1. System Identification and Hardware Architecture

Before opening the cabinet, capture every Siemens order number on the front bezel of each module. The order number, not the marketing label, is what determines compatibility, firmware class, and replacement part strategy. For the affected sterilizer the bill of materials is:

The EM 231 suffix 0HC22 designates the 4-channel, 12-bit variant (AI4 x 12 bit) without onboard RTD support. The 0XA0 ending indicates the standard firmware/revision tier. Verify these strings with a flashlight and a magnifier, since sticker legibility is often the first casualty after two decades in a steam-saturated environment. Cross-reference the MLFB against the official S7-200 System Manual to confirm the analog range, DIP-switch settings, and wiring diagram match the as-installed configuration.

2. Symptom Definition and First Observations

The reported symptom is specific and quantifiable: with the sterilizer idle and at ambient, the displayed atmospheric pressure on the Siemens touch panel should read 14.7 PSIA (or the equivalent kPa / bar unit the program has been scaled to). Instead, the value drifts across a 2-PSI window, with sample readings of 10.4, 10.7, and 12.3 PSIA captured in sequence. The wandering is uncorrelated with door state, jacket temperature, or cycle phase. Because the pressure value is a permissive for cycle start, every cycle is aborted or held by the HMI logic.

First observations that should be documented by the technician before touching any hardware:

• Trend the raw AIW (analog input word) value at the CPU using Micro/WIN's Status Chart. Note min, max, and standard deviation over a 5-minute window.
• Record the displayed engineering units on the HMI versus the raw counts.
• Photograph the wiring at the EM 231 terminal block, including shield and jumper locations.
• Confirm the 24 V DC supply at the terminal strip feeding the transducer is within +/- 5 % of nominal.
• Note ambient temperature and humidity - steam-saturated rooms drive condensation on terminal blocks and accelerate corrosion.

3. Differential Diagnosis - Eliminating the Wrong Suspect

A pressure-loop fault can originate in five distinct places. Each must be ruled out in turn. Skipping this ladder is how a vendor in the source case ended up replacing an EM 222 digital output module when the fault was on an analog input channel - a diagnostic dead-end that cost a full day of downtime.

3.1 Pressure Transducer

The transducer converts process pressure into a low-level analog signal, typically 4-20 mA or 0-10 V DC, proportional to the calibrated range. A healthy atmospheric reading should produce a stable output at the lower end of the span. Verification:

1. Disconnect the transducer wires at the EM 231 terminal block.
2. Connect a calibrated signal generator (Beamex, Fluke 754, or equivalent) set to the transducer's exact span.
3. Inject 4.000 mA, 12.000 mA, and 20.000 mA. Capture the corresponding AIW values.
4. If the AIW tracks the generator within 1-2 counts and the HMI displays the correct engineering units, the transducer-side wiring and the module's ADC conversion are both healthy. The fault is upstream or in the transducer itself.

In the source case, the vendor performed exactly this generator test on the analog side and the loop tracked correctly, eliminating the transducer and the field wiring. The original transducer was replaced anyway as a precaution; the fault persisted.

3.2 Field Wiring and Shielding

EM 231 inputs are sensitive to common-mode noise, particularly in 50/60 Hz industrial environments. Inspect for:

• Broken shield terminations at the terminal strip.
• Shared cable trays with VFD output cables or three-phase feeders.
• Corroded screw terminals - typical in sterilizer cabinets after a decade of steam exposure.
• 24 V DC loop supply sag under load.

Re-terminate and re-torque every screw to the specified 0.56 N-m (5 in-lb). If the symptom persists with a known-good generator on the terminals, the wiring is exonerated.

3.3 CPU and Program

The CPU 216 does not perform analog-to-digital conversion; the EM 231 does. A CPU fault can, however, corrupt the PPI handshake or drop expansion-module data. Confirm:

• SF (System Fault) LED is off on the CPU.
• BF (Bus Fault) LED is off.
• The EM 231's SF LED is off.
• Run / Stop switch behavior is normal.

A CPU-only fault typically manifests as a complete scan failure, not as wandering analog readings. The CPU is rarely the culprit for a single drifting channel.

3.4 Slot Configuration in the Program

The S7-200 maps analog I/O by physical slot, not by automatic discovery. If the EM 231 is moved from slot 3 to slot 1 (or vice versa) without editing the project's I/O configuration, the program will continue to read AIW4 (slot 3 channel 0) but the new module will physically reside at AIW0. The result is a "dead" channel or a wildly incorrect value. Slot reassignment is the most common configuration error during troubleshooting - the source technician tried other slots on the EM 231 module itself (different channels on the same module) which is correct, but moving the entire module between slots requires a program change.

To verify slot mapping, open the project in STEP 7 Micro/WIN and use System Block > Input Filter > Analog to confirm the module configuration matches the physical rack.

3.5 The EM 231 Itself

After exhausting all upstream possibilities, the EM 231 module is the prime suspect. Symptoms of a failing EM 231 include:

• One or more channels reading wildly while others remain stable.
• AIW values oscillating even with inputs shorted or driven by a precision generator.
• Elevated module temperature (warm to the touch beyond normal ambient +10 deg C).
• Visible PCB discoloration or capacitor leakage at the terminal end.

The source case is a textbook EM 231 failure: signal generator tests passed, transducer and wiring were verified, slot mapping was correct, and the fault followed the module. The first EM 222 that was swapped in by the vendor could not have influenced an analog channel - it is a digital output module with no ADC. Replacing it was a diagnostic non-event.

4. Root Cause - Why EM 231 Modules Fail After 15-20 Years

EM 231 analog input modules of the 6ES7 231-0HC22-0XA0 family use a successive-approximation ADC front end with input multiplexers and precision resistor networks. The failure mechanisms typical to this age class are:

1. Electrolytic capacitor drying: The on-board filter and reference capacitors lose capacitance over time, allowing ripple and drift into the reference voltage. This produces low-frequency wandering that masquerades as process noise.
2. Input protection clamp degradation: TVS diodes on each channel can become leaky, especially after ESD events or over-voltage transients from field wiring faults. A leaky clamp pulls the input bias and skews the conversion.
3. Solder joint fatigue: The through-hole terminal block pins develop micro-cracks from thermal cycling in steam-saturated cabinets, producing intermittent contact resistance.
4. Reference drift in the ADC's precision resistor ladder: After two decades of operation, the +/- 0.5 % resistors can drift, especially in high-humidity environments, producing a non-linear but stable offset.

All of these failure modes are consistent with the observed symptom set: wandering baseline, transducer swaps ineffective, signal generator injection partially corrective but not curative.

5. Replacement Procedure - Step by Step

Replacement of the EM 231 in a live S7-200 system is straightforward but requires sequencing to avoid corrupting the program image or losing the slot configuration.

1. Connect a programming workstation with STEP 7 Micro/WIN V4.0 SP9 (or latest revision) and the original program file. Upload the project from the CPU if a backup is not already archived.
2. Export the System Block to a file. Record all analog input filter, pulse catch, and retentive memory settings.
3. Place the CPU mode switch in STOP. Verify the STOP LED is on.
4. De-energize the 24 V DC loop supply feeding the transducer. Lock and tag the supply breaker.
5. Label each wire on the EM 231 terminal block. Photograph the wiring from three angles.
6. Loosen the terminal block screws and lift the field wires clear.
7. Remove the right-side expansion bus connector from the failed module.
8. Release the module from the DIN rail and remove it from the rack.
9. Inspect the backplane connector pins on the failed module for oxidation. Clean with DeoxIT D5 if recoverable; otherwise discard.
10. Set the DIP switches on the replacement EM 231 to match the original. The 6ES7 231-0HC22-0XA0 uses a 6-position DIP bank on the underside of the module to configure input range (0-10 V, 0-5 V, +/- 5 V, +/- 2.5 V, 0-20 mA, 4-20 mA). Confirm the channel-by-channel configuration against the as-found settings.
11. Mount the replacement EM 231 in the same physical slot. Reconnect the expansion bus connector.
12. Re-apply field wiring per the labels. Torque terminal screws to 0.56 N-m.
13. Restore the 24 V DC loop supply.
14. Place the CPU in RUN. Observe the SF and BF LEDs on the CPU and the EM 231 - both should extinguish within 5 seconds.
15. Open the Status Chart in Micro/WIN and trend AIW4 (the slot 3, channel 0 input from the pressure transducer).
16. With the sterilizer idle and at ambient, confirm the AIW value matches the expected engineering unit reading of 14.7 PSIA.

5.1 EM 231 DIP Switch Configuration Reference

For a 4-20 mA loop from a typical pressure transducer scaled 0-30 PSIA, SW3 set to 011 is the standard configuration. Verify against the program scaling constants in the source code (the S7-200 integer scaling formula is Engineering_Units = ((AIW - Offset) * Span) / Raw_Span + Zero_Eng).

6. Verification and Functional Check

After replacement, perform a three-tier verification:

1. Static verification: With the transducer at ambient, AIW value should be within +/- 2 counts of the value recorded before failure. HMI display should show 14.7 PSIA +/- 0.1.
2. Dynamic verification: Apply three calibrated pressures spanning 0-100 % of transducer range (e.g., 0, 15, and 30 PSIA using a dead-weight tester or portable pressure calibrator). Verify the HMI displays each value within the transducer's combined accuracy specification (typically +/- 0.25 % FS).
3. Cycle verification: Run a full sterilizer cycle from cold start. Confirm the chamber pressure tracks the recipe profile and the cycle completes without abort.

In the source case, the static verification was the tell-tale: "the old girl is up and running again after a year long furlough... I was almost giddy when I saw the PSIA read 14.7 again instead of 10.4, 10.7, 12.3."

7. S7-200 Lifecycle Status and Spare Parts Strategy

The S7-200 product family reached PM410 - Product Cancellation effective 01.10.2017. Per the Siemens Product Lifecycle model, PM410 means:

• No new manufacture.
• Spare-parts supply is no longer guaranteed by Siemens directly.
• Support is best-effort via legacy channels.

Refurbished and unused-new-old-stock modules are available through third-party channels (Radwell, PLC Source, and similar industrial surplus vendors). Verify each module's age, firmware revision, and 30-day DOA warranty before accepting shipment. Counterfeit modules have entered the secondary market; authentic Siemens modules have a laser-etched, not pad-printed, MLFB label on the front bezel.

Recommended minimum spare inventory for a critical sterilizer running on S7-200:

8. Touch Panel 6AV6 545 - HMI Considerations

The HMI is a SIMATIC TP 170A / TP 070 panel from the 6AV6 545 family. These panels are also past their production end-of-life and run ProTool / early WinCC Flexible. Replacement considerations:

• Project files for these panels are written in ProTool V6.0 SP3 or WinCC Flexible 2008 SP5. The panel must be flashed with a matching firmware image before downloading a project.
• A "naked" replacement panel will boot to a default screen with no project. Without the source ProTool project file, the panel cannot be made to display the original graphics.
• TP 170A is hardware-end-of-life. Recommended migration is to a SIMATIC Comfort Panel (TP700 Comfort or TP900 Comfort) on the existing S7-200 via the panel's PPI port. WinCC Comfort V16 can configure a Comfort panel to talk PPI to an S7-200 with the correct driver selection.

Always perform a backup of the panel project before any service activity. The panel stores the project in internal flash; a failed flash during firmware update can render the panel unrecoverable.

9. Diagnostic Matrix - Quick Reference

10. Migration Planning - The Longer-Term Path

Spare parts buy time; they do not eliminate risk. For a sterilizer approaching 20 years in service, plan a controls migration to a current-generation platform:

• Target controller: SIMATIC S7-1200 with CPU 1214C DC/DC/DC or CPU 1215C. The S7-1200 accepts the same 24 V DC architecture and uses TIA Portal V18 (or current), which is actively supported.
• Analog input: SM 1231 AI4 x 13 bit (6ES7 231-4HD32-0XB0) or AI8 x 13 bit, replacing the EM 231 with a drop-in channel count upgrade.
• HMI: SIMATIC KTP700 Basic or TP700 Comfort. Migrate the existing ProTool screens to TIA Portal using WinCC Comfort.
• Program conversion: STEP 7 Micro/WIN projects can be migrated to S7-1200 via the S7-200 Migration Tool built into TIA Portal. Ladder logic with standard instructions ports cleanly; SBR/INT calls, HSC configurations, and PPI-specific communications require manual rework.
• Validation: For medical / FDA-regulated sterilizers, plan a full IQ/OQ/PQ after migration. Pressure-loop calibration must be traceable to NIST.

11. Lessons Learned - Documentation Standards Going Forward

This failure illustrates three documentation gaps that catch every legacy-controls owner eventually:

1. No program archive. The owner did not have an offline copy of the Micro/WIN project. Without that file, a CPU failure means a complete re-engineering effort, not a module swap. Archive the project to a cloud repository after every change.
2. No MLFB inventory. The vendor guessed at module replacement using an EM 222 instead of an EM 231. A one-page rack diagram with MLFBs printed on it eliminates this class of error. Print it, laminate it, mount it inside the cabinet door.
3. No HMI project archive. The TP 170A project is equally irreplaceable without a backup. Export the ProTool source, commit to version control, and store with the PLC archive.

These three artifacts - PLC project, HMI project, rack diagram - are the minimum documentation needed to keep a 20-year-old controls system serviceable. Produce them before the next failure, not during it.

12. Frequently Asked Questions