Problem Overview
An ET 200S station with one or more PM-E DC24V power modules reports a group fault (SF – Sammelfehler) immediately after the configuration is downloaded to the head-end PLC. In the case documented here, the configuration contained more than five identical digital input modules and a single PM-E (order number 6ES7138-4CA01-0AA0) was flagged while its neighbours stayed green. The hardware diagnostic buffer in STEP 7 reported one or more of the following:
- Channel error on the PM-E
- Sensor or load voltage missing
- System data differs from the offline configuration
- Saved configuration differs from the one currently loaded in the PLC
The root cause is almost always wiring of the voltage group on the terminal modules (TM), specifically how the AUX1/AUX2 clamps are bridged from one TM to the next inside the same potential group. A mis-wired AUX bridge introduces a back-feed or a missing reference (M) to the PM-E, which the firmware interprets as a missing 2L+ supply on the load-voltage path. The SF LED on the power module, on every DI in the same potential group, and on the interface module turns on simultaneously.
ET 200S PM-E Module Identification
The PM-E is a pure power-feed / power-pass-through module in the SIMATIC ET 200S family. It does not perform digital I/O; it segments 24 V potential groups and supplies the downstream electronic modules with either 1L+ (sensor supply) or 2L+ (load supply). Three PM-E variants cover the typical 24 V application range:
| Order Number | Type | Rated Current | Diagnostic Output | Typical Use |
|---|---|---|---|---|
| 6ES7138-4CA01-0AA0 | PM-E DC24V | 5 A (load) / 10 A (overall) | SF LED + diagnostics byte | Standard potential-group segmentation |
| 6ES7138-4CB11-0AB0 | PM-E DC24..48V | 5 A / 10 A | SF LED + diagnostics byte | Wide-range DC feed |
| 6ES7138-4CA80-0AB0 | PM-E DC24V/8A RO | 8 A / 10 A | SF LED + relay output (RO) | Feed with status relay |
The variant in the field incident is the 6ES7138-4CA01-0AA0. Pin-compatibility across the family is preserved, but the diagnostic data set differs. The diagnostics structure for the 8 A relay variant is documented in the official Siemens attachment SIMATIC ET 200S Power module PM-E DC24V/8A RO manual (PDF); the same diagnostic bit layout (No load voltage / PWR) applies conceptually to the -0AA0.
Root Cause: AUX Clamp Bridging and Voltage Group Configuration
ET 200S terminal modules (TM-E, TM-P, TM-…-S) expose screw or spring terminals for the field wiring. Each TM has dedicated AUX1 (A3, A7) and AUX2 (A4, A8) clamps in addition to the 1L+, 2L+, M and chassis clamps. The AUX clamps are bridged from TM to TM by the TM itself – they are passive copper bars inside the terminal module that carry on to the next slot to the right.
This creates three rules that, when violated, generate the SF pattern described above:
- Potential group start. A new voltage group must begin with a PM-E (or PM-D) module. The TM immediately to the right of a PM-E is the first TM of the new potential group.
- AUX continuity inside a group. The AUX clamps (A1..A8) bridge through every TM inside one potential group. They are not allowed to bridge across a potential-group boundary.
- AUX must be referenced to the local M. Inside the same potential group, AUX is referenced to the local M of that group. If an AUX clamp is bridged to the M of an adjacent group, two M potentials are tied together – which is normally benign – but if it is bridged to the M of the power module itself, the load-voltage monitoring inside the PM-E sees a short and raises the No load voltage diagnostic.
In the documented case, the field wiring carried a bridge from AUX clamp A4 of the TM immediately right of the PM-E to pin 7 (M) of the power module. The bridge was originally installed to feed 0 V in series to another consumer that was later removed. Once the bridge was lifted, the SF cleared on every module in the potential group. The diagnostic Sensor or load voltage missing disappeared because the load-voltage (2L+) monitoring inside the PM-E was no longer being pulled to M through an unintended path.
PM-E Pinout and Voltage Group Architecture
The PM-E has eight user-relevant screw/spring terminals. The function of each pin is fixed by the firmware and must be respected when the TM is wired:
| Pin | Signal | Function | Required Wiring |
|---|---|---|---|
| 1 | 1L+ (sensor supply in) | 24 VDC for sensor / electronics | 24 VDC feed |
| 2 | 1L+ bused | Forwards 1L+ to next slot | Leave free or feed downstream sensors |
| 3 | M (electronics ground) | 0 V reference for 1L+ | 0 VDC |
| 4 | M bused | Forwards M to next slot | Leave free |
| 5 | 2L+ (load supply in) | 24 VDC for actuators | 24 VDC feed (may share with 1L+) |
| 6 | 2L+ bused | Forwards 2L+ to next slot | Leave free |
| 7 | M (load ground) | 0 V reference for 2L+ | 0 VDC; do NOT bridge to AUX clamps |
| 8 | FE / chassis | Functional earth | Bonded to cabinet PE |
The terminal module that follows the PM-E carries the I/O wiring. The TM has the same AUX clamps (A1..A8) that forward passively. The first TM after a PM-E always opens a new potential group – even if 1L+ and 2L+ are physically the same 24 VDC source.
Diagnostic Procedure in STEP 7 / TIA Portal
STEP 7 V5.x and the TIA Portal expose three independent diagnostic sources. Read all three before changing wiring.
1. Online > Hardware Diagnostics (F8)
Open the project, go online with the CPU, and double-click the ET 200S station in the device view. The "Module Information" window lists every slot and its current status. For the PM-E the right pane shows:
- Module status: OK / Fault / Not reachable
- Channel diagnostics available: Yes / No
- SF LED: ON / OFF
- Maintenance / fault LEDs: per slot
2. Diagnostic Buffer of the CPU
CPU > Diagnostic Buffer contains the chronological list of events. Filter on the ET 200S DP/PN address and look for entries of class "Channel error", "Station failure" or "Module returned". The time-stamped entries tell you whether the fault existed before the download (configuration mismatch) or appeared afterwards (wiring / commissioning).
3. Module Information > Diagnostics
Per-slot diagnostics in STEP 7 V5.5 / TIA V13+ exposes the raw diagnostic bytes. For PM-E the relevant bit fields are:
| Byte | Bit | Meaning | Likely Root Cause |
|---|---|---|---|
| 0 | 0 | Module fault | Hardware defect – replace module |
| 0 | 1 | Internal error | Firmware – power-cycle, reflash |
| 0 | 2 | External error | Wiring (this case) |
| 0 | 3 | Channel error present | Detailed in bytes 4..7 |
| 4 | 0..7 | Channel error type | 0x10 = No load voltage, 0x30 = overload |
| 5 | 0..7 | Channel number | 0xFF = applies to all channels of the module |
If the diagnostic buffer reports 0x10 No load voltage on channel 0xFF, the firmware has detected that 2L+ is present at pin 5 of the PM-E but the load-voltage monitor inside the module cannot see a healthy current path – consistent with an AUX bridge tying 2L+ or its return back to M incorrectly.
Terminal Module Compatibility Matrix
The AUX clamps only bridge when the correct TM is installed to the right of the PM-E. The matrix below summarises the TM part numbers and their AUX-bridging behaviour for the PM-E DC24V (6ES7138-4CA01-0AA0):
| Terminal Module | Screw (S) / Spring (C) | AUX1 Bridged | AUX2 Bridged | Usable with PM-E DC24V |
|---|---|---|---|---|
| TM-E15S24-01 | S | Yes (A3/A7) | Yes (A4/A8) | Yes – standard |
| TM-E15C24-01 | C | Yes | Yes | Yes – spring |
| TM-E15N24-01 | S | No | No | Yes – AUX isolated per slot |
| TM-P15S23-01 | S | No | No | Yes – pass-through, AUX open |
| TM-E15S24-A1 | S | Yes | Yes | Yes – AUX isolated to right |
If a TM-P (pass-through) is inserted where a TM-E (AUX-bridging) is expected, the AUX path is broken and downstream modules may float; if a TM-E is inserted where the design called for an isolated AUX, the bridge silently ties two groups together and produces the SF symptom on the adjacent PM-E.
Step-by-Step Resolution Procedure
- Power-cycle the ET 200S station with 24 VDC removed from the PM-E for at least 5 s. Re-apply and watch the LED sequence on the PM-E and on the head-end (IM 151 / IM 151-1).
- Verify 24 VDC at the PM-E terminals. Measure between pin 2 (1L+) and pin 3 (M), and between pin 6 (2L+) and pin 7 (M). Tolerances per Siemens datasheet: 20.4 V to 28.8 V DC. If either pair is missing or reversed, correct the feed before continuing.
- Open the wiring diagram for the potential group. Identify every TM to the right of the PM-E up to the next PM-E. Draw the AUX buses (A3/A7 and A4/A8) on paper.
- Look for unintended bridges between AUX and pin 7 (M) of the PM-E. A stranded jumper from A4 to pin 7 was the actual defect in the field case. Remove any bridge whose only purpose was to feed a now-disconnected consumer.
- Confirm AUX-to-next-TM continuity with a continuity tester on the unused AUX clamps. They must read ~0 Ω from TM n to TM n+1 within the same potential group, and open to the TM on the other side of the next PM-E.
- Re-download the STEP 7 / TIA configuration. Use "Download to target device > Hardware and software" so the slot table is rewritten. A configuration-only download will not clear a wiring-induced diagnostic.
- Go online and read the diagnostic buffer. The entry "Channel error – No load voltage" must be gone and the SF LED on the PM-E and on every DI in the same potential group must be off.
Verification and Hardware Diagnostics Output
After the AUX bridge is removed and the configuration is re-downloaded, run a clean verification sequence. Acceptable results:
- SF LED on PM-E = OFF
- SF LED on every DI / DO in the same potential group = OFF
- SF LED on the head-end IM 151 = OFF
- CPU diagnostic buffer shows "Module OK – no fault" for the affected slot
- I/O image updates correctly under process simulation (force a 1 on each DI and confirm in VAT)
For fail-safe ET 200S stations, the fail-safe channel LED and the SF LED are documented in the TIA Portal F-modules manual – Error diagnostics (Siemens docs.tia.siemens.cloud). The diagnostics structure is consistent: a red SF LED plus a red channel LED identifies the affected channel, and the detailed bit pattern can be read from the safety diagnostics.
Related SF Error Conditions
Other wiring mistakes that present with the same SF signature on a PM-E (or on the entire potential group) and must be ruled out before replacing hardware:
| Symptom | Most Likely Cause | Check |
|---|---|---|
| SF on PM-E only, PWR ON | 2L+ missing or AUX bridge to M | Measure 2L+ vs M at pin 6/7; check bridges |
| SF on PM-E and on every module in group | M reference lost – TM inserted in wrong slot | Verify TM order against HW catalog |
| SF on PM-E only at startup, clears after warm restart | Configuration mismatch – slot occupied but not configured | Compare GSD/HW catalog with physical build |
| SF on PM-E, plus "Station failure" on IM 151 | PROFIBUS / PROFINET drop-out, not wiring | Check bus terminator, address, cable |
| SF on PM-E, plus channel-diagnostic 0x30 overload | Load current > 5 A (or > 8 A on -CA80) | Sum Iout of downstream actuators |
| SF only after firmware update | FW > HW revision incompatibility | Roll back firmware or upgrade I/O |
Field-Commissioning Notes
- One PM-E per group is mandatory. Two PM-E modules in the same potential group – even at opposite ends – will fight each other on the 2L+ monitor and raise the SF.
- Reverse polarity of M (pin 7) does not blow a fuse on the PM-E itself, but it does turn on the SF immediately. Always meter 24 VDC vs 0 VDC at the screw terminal before applying field wiring.
- Shielded TM (TM-E15S24-S) variants route the shield clamps (S1..S4) differently from the AUX clamps. Mixing a shielded and a non-shielded TM inside one group leaves the shield floating and can show up as a channel error on neighbouring analog modules rather than on the PM-E.
- Hot-swap of a TM in a live ET 200S rack is not supported on -0AA0 hardware. Always power down the segment when re-terminating AUX bridges.
- Document the bridges. Mark each AUX jumper on the cabinet drawing; an undocumented bridge installed by a previous installer is the single most common root cause of SF on a PM-E.
When to Replace the Module
If the diagnostic buffer reports Module fault (byte 0, bit 0) or Internal error (byte 0, bit 1) after the wiring has been verified and the firmware is current, the PM-E itself is defective. The bench-test procedure is straightforward: install the suspect PM-E in a known-good ET 200S rack with a known-good TM-E15S24-01, a known-good 24 VDC supply on pin 2 and pin 5, no AUX bridges – and a single DI. An SF with this baseline configuration points at the module. Order a replacement 6ES7138-4CA01-0AA0 (or the -CA80-0AB0 variant if the application requires the 8 A / relay-output function).
Standards and Reference Material
- SIMATIC ET 200S distributed I/O – Power module PM-E DC24V (6ES7138-4CA01-0AA0) operating instructions, available on the Siemens Industry Online Support portal.
- SIMATIC ET 200S distributed I/O – Power module PM-E DC24V/8A RO (6ES7138-4CA80-0AB0) operating instructions: 43582121 (PDF).
- TIA Portal F-modules manual – Error diagnostics: docs.tia.siemens.cloud/r/fail_safe_modules.
- EN 61131-2 – Programmable controllers, equipment requirements (referenced for 24 VDC tolerance and EMC).
Frequently Asked Questions
What does the SF LED on a Siemens ET 200S PM-E mean?
SF (Sammelfehler / group fault) is lit whenever the PM-E firmware detects either an internal fault, an external fault such as missing 2L+ load voltage, or a configuration mismatch. Read the module's diagnostic bytes in STEP 7 or TIA Portal under "Module Information > Diagnostics" to identify the cause.
Why is the SF LED on my PM-E on, the PWR LED is on, and voltage is present at pins 2 and 6?
The most common cause is an unintended bridge between an AUX clamp (A3, A4, A7 or A8) on the downstream TM and pin 7 (M) of the PM-E, or between two M potentials of different power modules. Lift every undocumented AUX-to-M bridge inside the affected potential group and re-download the configuration.
Which terminal module (TM) belongs to the right of the PM-E DC24V (6ES7138-4CA01-0AA0)?
Use TM-E15S24-01 (screw) or TM-E15C24-01 (spring) for standard AUX-bridging applications, or TM-E15N24-01 / TM-E15S24-A1 when the AUX bus must be isolated inside the same potential group. The TM part number is printed on the front flap; verify it against the hardware catalog in STEP 7 or TIA Portal before commissioning.
Can I clear the SF diagnostic without re-wiring by downloading the configuration again?
No. A configuration download clears configuration mismatch diagnostics only. A wiring-induced No load voltage diagnostic is generated by the firmware on every power-up and will re-appear after every restart until the AUX bridge is removed.
How do I tell from the diagnostic buffer whether the SF is wiring-related or a module defect?
Read diagnostic byte 0 bit 2 (External error) and byte 4 (Channel error type). "External error" set with channel type 0x10 (No load voltage) is wiring. "Module fault" (byte 0 bit 0) with no external-error bit points at the PM-E itself; replace the module.