Resolving S7-1200 SM 1222 6ES7 222-1HH32-0XB0 No Output Voltage

Problem Statement

After installing two Siemens SM 1222 digital output modules (order code 6ES7 222-1HH32-0XB0) and one SM 1223 digital input/output module (6ES7 223-1PL32-0XB0) on an S7-1214C CPU, the engineer observed that the output LEDs on the SM modules illuminated when the corresponding bits were set in the process image, yet measuring the output terminals against M produced 0 V. The CPU's own onboard outputs operated normally, and the SM 1223 inputs registered field signals correctly. Forcing a bit to DQb.0 in TIA Portal produced the same symptom: LED on, no voltage at the terminal. The fault appeared on both SM 1222 modules. After routing the 24 VDC rail of the cabinet to the module's L+ and M terminals, all 16 outputs per module switched correctly.

This article documents the diagnostic path, the underlying root cause, and the corrected wiring for the affected hardware. It also captures the current-loading rules, group structure, and common S7-1200 signal-module pitfalls that should be checked whenever a "LED on but no output" symptom is reported.

Affected Hardware

The MLFB suffix "32" identifies the V3 generation of the S7-1200 SM, supported on CPU firmware V3.0 and later. The CPU in the reported case was manufactured in Germany; the SM modules were manufactured in China. Manufacturing origin is irrelevant to the failure mode — the root cause is wiring, not hardware provenance. Genuine Siemens modules from either production site are tested to the same acceptance criteria at the factory.

Root Cause: Module Electronics Supply (L+/M) Not Energised

The S7-1200 signal module requires two distinct 24 VDC rails to switch an output:

1. Module electronics supply on terminals L+ and M. This rail powers the backplane transceiver, the relay coils (or the output-driver transistors), and the status LEDs. It is sometimes called the "logic supply" or "module supply". Operating range is 20.4 V to 28.8 V DC.
2. Load supply on terminals 1L, 2L, 3L, 4L (relay SM) or 1L / 2L (transistor SM). This is the rail that the relay contact — or the source-driver high-side FET — actually switches to the field device. It is galvanically isolated from the module electronics supply and may be any voltage the contact is rated for (for HH32 relay contacts, that is 5 V to 30 V DC or 5 V to 250 V AC).

With the module supply unpowered, the following chain of events occurs on the affected SM 1222 / SM 1223:

1. The CPU can still write the process image and the SM's I/O ASIC latches the requested state — so the on-board LED logic may still appear to track the commanded value (the status LEDs are typically driven from the logic side, not the contact side).
2. The relay coils have no energy and the contacts stay open. With a transistor SM the high-side drivers are biased off, so the output terminal sits at 0 V regardless of the commanded state.
3. Forcing a bit in TIA Portal → Online & diagnostics → Force table succeeds at the data-link level (the LED toggles, the watchdog does not trip), but the physical output does not change because the driver stage is unpowered.
4. No SF (system fault) LED is raised and no diagnostic interrupt is queued — the CPU has no way to know the module is unpowered, because the backplane is still functional.

The original wiring in the reported case connected 230 VAC to the 3L terminal of the SM 1222 in the mistaken belief that this would switch the relay output. The 24 VDC electronics supply on L+ / M was never wired. The fix was to route a clean 24 VDC source to the module's L+ and M terminals; the load-supply terminals were then wired with the actual field voltage, and all 16 outputs of each SM 1222 began to switch.

S7-1200 Signal Module Power Architecture

The S7-1200 separates power domains deliberately. The diagram below shows the corrected wiring for an SM 1222 6ES7 222-1HH32-0XB0, which is the same architecture used by the SM 1223 6ES7 223-1PL32-0XB0 relay outputs.

Key observations from the diagram:

• The cabinet 24 VDC rail feeds the CPU's L+/M power input.
• The CPU's sensor power output (or a separate 24 V rail, recommended) feeds the SM's L+ and M. This is the rail that was missing in the original wiring.
• The four load-supply terminals 1L..4L on the SM 1222 each connect to the source that the corresponding group of four relay contacts switches. Each group is electrically isolated; the rail voltage on each nL can be different (24 VDC, 120 VAC, 230 VAC, etc., within the contact rating).

Diagnostic Methodology

Use this sequence whenever a S7-1200 SM output "does nothing" while the LED appears correct.

1. Confirm the CPU is in RUN and the slot is mapped. In TIA Portal, open Online & diagnostics → Diagnostics status for the affected SM. The status should read OK with no pending diagnostic interrupts. If the slot shows Fault or Not accessible, the wiring is a secondary issue — resolve the backplane problem first.
2. Measure the SM supply terminals. Place a DMM between L+ and M on the SM itself (not the upstream rail). The expected reading is 20.4 V to 28.8 V DC. A reading of 0 V is the failure mode that caused the original incident.
3. Measure the load-supply terminals. With the load supply rail de-energised for safety, place a DMM between each nL terminal and the corresponding DQ terminal with the output commanded ON. The expected reading is closed contact (typically < 1 Ω). If the reading is open (> 1 MΩ) while the LED is on, the relay coil is not energising — go back to step 2.
4. Force the output. In TIA Portal, use Online & diagnostics → Force table to force a single output. Watch the SM LED. If the LED tracks the force but the contact does not close, the module's internal driver or coil is unpowered or damaged. If the LED does not track the force, the slot mapping or firmware is wrong.
5. Test other outputs in the same group. Force a different DQ in the same group. If the other outputs work, you have an over-current or short on the failing channel that may be dragging the group rail down. If all outputs in the group fail, the group supply (nL) is missing or the module is defective.
6. Test outputs in a different group. Force an output in 2L's group. If that group works, the problem is isolated to the 1L/2L/3L/4L rail and its wiring — not the module supply.
7. Inspect for DOA hardware. If steps 1–6 all point to a healthy module supply and healthy load supply, but a specific DQ still does not switch, the module is defective. Tag it for RMA. Do not chase a wiring fault that does not exist.

Step-by-Step Wiring Procedure

The procedure below restores the missing 24 VDC module supply to the SM 1222 / SM 1223 and verifies correct operation. Power down the cabinet before touching terminals.

1. Lock out and tag out (LOTO) the cabinet. Open the main breaker and verify zero energy with a known-good DMM at the field bus terminals.
2. Identify the SM terminals. On the SM 1222 HH32, the module supply terminals are L+ and M on the upper connector. The load-supply terminals 1L..4L are below them. The 16 output terminals DQ a.0..b.7 are on the right-hand connector.
3. Route a 24 VDC source to L+ and M. Use a dedicated 24 V PSU sized for the sum of all SM electronics (≈ 200 mA per SM 1222 HH32, ≈ 180 mA per SM 1223 PL32 at 24 V). Run the L+ and M conductors as a twisted pair back to the PSU to minimise noise pickup on the backplane.
4. Wire the load-supply terminals 1L..4L. Connect each nL to the source that the corresponding group of four contacts switches. For DC loads, observe polarity only if the load is polarity-sensitive (most relay contacts are not). For AC loads, no polarity.
5. Wire the field loads to the DQ terminals. Run the load conductor from DQ a.x / DQ b.x through the field device back to the appropriate supply return. For DC sourcing transistor outputs, this is the equivalent of a low-side switch — the load is between DQ and the positive rail, and M returns to the rail negative.
6. Tighten terminals to the specified torque (typically 0.6 N·m for the S7-1200 spring terminals, 0.5 N·m for screw terminals — verify against the S7-1200 system manual).
7. Restore cabinet power. Watch the SM LEDs at power-up. The PWR LED on the SM should illuminate within < 1 s. A SF LED or a flashing PWR LED indicates a hardware fault or under-voltage — re-check the supply rail at the SM terminals.
8. Run a force-table test. Force each output ON for 2 s, then OFF. Verify the LED tracks the command and the contact closes. Cycle through all 16 outputs of each SM.
9. Run a real-load test. Replace the force with a controlled program (e.g. a 5 s toggle in OB1 or a watch table) and verify the field device energises and de-energises cleanly.

Output Current Limits and Group Loading

Relays and transistors fail differently. The relay output modules (HH32, PL32) are limited by contact rating; the transistor modules (BH32, BL32) by on-resistance and thermal limits. The table below summarises the electrical limits you must respect; for the binding values, always consult the S7-1200 Programmable Controller system manual for your specific MLFB.

Verification

After the wiring repair, run a structured verification cycle before releasing the panel:

1. Voltage check at the SM terminals. With the cabinet powered, measure L+ to M at the SM itself. Expect 20.4 V to 28.8 V DC. Repeat for each load-supply terminal nL to its return. Record all readings.
2. Force-table walk-through. Force each of the 16 outputs ON for 2 s in turn. Confirm the corresponding LED and the field device respond. Capture the readings with a screen recorder for the commissioning report.
3. Group-load check. If the application drives all four outputs of a group simultaneously, measure the group current with a clamp meter. Confirm the steady-state current is ≤ 8 A and the inrush is within the lamp-load rating.
4. Module electronics temperature. After 30 min of operation with the expected load profile, measure the front face of the SM with an IR thermometer. A reading above 60 °C is a sign of over-loaded or improperly tightened terminals.
5. Diagnostic buffer sanity check. In TIA Portal, open the CPU's diagnostic buffer and confirm no entries for the SM slots. Any Wire break, Short circuit, or Load voltage missing entry (where supported by your firmware) needs to be investigated before sign-off.

Defective-on-Arrival (DOA) and Warranty Handling

Hardware can be DOA. The diagnostic path above should rule out wiring, supply, and configuration issues first; if a module still fails to switch a single output after the L+/M rail and the group load rail are confirmed healthy, the module is defective. Field-engineer practice:

• Document the failure with the diagnostic buffer export, a wiring photo, and the DMM readings at L+/M and at the failing DQ terminal.
• Open a warranty case with the Siemens regional representative. The MLFB and serial number are on the front of the module and in the device master in TIA Portal.
• Do not attempt to repair the module. The relay contacts and the output-driver PCB are not field-serviceable; opening the housing voids the warranty.
• While waiting for the replacement, run the application on a reduced I/O count if the program allows. Forcing outputs is a diagnostic action, not a workaround for production.

Common Wiring Pitfalls on S7-1200 Signal Modules

Quick Reference: MLFBs Mentioned

• 6ES7 214-1AG40-0XB0 — S7-1214C DC/DC/DC (24 V supply, 24 V transistor outputs, 24 V inputs)
• 6ES7 214-1BG40-0XB0 — S7-1214C AC/DC/RLY (120/230 V supply, 24 V transistor outputs, relay outputs)
• 6ES7 214-1HG40-0XB0 — S7-1214C DC/DC/RLY (24 V supply, 24 V transistor outputs, relay outputs)
• 6ES7 222-1HH32-0XB0 — SM 1222, 16 DO relay, 2 A
• 6ES7 223-1PL32-0XB0 — SM 1223, 16 DI 24 V / 16 DO relay

FAQ