Wire Break Detection on Siemens 6ES7326-1BK01 F-DI Inputs

1. Problem Statement: Wire Break Detection on F-DI Channels

Modern safety-integrated PLC systems (SIL 2/3, PL d/e per IEC 61508 and EN ISO 13849-1) must detect not only the operational state of an emergency-stop or guard-door contact but also the integrity of the wiring that connects the contact to the safety input module. A wire break on a standard digital input looks identical to a healthy open contact, so the safety function cannot distinguish a released E-stop from a snapped cable in the field.

The Siemens SM 326F safety input module (MLFB 6ES7326-1BK01-0AB0) is a 24-channel, 24 V DC F-DI designed for S7-300F and ET 200M systems. Its on-board diagnostic evaluation can flag wire break, short circuit, and cross-circuit faults, but the standard 24 V digital inputs (channels 0-23) of this module are designed for positively opening (NC) contacts as defined in EN ISO 14119. A normally open (NO) contact on a standard channel cannot be monitored for wire break because the channel sees an identical high-impedance state in both "contact open" and "cable broken" conditions.

The remaining two channels (the dedicated NAMUR pair wired per the user's application) are configured for NAMUR sensors per DIN 19234 / IEC 60947-5-6 current-loop operation. These channels actively source a defined test current, measure the loop current, and produce a deterministic evaluation of the connected sensor's state, including a wire break condition. The user's question therefore has only one production-grade answer: use the SM 326F NAMUR inputs with a NAMUR-formatted sensor, or implement a passive resistor network that translates a wire break into a measurable voltage excursion on a standard channel — the latter is non-SIL and must be carefully considered.

2. SM 326F (6ES7326-1BK01-0AB0) Module Specifications

The SM 326F is the safety-relevant digital input module for the S7-300F / ET 200M family. Its key parameters relevant to wire break detection are summarized below. Spec values follow the manufacturer's manual, the Siemens S7-300/S7-300F Safety Integrated manual entry 19026151, which is the primary document for hardware design verification.

3. Why NC Contacts Are Standard in Safety Engineering

EN ISO 14119 and the long-standing convention codified in IEC 60204-1 require safety-related switching elements to be positively opening (also called "force-guided" or "direct-opening action"). A positively opening contact is mechanically constrained so that, when the actuator is withdrawn, the contacts are mechanically pulled apart by a rigid link rather than relying on spring tension alone. This construction guarantees that a contact that welds in the closed position cannot release without mechanical failure, which is detectable at the next test cycle.

In F-DI modules such as the SM 326F, the standard channels implement this expectation by sourcing a small test pulse through the input. With an NC contact wired between the input and M (24 V return), the module measures the current loop and distinguishes three states:

• Healthy closed contact → test current flows → "1" (safe state NOT active)
• Healthy open contact → test current interrupted → "0" (safe state ACTIVE, e.g., E-stop pressed)
• Wire break → test current interrupted → also "0", but with a diagnostic flag

When you replace the NC contact with a NO contact, the table becomes:

• Healthy open contact → test current interrupted → "0" (safe state ACTIVE)
• Healthy closed contact → test current flows → "1" (safe state NOT active)
• Wire break → test current interrupted → "0", but with a diagnostic flag — identical to the safe state

The risk: a wire break on a channel that is supposed to read "1" (operational, guard closed) will silently look like the E-stop has been pressed. The system will trip on every cycle. More dangerously, a wire break on a channel that is supposed to read "0" (E-stop pressed) will look identical to a healthy "0", and the module will not flag it because the test current is already absent in the desired state. To get unambiguous wire break coverage in both states, you need either the dedicated NAMUR input or a passive resistor network that pulls the line to a known potential when the contact is open.

4. NAMUR Sensor Solution per DIN 19234

The cleanest engineering answer is to use a NAMUR sensor on the SM 326F's NAMUR channels. A NAMUR sensor is a two-wire, current-loop device with the following behavior, defined by IEC 60947-5-6:

• The module supplies a nominal 8.2 V DC open-circuit voltage at the input.
• A "damped" (target present) sensor draws approximately 2.5-3.5 mA.
• An "undamped" (target absent) sensor draws approximately 0.4-1.0 mA.
• A wire break drops the loop current below 0.1-0.2 mA.
• A short circuit forces the loop current above 6 mA.

The module's internal evaluation produces four distinguishable states: damped (logic 1, no fault), undamped (logic 0, no fault), wire break (fault), and short circuit (fault). All four are reported individually to the F-CPU's diagnostic buffer and the standard channel diagnostic image (input value, channel fault, wire break bit, short circuit bit).

NAMUR loop current evaluation (SM 326F Ch24/25):
  I_loop >= 6.0 mA   → SHORT CIRCUIT  (fault: SC bit set)
  2.1 mA < I_loop < 6.0 mA → LOGIC 1 (damped, healthy)
  1.2 mA < I_loop < 2.1 mA → indeterminate
  0.1 mA < I_loop < 1.2 mA → LOGIC 0 (undamped, healthy)
  I_loop <= 0.1 mA   → WIRE BREAK     (fault: WB bit set)

For the user's application (a guard-door interlock on a S7-315F with SM 326F), the path is:

1. Replace the NO mechanical contact with a NAMUR-format proximity sensor or a NAMUR-formatted reed switch module (e.g., Pepperl+Fuchs NCN3 or SJ3.5 series, or ifm NK series — any sensor marked "NAMUR" or "DIN 19234").
2. Wire the two leads of the NAMUR sensor to the SM 326F channel 24 or 25 terminal and the matching M terminal. Polarity is not observed; NAMUR sensors are polarity-insensitive.
3. In STEP 7 (with the S7 Distributed Safety or F-Configuration Pack option installed), open the hardware configuration (HW Config) and assign the relevant channels to "NAMUR sensor with diagnostic evaluation" in the module's properties. The PROFIsafe / F-parameters dialog accepts this assignment.
4. In the safety program (F-FB / F-FC), the channel value and the diagnostic bits are available in the F-I/O DB. Use the wire break bit (e.g., DIAG_NAMUR_WIRE_BREAK) and the short circuit bit to drive a passivation request to the F-CPU.

F-I/O DB structure (illustrative, F-CONF Pack format):
  F_DATA_CH24   : BOOL        -- process value, channel 24
  DIAG_CH24     : BOOL        -- 1 = any diagnostic event on ch24
  WB_CH24       : BOOL        -- 1 = wire break detected
  SC_CH24       : BOOL        -- 1 = short circuit detected
  PASS_ON       : BOOL        -- passivation request
  QBAD_CH24     : BOOL        -- 1 = channel value invalid

5. Resistor Network Workaround (Non-NAMUR, Non-SIL)

When a NAMUR sensor is not available, the same wire-break capability can be approximated with a passive resistor network. This is a long-standing field workaround. The trade-off is that the resulting circuit is not type-approved to SIL 3 by the manufacturer; it must be evaluated by the integrator for the specific safety class required, including consideration of resistor failure modes (open, short, drift).

The principle: a pull-up resistor (R_p) holds the F-DI input to 24 V (logic "1") through a current-limiting resistor (R_s). The NO contact, when closed, shorts the input to M through R_s only, pulling the input to a low voltage (logic "0"). A wire break anywhere in the loop drops the test current to zero and lets the pull-up drive the input to 24 V, which is detectable as a stuck-"1" condition by the safety program.

A typical component selection for 24 V DC F-DI (logic "1" = +15 to +30 V, "0" = -30 to +5 V, input current typ. 9 mA):

Closed-contact path current: I_closed = 24 V / (R_p + R_s) = 24 V / 12.2 kΩ ≈ 1.97 mA. The input voltage at the F-DI terminal: V_in = I_closed × R_s = 1.97 mA × 2.2 kΩ ≈ 4.33 V, which falls inside the "0" range of the SM 326F (-30 V to +5 V). The module reads "0" correctly.

Open-contact path current: I_open = 0 mA (contact open, no other path). The input is pulled to +24 V by R_p, with the module's internal input impedance in parallel. The F-DI reads "1".

Wire-break path: same as open-contact, but the safety program must distinguish "wire break" from "intentional open". This requires a separate test strategy — typically a periodic closure of the contact, or a second channel that toggles to confirm the wiring is intact. Without that test strategy, the workaround only covers one direction of the failure.

6. Wiring Topologies (Diagrams)

6.1 NAMUR Sensor on SM 326F Channel 24

6.2 NO Contact with Resistor Network (Standard Channel)

6.3 Standard NC Contact (Reference, for Comparison)

7. STEP 7 / Distributed Safety Configuration

With the SM 326F installed and addressed, the NAMUR inputs are configured in HW Config and the safety logic is built in the F-FB / F-FC blocks of STEP 7 with the Distributed Safety (S7 F-Configuration Pack or S7 F/FH Systems) optional package.

7.1 Hardware Configuration Steps

1. Open SIMATIC Manager → your S7-300F station → HW Config.
2. Open the slot containing the 6ES7326-1BK01-0AB0 and double-click to open properties.
3. On the Parameters tab, navigate to the input channel group that includes the NAMUR channels.
4. Set Sensor type = "NAMUR sensor" (not "24 V DC standard") for the channels that will carry NAMUR signals.
5. Set Diagnostic evaluation = "Active" so the module reports wire break and short circuit in the diagnostic buffer.
6. Set Discrepancy time if the application uses two redundant channels (default 50 ms; range 1-65535 ms).
7. Compile and download the hardware configuration to the F-CPU. The CPU will perform a safety startup; the new parameters are activated after the next STOP→RUN transition or via the standard "Restart" mechanism.

7.2 Safety Program Integration

The F-I/O DB exposes the process value and the diagnostic flags. A typical F-FB call uses the Q_BAD output of the F-channel driver to drive passivation and to latch a maintenance request. The wire break bit is mapped to an HMI tag for operator display.

// F-I/O DB (F-CONF Pack convention, example name DI_FDB):
  F_DATA_CH24        : BOOL    // process input, NAMUR channel 24
  DIAG_CH24_WB       : BOOL    // wire break
  DIAG_CH24_SC       : BOOL    // short circuit
  DIAG_CH24          : BOOL    // any channel fault
  QBAD_CH24          : BOOL    // 1 = value invalid (passivated)
  PASSIVATION_ON     : BOOL    // aggregated passivation command

In the F-FB, evaluate:

// ST snippet — F-runtime (safety program)
IF DIAG_CH24_WB OR DIAG_CH24_SC THEN
    // Passivate this channel; downstream safety function takes safe state
    PASSIVATION_ON := TRUE;
    QBAD_CH24      := TRUE;
    // Operator message (acknowledged only after re-test)
    HMI_MSG_WIREBREAK_CH24 := TRUE;
END_IF;

The passivation automatically drives the F-CPU's safety outputs to their safe state (typically de-energize). The system must remain in safe state until the wire break is physically repaired and the F-channel is reintegrated via the standard reintegration procedure (operator confirmation in the HMI, or automatic if so configured).

8. Comparison of Detection Methods

The table below compares the three practical options for wire break coverage on a Siemens SM 326F application. Use it for design selection.

9. Commissioning and Verification

After installation and parameter download, perform a structured verification of the wire break function on every channel. A 100% test is mandatory in safety-integrated systems.

Test procedure per channel:

1. With the system in a safe state, connect the field device (NC contact, NAMUR sensor, or NO contact + resistor network) per the wiring diagram.
2. Drive the F-CPU to RUN. Verify the process value matches the expected idle state (e.g., "1" for guard closed, "0" for guard open).
3. Disconnect one wire at the field end (simulate wire break at the field terminal). Verify that within the configured discrepancy time the F-CPU reports:
   • DIAG_CHxx_WB = 1
   • QBAD_CHxx = 1
   • The downstream safety actuator de-energizes
4. Reconnect the wire. Press the operator reintegration button on the HMI. Verify the F-CPU reintegration completes without error and the process value returns to expected.
5. Repeat for short-circuit test: connect the input directly to +24 V (or to M for cross circuit). Verify the corresponding DIAG bit and QBAD behavior.
6. Document the test result per channel, signed by the responsible safety engineer. File the test record with the project documentation per EN ISO 13849-2 / IEC 61508 SIL verification requirements.

10. Diagnostic Event and Error Code Reference

When a wire break is detected, the SM 326F enters the event in the F-CPU's diagnostic buffer. The event IDs are documented in the module manual. Typical examples (verify against the live firmware release notes for the version installed):

11. Troubleshooting Matrix

Use the matrix below as a fast field reference. Always cross-check the actual error code against the module manual for the installed firmware.

12. Broader Safety Management Context

Wire break detection on individual safety I/O is one layer of a multi-layered safety program. Per the OSHA hazard-identification framework, the technical control (the F-DI module, the NAMUR sensor, the wiring) is the engineering layer; it must be backed by administrative controls (regular inspection, proof testing, training) and, where residual risk remains, PPE. The reference at OSHA Safety Management - Hazard Identification and Assessment outlines the full hierarchy: collect existing information about workplace hazards, inspect the workplace for safety hazards, and identify health hazards as a starting point for any retrofit that introduces new safety I/O. When a new F-DI channel is added or a sensor topology changes (for example, swapping an NC contact for a NAMUR sensor), the hazard analysis must be re-performed and the resulting safety requirements (SIL target, PFH budget, proof test interval) re-validated.

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