S5-135U 6ES5 430-4AU13 DI Card: Terminal 3 GND vs 24VDC

Module Identification and Context

The 6ES5 430-4AU13 is a 32-channel, 24 V DC digital input (DI) module for the SIMATIC S5-135U PLC family. It belongs to the 6ES5 430 sub-range that occupies a single slot in the S5-135U central rack (CR) or expansion rack (ER) and is addressed by the CPU through the S5 backplane I/O bus. In migration projects that move control logic from an S5-135U to an S7-300 (or to an S7-1500 with an ET 200 SP migration gateway), this DI card is one of the most common termination points re-wired during a planned shutdown, because the field wiring harness already exists in the cabinet and is typically the only piece of the legacy system that physically survives the conversion.

Pin 3 of the 40-pin front connector is the module common (M terminal). The originating question was triggered when one terminal-3 conductor was found landed on cabinet ground (0 V) rather than on the more familiar +24 V rail. Both wirings are legal as long as the field devices connected to the card (PNP sensors, mechanical limit switches, dry contacts, NPN sensors) match the polarity the module expects. Pin 3 alone is therefore not a wiring "error" - it is a wiring "choice" dictated by sensor family. A mis-paired sensor on a correctly-wired M terminal will still misbehave; a mis-paired M terminal on correctly-polarised sensors will look like every input is stuck at "0".

Terminal 3 Function: Module Common (M)

Each of the 32 input channels on the 6ES5 430-4AU13 is an optically-isolated input that returns to a shared M rail inside the module. The 40-pin front connector routes that M rail to pin 3 (and, on the 6ES5 430-4 variants, also to pin 4 as a parallel feed to reduce voltage drop across long field harnesses). External field wiring must complete the circuit from the sensor back to pin 3; otherwise the optocoupler LED inside the module never energises and the input reads "0" regardless of the physical state of the sensor.

Two valid wiring conventions exist for any S5 24 V DC DI module:

• PNP / sourcing convention: Pin 3 = 0 V (GND). The sensor switches +24 V to the input pin when active. Current flows from +24 V (sensor supply) → through the optocoupler LED inside the module → out pin 3 → back to the 0 V supply reference. This is the dominant convention in European machinery and is the Siemens default in most S5 and S7 wiring diagrams.
• NPN / sinking convention: Pin 3 = +24 V. The sensor switches 0 V (GND) to the input pin when active. Current flows from +24 V at pin 3 → through the optocoupler LED → out the input pin → into the sensor → to 0 V. This convention is more common in Asian-built machinery and on certain hydraulic and pneumatic valve manifolds.

Mixing PNP and NPN sensors on the same module is allowed only when the sensor commons are isolated and the field installer routes them onto separate M terminals - which is not what pin 3 of the 6ES5 430-4AU13 provides. A practical field rule: choose one convention for the entire module and wire every sensor on that module accordingly. If the cabinet has mixed sensors, split them across two 6ES5 430 cards (or across the available input groups of an S7-300 SM 321 during migration) so that each module honours only one polarity.

PNP vs NPN: Current-Path Comparison

The functional difference between connecting pin 3 to GND or to +24 V is not in the module itself - the optocoupler LED is bidirectional within its reverse-polarity tolerance - it is in which device sources the switching current. The diagram below uses a 4-channel excerpt for clarity; pin 3 is the module common and pins 8/9/10/11 are four of the 32 input pins on the 40-pin front connector.

In the PNP case the sensor is the current source; in the NPN case the module is the current source through pin 3. From the optocoupler's point of view the LED sees the same forward drop either way; the only thing that changes is which end of the loop is held at supply potential.

What "Polarity Reversal for up to 8 Inputs per Module Is Permissible" Means

The Siemens manual note that prompted the field report - "Polarity reversal for up to 8 inputs per module is permissible" - is a damage-tolerance statement, not a recommendation. The 6ES5 430-4AU13 input optocouplers include a parallel reverse-polarity protection network (a clamping diode in series with a current-limiting resistor) that absorbs the reverse voltage if a wireman swaps +24 V and 0 V at the field device. Up to eight such reversals per module will not damage the card under the rated input conditions.

This does not mean that a reversed input will read the correct logic state. A reversed-polarity input will:

• Not illuminate the green input-status LED on the module front panel, even when the sensor contact is closed.
• Read as a logic "0" in the S5 process image (or in the S7 input process image after migration) even when the physical sensor is active.
• Not generate an SF (system fault) on the CPU - the module will appear healthy on the diagnostic scan, which is what makes the symptom so hard to find.
• Eventually fail if more than 8 inputs are reversed and the protection network is thermally overloaded (typical limit: the short-circuit current of the supply multiplied by the number of reversed inputs, derated by the protection resistor's continuous dissipation).

Treat the 8-input tolerance as a safety margin during commissioning, not as a design parameter. Each reversed input must still be re-paired before the line is signed off.

Reading the 40-Pin Front Connector

The 6ES5 430-4AU13 uses the standard Siemens 40-pin front connector (6ES5 490-7xx series). Pin 1 is at the lower-right of the connector when the cable entrance is to the left; pin 2 sits diagonally above pin 1; pin 40 sits diagonally above pin 39 at the upper-right. The mapping most often reproduced for the 430 sub-range is summarised below; the values for a specific build revision should always be confirmed against the wiring diagram supplied with the cabinet before any conductor is moved.

Bytes I 0.0 - I 3.7 are the four input bytes the CPU reads through the process image; byte order depends on the S5 CPU model and the slot address configured in COM 135U or STEP 5. The exact byte offsets (32, 33, 34, 35 or 64, 65, 66, 67 etc.) must be looked up from the existing program (e.g. via cross-reference in STEP 5) before any re-termination.

Sensor Compatibility Matrix

Use the matrix below to validate a planned sensor family against the planned pin-3 polarity before the harness is re-terminated. A "yes" cell means the module will read that sensor family correctly; a "no" cell means the input will be stuck at 0 regardless of contact state and must be re-wired (or the sensor replaced).

IEC 61131-2 Type 1 digital inputs are specified for a rated voltage of 24 V DC, an ON-state transition range of 11-30 V (Type 1) or 15-30 V (Type 3), and an OFF-state range below 5 V. The 6ES5 430-4AU13 falls in the same family, so any IEC 61131-2 Type 1 PNP or NPN device will be electrically compatible regardless of manufacturer (Siemens, Sick, IFM, Pepperl+Fuchs, Turck, Balluff, Omron).

S5-135U to S7-300 Migration: Recreating the Wiring on SM 321

During an S5-135U to S7-300 migration the field wiring harness almost always survives. The new S7-300 SM 321 DI module (e.g. 6ES7 321-1BL00-0AA0 for 32-point 24 V DC, or 6ES7 321-1BH02-0AA0 for 16-point) is wired with the same conventions:

1. Document the legacy pin-3 polarity on each 6ES5 430-4AU13 before de-energising. Photograph the connector with the cable entrance visible and a ruler in frame.
2. Cross-reference each input byte in the STEP 5 program to its field device, using the IB cross-reference. Print the list and tape it to the cabinet.
3. Pull the S5 front connector off the 6ES5 430-4AU13 and label every conductor with its pin number using a Brady or Phoenix marker.
4. Choose the SM 321 variant that matches the existing pin-3 polarity and the input count. If the legacy wiring used pin 3 = GND, select an SM 321 with PNP-compatible inputs (the default 6ES7 321-1BL00 is PNP). If the legacy wiring used pin 3 = +24 V, you may need an SM 321 with hardware-configurable input type, or you may re-terminate the harness to swap pin 3 to GND and re-verify polarity at every sensor.
5. Re-pin the front connector from the 40-pin S5 footprint to the 20-pin or 40-pin S7 footprint as required by the SM 321 variant. Maintain the pin-3 polarity chosen in step 4.
6. Configure the input byte address in HW Config (TIA Portal or STEP 7 Classic) to match the legacy address from step 2. For an SM 321-1BL00 in slot 4 of an S7-300, the default input addresses are IB 0 through IB 3 (32 points); for an SM 321-1BH02 in slot 4, IB 0 and IB 1 (16 points).
7. Loop-test every channel with a hand-held pushbutton wired from +24 V (PNP) or 0 V (NPN) into the input pin, returning to pin 3. Verify in the S7 online monitor (TIA Portal > Online & Diagnostics, or STEP 7 > Monitor/Modify) that the correct input bit toggles.

Field Commissioning Checklist

Run the checklist below before energising the cabinet after any re-termination of pin 3.

1. Verify cabinet ground (PE) and the 0 V rail (M) are bonded at exactly one point - typically the 24 V DC power supply negative terminal. Multiple bonds create ground loops that bias 0 V relative to the sensor supply and produce phantom "1" states on PNP inputs.
2. Confirm the 24 V DC supply is within the 20.4-28.8 V range that Siemens specifies for the 6ES5 430 family at full load and full input current.
3. Measure the voltage between pin 3 and the sensor's reference rail with a calibrated Fluke 87V (or equivalent) on the 24 V DC scale. The reading should match the design intent: 0 V for PNP, 24 V for NPN.
4. For every input channel, force the sensor (by hand or with a control signal) and verify the green LED on the 6ES5 430-4AU13 front panel illuminates. No LED = no module-side loop, regardless of whether the process image reports a "1".
5. Cross-check the LED against the S5 process-image value in STEP 5 > Test > Status. A green LED with a "0" in the process image indicates a configuration error in COM 135U (wrong slot address) or a CPU scan fault. A "1" in the process image with no LED indicates a reverse-polarity input - the protected state the module allows up to 8 times.
6. Record the polarity of every channel on the as-built drawing and lock the wiring harness to prevent later disturbance.

Diagnostic Procedure When an Input Reads "0"

Walk the tree below in order. Each step is faster than the one below it; stop at the first match.

Comparison with Modern S7-300 SM 321 Modules

For projects that target an S7-1500 with an ET 200 SP migration gateway (e.g. 6ES7 678-2AC42-1AA0), the field wiring can be reused almost verbatim. The migration gateway presents PNP-compatible 24 V DC inputs on its 16-pin push-in terminals; pin-3 polarity must still be checked because the gateway cannot tolerate a reversed module common like the legacy S5 module can.

Common Pitfalls and Safety Notes

• Never assume pin 3 is +24 V. Open the cabinet drawing first; the legacy wireman may have chosen PNP convention with pin 3 grounded.
• Never re-energise a 6ES5 430-4AU13 without first verifying that no more than 8 inputs are reverse-polarised. The protection network is rated for that count under the published derating curve; exceeding it during a bench test is the most common way to destroy the card before it ever sees production.
• Never share pin 3 across multiple 6ES5 430 cards with mixed polarity. Two cards wired with opposite pin-3 conventions and a common field ground will short the 24 V rail through the card commons and trip the power supply.
• Always re-torque the front-connector screws to 0.6-0.8 N·m after any re-termination. Loose connectors produce intermittent "0" states that look like noise but are actually micro-arcing on the pin.
• Label every conductor with pin number and signal name before pulling the connector. The 40-pin connector is dense enough that hand-tracing later takes hours.
• Lock out / tag out the 24 V DC supply before touching pin 3. The S5-135U backplane remains live as long as the CPU is in RUN; field voltage is not.

Documentation and Further Reading

The official Siemens support portal hosts the S5-135U system manual and the 6ES5 430 module manual as PDF downloads. Search for the order number (6ES5 430-4AU13) on the Siemens Industry Online Support portal to retrieve the matching wiring diagram, the slot-address assignment table, and the FW version notes. For migration projects that bring the legacy S5 to a modern HMI such as a Pro-face GP-4500 series, the manufacturer-supplied S5 CPU Direct Driver manual documents the electrical interface between the HMI port and the S5 CPU programming port - the 6ES5 430-4AU13 wiring is independent of that interface, but the manual is useful background for the migration team: SIMATIC S5 CPU Direct Driver Manual (Pro-face).

For IEC 61131-2 Type 1 / Type 3 input voltage thresholds and for the standard reference on PLC digital-input behaviour, consult the IEC 61131-2:2017 standard directly through your national member body.