Wiring KNF Simdos 02 to Siemens S7-300 SM323: Logic and I/O

Overview

This field guide details how to wire, configure, and program a KNF Neuberger Simdos 02 diaphragm metering pump with a Siemens SIMATIC S7-300 programmable logic controller (PLC) using the SM 323 digital input/output module (order number 6ES7323-1BL00-0AA0). The procedure also applies to the larger Simdos 10, Simdos 20, and Simdos 100 variants with appropriate flow-rate adjustments.

The Simdos 02 is engineered for laboratory and low-flow process dosing at 0.03 to 2 ml/min. It exposes two digital inputs (IN1 configurable as Start/Stop or volume pulse, IN2 reserved for prime) and one digital output (O1, the Running signal). Its digital inputs follow a low-active convention: a voltage below 0.8 V DC at the input terminal is interpreted as the "active" or "on" state, while a voltage above 2 V DC is interpreted as "inactive" or "off." This is the opposite of the standard PNP sourcing convention used by virtually all Siemens SIMATIC digital output modules, including the SM 323, so the polarity of control must be inverted in either the wiring, the program, or an interposing stage.

The O1 feedback is implemented as an NPN open-collector transistor capable of pulling its terminal to 0 V DC when the pump is running, and otherwise high-impedance. S7-300 SM 323 inputs are PNP (sourcing) type and require +24 V DC to register a logic "1", so the O1 line must be pulled up to +24 V DC through an external resistor, and the resulting input bit must be inverted in the PLC logic if you want a "Running" tag to read TRUE while the pump is running.

After following the wiring, programming, and verification steps in this article, the SM 323 will reliably command the Simdos 02 to start, stop, and prime, and will deliver a valid Running feedback to the S7-300 CPU for HMI display, interlocking, and batching logic.

Hardware Identification and Specifications

Verify the SM 323 variant by reading the front-panel label and the order code printed on the side of the module. Do not mix the SM 323 with the SM 322 (output only) or SM 321 (input only) - while the connectors look identical, the pin assignments differ.

SM 323 Module Signal Characteristics

The SM 323 (6ES7323-1BL00-0AA0) is a combined digital input and digital output module occupying one slot in the S7-300 rack. The 16 inputs are isolated from the 16 outputs by an optocoupler barrier, and the entire module is galvanically isolated from the backplane bus. The 16 outputs are implemented as PNP transistors (IEC 61131-2 type, 0.5 A, sourcing) that switch the load to +24 V DC. The 16 inputs are PNP type 1, sensing +24 V DC as logic "1" and 0 V DC (or open circuit) as logic "0".

The 0.5 ms output response time is what allows the SM 323 to deliver the 200 ms pulses mentioned in the field report. Even with a 4.8 ms input filter configured for noisy environments, a 200 ms pulse will be registered reliably.

Simdos 02 Signal Logic: The Low-Active Input Convention

The Simdos 02 input stage is designed to be driven by a simple switch contact or a PLC output stage, not by a sourcing transistor. Internally, each input (IN1, IN2) is pulled toward the local reference through a high-value resistor, and the input is recognized as "active" whenever an external device pulls the terminal below 0.8 V DC, i.e., toward 0 V (ground). A voltage above 2 V DC on the terminal, including the floating state of an open switch, is recognized as "inactive." This logic is sometimes called sinking input, low-side switching, or active-low.

That is the entire reason an integrator observes "I1 & I2 are high when the signal is under 0.8 V and low when higher than 2 V." The pump is reading "high" with respect to its own input pin when the pin is near ground, which is the opposite of the PLC's "high = 24 V" convention.

The configuration menu accessed via the membrane keypad on the pump face allows IN1 to be set to one of two modes:

• Pulse mode (default): Each falling edge below 0.8 V DC at IN1 corresponds to a fixed dispense volume set in the menu (calibration value, factory default 1 mL per pulse). This mode is used for volume dosing, and the integrator counts pulses in the PLC.
• Level mode: IN1 acts as a latch - a sustained low level runs the pump at the configured flow rate, a sustained high level (above 2 V) stops the pump. This is the mode required for the SM 323 Start/Stop control described in the field report.

IN2 is dedicated to the prime function in either mode. Drive IN2 low (0 V) to actuate the prime cycle, then return it to high (24 V) to stop priming.

Prerequisites

Before opening any terminal block, confirm the following:

• The S7-300 rack is powered down and the 24 V DC supply to the SM 323 and to the Simdos 02 is locked out and tagged out per site procedure.
• STEP 7 V5.5 (or later) with the S7-300 module support package installed, OR TIA Portal V13 SP1 (or later), is available on the engineering station.
• HW Config reflects the actual rack layout, with the SM 323 inserted at the slot you intend, and with the module's 24 V DC supply terminal (terminal 19 / 39 on the front connector) wired to the same 24 V DC bus that feeds the Simdos 02.
• The Simdos 02 operating instructions are at hand, especially the menu map for IN1 mode (level vs. pulse) and the calibration routine for the dispense volume.
• A digital multimeter with diode-test and continuity functions is available, as well as a clamp-on DC ammeter for verifying input current (the SM 323 input draws roughly 7 mA per energized channel).
• The integrator has confirmed that the Simdos 02 variant in his possession exposes the digital I/O terminal block (not the analog 4-20 mA variant).

Wiring: PLC Outputs to Simdos 02 Inputs (IN1, IN2)

The SM 323 outputs are PNP sourcing. With no relay and no external inverter stage, the wiring is as follows:

• SM 323 Output Q0.0 (Start/Stop) → Simdos IN1 terminal.
• SM 323 Output Q0.1 (Prime) → Simdos IN2 terminal.
• Simdos GND terminal → SM 323 24 V supply 0 V (front connector terminal 20 or 40).
• Simdos +24 V terminal → external 24 V DC supply +V (separate from the SM 323 load supply, if local practice dictates).

With this wiring, the PLC bit semantics are inverted:

• To start the pump, set Q0.0 := FALSE. The SM 323 output transistor turns OFF, the Simdos input is recognized as active (below 0.8 V), and the pump runs.
• To stop the pump, set Q0.0 := TRUE. The SM 323 output sources +24 V to the terminal (above 2 V), satisfying the "inactive" condition.

For clarity at the HMI level, expose two intermediate boolean tags in the symbol table:

// Symbol table (STEP 7)
Pump_Run_Command    BOOL  %M100.0   // 1 = operator wants pump to run
Pump_Start_Stop_Q   BOOL  %Q0.0      // Inverted: 0 = run, 1 = stop
Pump_Prime_Q        BOOL  %Q0.1      // Inverted: 0 = prime, 1 = idle
Pump_Running_FB     BOOL  %I0.0      // Inverted feedback from O1
Pump_Running        BOOL  %M100.1   // True while pump is actually running

OB1 logic (Statement Text):

// Drive the Simdos 02 Start/Stop input
// TRUE command -> 0 V at IN1 -> pump runs
// FALSE command -> 24 V at IN1 -> pump stops
      A   Pump_Run_Command
      NOT
      =   Pump_Start_Stop_Q

// Drive the Simdos 02 Prime input
      A   Pump_Prime_Command
      NOT
      =   Pump_Prime_Q

// Invert the O1 feedback
// 0 V at O1 (pump running) -> SM323 input reads 0
// 24 V at O1 (pump stopped) -> SM323 input reads 1
      AN  Pump_Running_FB
      =   Pump_Running

// Latch the running state with a 200 ms debounce
      A   Pump_Running
      L   S5T#200MS
      SE  Pump_Running_Timer
      A   Pump_Running_Timer
      =   Pump_Running_Stable

The 200 ms debounce absorbs the contact-bounce of the Simdos 02 internal relay and the SM 323 input filter. A 200 ms pulse width is also the minimum recommended by KNF for stable edge detection when IN1 is in pulse mode.

Topology Diagram

Figure 1 - Direct wiring between SM 323 and Simdos 02. The 4.7 kΩ pull-up at the O1 terminal ensures the SM 323 input is driven high when the open-collector is OFF.

Wiring: Simdos 02 Open-Collector Output to SM 323 Input

The Simdos 02 O1 (Running) terminal is an NPN open-collector output. Its internal transistor can only pull the terminal to 0 V; it cannot source current. The S7-300 SM 323 expects a sourcing signal (24 V = "1"). The interface between them is therefore a single pull-up resistor.

Recommended wiring:

• Simdos 02 O1 terminal → SM 323 input I0.0.
• Pull-up resistor 4.7 kΩ to 10 kΩ, 0.25 W, from O1 line to the SM 323 24 V supply L+ (terminal 19 or 39 of the SM 323 front connector).
• Simdos 02 0 V (GND) → SM 323 0 V (M, terminal 20 or 40).

The 4.7 kΩ value is the conservative choice for a 24 V supply with 7 mA of input current drawn by the SM 323 input. The 10 kΩ value is acceptable for slower applications; the rise time is limited by the input capacitance of the SM 323 (a few hundred pF) and the 10 kΩ resistor, settling in under 1 ms.

Truth table for the wired interface:

The PLC must invert this bit to recover the intuitive meaning. The block shown in the previous section (AN Pump_Running_FB) handles that inversion in a single rung.

STEP 7 Programming: Logic Inversion in OB1

The ladder logic below implements the full Start/Stop, Prime, and Running feedback chain. It is written for STEP 7 V5.5 but the TIA Portal equivalent in SCL is functionally identical.

Ladder View

Network 1: Start/Stop latch with E-Stop and fault interlock
  --|P|-- Pump_Start_PB        --|/|-- Pump_Stop_PB        --(S)-- Pump_Run_Command
  --|P|-- Pump_Stop_PB         --|/|-- E_Stop_OK
                                 --|/|-- Pump_Fault         --(R)-- Pump_Run_Command

Network 2: Prime output to pump (inverted, momentary)
  --| |-- Pump_Prime_PB         --( )-- Pump_Prime_Command
                                  |   |
                                  +-( T1 )-- 2 s self-hold
  --|/|-- T1.Q                  --(R)-- Pump_Prime_Command

Network 3: Invert Q0.0 and Q0.1 for the pump input stage
  --| |-- Pump_Run_Command      --( )-- Pump_Start_Stop_Q   // to Q0.0
  --| |-- NOT_Pump_Start_Stop_Q --( )-- "Pump_Start_Stop_Q_Inv"
  --| |-- Pump_Prime_Command    --( )-- Pump_Prime_Q         // to Q0.1
  --| |-- NOT_Pump_Prime_Q      --( )-- "Pump_Prime_Q_Inv"

Network 4: Invert feedback and debounce
  --|/|-- I0.0                  --( )-- Pump_Running
  --| |-- Pump_Running          --(SE T#200MS)-- Pump_Running_Deb
  --| |-- Pump_Running_Deb      --( )-- "Pump_Running_Stable"  // to HMI

For TIA Portal users, the equivalent SCL in OB1 reads:

// Start/Stop latch with E-Stop and fault interlock
IF "Pump_Start_PB" AND NOT "Pump_Stop_PB" AND "E_Stop_OK" AND NOT "Pump_Fault" THEN
    "Pump_Run_Command" := TRUE;
END_IF;
IF "Pump_Stop_PB" OR NOT "E_Stop_OK" OR "Pump_Fault" THEN
    "Pump_Run_Command" := FALSE;
END_IF;

// Prime with 2 s self-hold
IF "Pump_Prime_PB" THEN
    "Pump_Prime_Command" := TRUE;
    "Pump_Prime_Timer".TON(IN := TRUE, PT := T#2s);
ELSE
    "Pump_Prime_Command" := FALSE;
    "Pump_Prime_Timer".TON(IN := FALSE, PT := T#2s);
END_IF;

// Inverted output to pump inputs
"Pump_Start_Stop_Q"  := NOT "Pump_Run_Command";   // Q0.0
"Pump_Prime_Q"        := NOT "Pump_Prime_Command"; // Q0.1

// Inverted feedback from pump output
"Pump_Running"        := NOT "I0.0";               // I0.0 raw from SM 323
"Pump_Running_Deb"    := "Pump_Running" AND "Pump_Running_TON".Q;
"Pump_Running_TON".TON(IN := "Pump_Running", PT := T#200ms);

Note the use of separate tags (Pump_Run_Command vs. Pump_Start_Stop_Q) so the HMI and the rest of the program see the intuitive TRUE = "running" semantics, while the physical output and input bits are explicitly inverted at the I/O boundary. This makes the application code readable and keeps the inversion localized.

Hardware Inversion Alternatives

For sites that prefer not to embed logic inversion in software - typically for fail-safe-stop reasons, or to provide galvanic isolation between the PLC and the pump - the same result can be achieved with an interposing relay. The recommended topology is a single-pole normally-closed relay per pump input, with the relay coil driven by an SM 323 output.

Relay Topology

• SM 323 Q0.0 → 24 V relay coil K1 → 0 V return.
• K1 normally-closed contact → Simdos 02 IN1.
• Simdos 02 GND → 0 V common (must be the same as the relay coil 0 V).

Operation:

• When the SM 323 output is OFF (relay de-energized), the NC contact is closed and shorts IN1 to 0 V → the pump sees the "active" condition (below 0.8 V) and runs.
• When the SM 323 output is ON (relay energized), the NC contact opens and the Simdos internal pull-up pulls IN1 up to 24 V → the pump stops.

The fail-safe-stop behavior is the key consideration: if the SM 323 loses power, the relay drops out, the NC contact re-closes, and the pump starts. This is the opposite of the usual fail-safe expectation and must be documented in the HMI and the FDS. If the site requires fail-safe-stop, drive the SM 323 output HIGH in the run state (relay energized) and accept the inverted software semantics, then place a normally-open contact in series with IN1 in addition to the NC contact if double-action safety is required. Confirm the resulting logic with the safety review board before commissioning.

For most lab and pilot dosing applications, the software-only approach is recommended because the SM 323 output current limit (0.5 A) and the Simdos input current (~10 mA) are well within safe operating margins, and the inversion logic is trivial.

Volume Pulse Counting with the FM 350 Counter Module

When the Simdos 02 is configured in pulse mode (IN1 = pulse, the default factory mode), every falling edge at IN1 corresponds to a fixed dispense volume - typically 1 mL per pulse at the factory default calibration. If the application requires tight volume accounting (for example, batching by total volume dispensed, or closed-loop flow verification), the field report correctly notes that an FM 350 counter module may be required because the Simdos pulse rate can exceed the SM 323 input scan rate at high flow rates.

For the Simdos 02 specifically, the pulse rate is well under 1 Hz, so the standard SM 323 DI is fully adequate. The FM 350-1 (6ES7350-1AH03-0AE0) becomes relevant if the integrator scales up to a Simdos 100, or if the calibration is re-tuned to deliver many small pulses per millilitre.

Wiring to the FM 350-1 is similar to the SM 323 case, but the FM 350-1 inputs are differential and accept 24 V sourcing signals directly, with a maximum count frequency of 500 kHz. The pull-up resistor used for the SM 323 input is not required for the FM 350-1 - the FM 350-1 has internal pull-ups and a 24 V threshold per channel. Configure the FM 350-1 channel in HW Config as "Count continuously" or "Count once" depending on whether the application requires a running total or a batched count.

For applications that only need a "Running" feedback (not volume counting), the SM 323 input wiring shown in the previous section is correct and complete; do not introduce the FM 350 unless a volume total is genuinely required.

Commissioning and Verification Procedure

Once the wiring and the program are complete, work through the following checklist before placing the pump into service. The procedure assumes a STEP 7 V5.5 environment; TIA Portal is functionally equivalent.

1. Power up the rack. With the CPU in STOP, energize the 24 V supply to the SM 323 and to the Simdos 02. Verify that the SF (system fault) LED on the SM 323 is OFF. If it is ON, open HW Config and confirm the slot assignment matches the physical module.
2. Verify polarity with a multimeter. Measure the voltage at the Simdos 02 IN1 terminal with the SM 323 output OFF. You should see less than 0.8 V DC (the SM 323 output transistor is OFF and the input is being pulled low through the Simdos input stage). Then force Q0.0 in the variable table and measure again - you should see approximately +24 V DC. If the voltages are inverted, re-check the GND bonding.
3. Test the prime function. From the variable table, force Q0.1 OFF (which the program inverts to drive IN2 low). The pump should enter prime mode. Verify that the prime cycle can be interrupted by forcing Q0.1 ON. If the pump does not respond, re-check the IN2 wiring polarity.
4. Test the Running feedback. Force Q0.0 ON (the program will invert it to drive IN1 high and stop the pump). The I0.0 raw bit should read 1 (high). Then force Q0.0 OFF; the I0.0 raw bit should read 0 (low) and the Pump_Running tag should evaluate to TRUE. If I0.0 floats or flickers, re-check the pull-up resistor on the O1 line.
5. Test the Start/Stop from the HMI. Click Start on the HMI. Pump_Run_Command should latch TRUE, Q0.0 should go FALSE, the pump should start, and Pump_Running should go TRUE within ~200 ms. Click Stop; Pump_Run_Command should drop, Q0.0 should go TRUE, and the pump should stop within ~200 ms.
6. Test the E-Stop and fault interlock. Activate the E-Stop. Pump_Run_Command should drop immediately, the pump should stop, and the HMI should display the E-Stop banner. Reset the E-Stop and clear the fault before attempting to start again.
7. Test the volume calibration (if used). With the Simdos in pulse mode, dispense 100 mL into a graduated cylinder and verify the FM 350-1 count matches the calibration value. Adjust the calibration constant in the pump menu if the discrepancy exceeds ±2 %.
8. Document the inverted logic. Add a comment to every inverted bit in the program, and add a notice to the operator HMI screen explaining that the pump runs when its Start/Stop LED is OFF (a counterintuitive indication if the integrator is used to seeing the LED come on when the pump is running).

If all eight steps pass, the pump is ready for production use. If any step fails, refer to the troubleshooting matrix in the next section.

Troubleshooting Matrix

Frequently Asked Questions