Configuring S7-1200 Analog Inputs for pH and TDS Sensors

Water Quality Monitoring Architecture on S7-1200

A bench-top or skid-mount water quality station typically combines three transducers: a pH probe, a Total Dissolved Solids (TDS) probe, and a temperature sensor for compensation. Each transducer emits a low-level analog voltage that must be conditioned, scaled, and (for pH) linearized before it is usable. The Siemens S7-1200 CPU 1215C is a strong fit for this application because it integrates the CPU, 14 discrete inputs, 10 relay outputs, and 2 analog inputs in a single 110 mm wide package. The AC/DC/Relay variant used in the project is SIMATIC S7-1200 Programmable Controller System Manual (entry ID 109972930) device 6ES7215-1BG40-0XB0 (firmware 4.6 or later recommended for current TIA Portal V18+ projects).

The two on-board analog inputs of the CPU 1215C accept 0–10 V unipolar signals at a 10-bit resolution. Because the DFRobot SEN0161 pH probe outputs 0–3 V and a typical TDS probe outputs 0–2.3 V, both are well within the 0–10 V common-mode window — so there is no electrical incompatibility. The challenge is not wiring or input range, but usable resolution. A 10-bit converter spread across the full 0–10 V span can only resolve the lowest 30 % of the range to roughly 300 discrete values, which limits pH resolution to about 0.046 pH per LSB. If the design target is ±0.1 pH accuracy at 25 °C, the on-board AI is borderline. Adding a Signal Board SB 1231 (6ES7231-4HA30-0XB0) with a 12-bit ADC and selectable ±2.5 V range will quadruple the resolution of the pH channel and is the recommended upgrade path for the project described here.

pH and TDS Sensor Output Characteristics

The DFRobot SEN0161 / SEN0161-V2 is a combination electrode pre-amplified to a 0–3.0 V single-ended output. The midpoint of the probe output corresponds to pH 7 (neutral) and is trimmed to 2.50 V ± 0.05 V at the factory. The voltage-to-pH relationship is governed by the Nernst equation and approximates 59.16 mV per pH unit at 25 °C. Once the probe is referenced to 2.5 V at pH 7, the relationship is linear over the calibrated range:

Working backwards to recover pH from voltage:

At the extremes of the 0–3 V output window the probe nominally covers pH 0 to pH 14, although the practical useful range is pH 2 to pH 12 because the amplifier saturates near the rails. Quoted accuracy for the SEN0161-V2 is ±0.1 pH at 25 °C with a 2-minute response time to 95 %. The connector is a BNC on the probe side and a 3-pin PH2.0 on the pre-amplifier output (Signal, V+, GND). A complete datasheet is hosted at the DFRobot SEN0161-V2 product wiki.

The TDS probe (DFRobot SEN0244 / Gravity analog TDS) outputs 0–2.3 V corresponding to 0–1000 ppm nominally. The output is non-linear in raw millivolt form, but the manufacturer calibration curve is sufficiently linear over 0–1000 ppm that a two-point calibration is acceptable for monitoring. A common pinout is the same 3-wire PH2.0 (Signal, V+, GND).

S7-1200 1215C On-board Analog Input Specifications

The S7-1200 1215C ships with two single-ended analog inputs on the bottom terminal block (pins 2, 3, 4, 5). Each input can be software-configured per channel to one of the following ranges in the TIA Portal device configuration:

• 0 – 10 V (unipolar, default)
• 0 – 20 mA (when wired to current source through 50 Ω internal burden)

Resolution of the on-board AI is 10 bits. The 10-bit raw value is mapped linearly to the S7-1200 integer range 0 – 27648, where 0 corresponds to 0 V and 27648 corresponds to 10 V. The S7-1200 System Manual entry ID 109972930 documents the analog input wiring and configuration in chapter 6. The 0 – 27648 convention is consistent with every Siemens S7 analog input since the S7-200 era and is what enables drop-in scaling blocks such as NORM_X and SCALE_X to be used identically across signal boards, signal modules, and CPUs.

Because the probe signals are 0–3 V and 0–2.3 V, they only ever consume 30 % and 23 % of the 0–10 V input window. Two consequences follow:

1. Wasted LSBs. A 0–3 V pH signal spans only 0 – 8288 counts (0 – 3 × 27648 / 10). The remaining 19360 counts (3 – 10 V) are never used.
2. Coarse resolution. The 10-bit ADC has 1024 LSBs total, but only ~307 LSBs are useful in the 0–3 V range. This is the resolution penalty that the discussion contributors flagged.

Resolution Penalty: 10-bit ADC vs 12-bit SB 1231

The 10-bit resolution of the on-board AI is the dominant source of error in this project. The table below quantifies the penalty for the chosen sensors.

A 0.046 pH per LSB resolution is below the SEN0161's own ±0.1 pH accuracy spec, but barely. A 4 ppm per LSB TDS resolution will be visible as quantisation noise in the trend. If a stable 0.01 pH or 1 ppm display is required, the on-board AI is not the right tool.

Adding an SB 1231 (1 AI, voltage, 12-bit + sign) configured for the ±2.5 V range improves this dramatically. The 12-bit ADC produces 4096 LSBs over ±2.5 V (2.5 V half-scale = 2048 LSBs unipolar-equivalent). In practice, the SB 1231 raw value range for a 0 – 2.5 V unipolar source maps to 0 – 27648 (with the negative half of the bipolar range unused). Effective LSB size:

The improvement factor on the 0–3 V pH signal is approximately 16× when the SB 1231 is configured for ±2.5 V versus using the on-board AI. Caveat: the ±2.5 V range clips a 0–3 V signal at 2.5 V, so a voltage divider or a different probe with a 0–2.5 V output is required. The cleanest solution is the SEN0169 (0–2.5 V pH probe) or a 2:3 resistor divider at the SB 1231 input.

I/O Expansion: SM 1231 vs SB 1231 Selection Matrix

The S7-1200 supports one signal board (plugged onto the CPU front face) and up to eight signal modules (on the right-hand expansion bus). For a pH + TDS station the IO count is small, so a single SB 1231 + the two on-board AIs covers the build with no extra DIN-rail width. The table below maps the common expansions.

For this project the recommended configuration is: on-board AI0 → TDS sensor, on-board AI1 → spare or temperature (PT100 via RTD module), and SB 1231 on the CPU front → pH sensor (configured for ±2.5 V range with a 5:6 voltage divider at the input). If a divider is undesirable, use the SM 1231 AI4 and configure the pH channel to ±5 V to take the 0–3 V signal directly without a divider.

Sensor Signal Conditioning and Wiring

Wiring the SEN0161-V2 pH probe and the Gravity TDS probe to the S7-1200 1215C requires only the three PH2.0 wires per probe plus a 0 V reference tied to the PLC analog common (terminal M). The S7-1200 on-board AI requires a single-ended 0 V referenced signal, so the probe GND must return to the same M terminal on the CPU that the AI negative pin returns to. Mixing the analog ground with a digital 24 V return will inject switching noise into the millivolt-level pH signal.

Pin-by-pin wiring table for the on-board AI:

Scaling Math: Converting Raw Counts to Engineering Units

The S7-1200 on-board AI returns a raw integer in the range 0 – 27648. The project goal is to convert that integer to either a pH value or a ppm value. The formula is the same one that the project discussion used:

For the pH sensor on the on-board AI (0 – 10 V input range, sensor spans 0 – 3 V):

Voltage_at_AI0  = (Raw_Count / 27648) × 10 V
pH_Raw_Span      = (3.0 V / 10 V) × 27648 = 8288.4 counts
pH               = 7 + (2.50 − Voltage_at_AI0) / 0.05916

The project discussion correctly calculated 0 – 8313 as the count span for 0 – 3 V. The remainder of the AI range is unused, and any value above 8288 must be flagged as an overrange condition rather than treated as additional measurement range. The on-board AI does not throw an overrange diagnostic; the application code must check the raw value against 8288 and alarm accordingly.

For the TDS sensor on the on-board AI (sensor spans 0 – 2.3 V):

TDS_Raw_Span = (2.3 V / 10 V) × 27648 = 6359 counts
TDS_ppm       = (Raw_Count / 27648) × 10 V × (1000 ppm / 2.3 V)
             = (Raw_Count / 27648) × 4347.83 ppm

If an SB 1231 is used for the pH channel in the ±2.5 V range, the SB 1231 raw value range is 0 – 27648 over 0 – 2.5 V (i.e. the unused negative half of the bipolar range is not signal-relevant). A 5:6 divider at the input brings 0 – 3 V down to 0 – 2.5 V, and the SB 1231 returns 0 – 27648 over the full 0 – 3 V probe output. The S7-1200 then scales:

pH = 7 + (2.50 × 27648/27648 − Raw_Count × 2.5/27648) / 0.05916
   = 7 + (2.50 − Raw_Count × 9.04e-5) / 0.05916

TIA Portal Hardware Configuration

Configure the S7-1200 in TIA Portal V18 or later as follows.

1. Add the CPU 1215C AC/DC/Rly (6ES7215-1BG40-0XB0) to the project. TIA Portal will auto-populate the device configuration with the on-board AI0 and AI1 at 0 – 10 V default.
2. In Properties → Analog inputs, set AI0 to 0 – 10 V and enable the diagnostics for broken-wire / overrange if desired. AI1 is configured the same way if used.
3. If the SB 1231 is fitted, drag the Signal Board → AI1 × 12-bit (catalog 6ES7231-4HA30-0XB0) from the hardware catalog onto the CPU's signal board slot. Set its range to ±2.5 V.
4. Set the analog input smoothing to None for trend recording, or to Weak / Medium / Strong if the sensor wiring is exposed to 50/60 Hz noise. Smoothing reduces the effective update rate.
5. Compile the hardware configuration and download to the CPU.

PLC Program: NORM_X, SCALE_X, and pH Linearization

Use the NORM_X and SCALE_X instructions (block numbers FC106 / FC105 in legacy S7-200/300 style, or the standard TIA Portal NORM_X / SCALE_X boxes) to convert the raw count into the engineering range. A compact SCL (Structured Control Language) FB that runs each scan is the cleanest approach for pH because the pH conversion is non-linear only when the Nernst slope is used directly — once the pH is reduced to a linear voltage, the SCALE_X is straightforward.

Data block DB10 (pH_Sensor_Data):

Raw_Count        : INT    // IW96 from SB 1231, or IW64 from on-board AI1
V_Span_Min       : REAL = 0.0      // Volts
V_Span_Max       : REAL = 3.0      // Volts
Counts_Min       : INT  = 0
Counts_Max       : INT  = 8288    // For 0–3 V on 0–10 V AI
pH_Midpoint_V    : REAL = 2.50
Nernst_Slope_V   : REAL = 0.05916
pH_OUT           : REAL
V_OUT            : REAL

SCL code in OB1:

// 1. Normalize the raw count to 0.0 – 1.0
V_OUT := (INT_TO_REAL(DB10.Raw_Count) / 27648.0) * 10.0;

// 2. Clip to the sensor window
IF V_OUT > DB10.V_Span_Max THEN V_OUT := DB10.V_Span_Max; END_IF;
IF V_OUT < DB10.V_Span_Min THEN V_OUT := DB10.V_Span_Min; END_IF;

// 3. Convert voltage to pH using the calibrated midpoint and Nernst slope
DB10.pH_OUT := 7.0 + (DB10.pH_Midpoint_V − V_OUT) / DB10.Nernst_Slope_V;

// 4. Overrange flag — should never exceed 3.0 V on a healthy probe
DB10.pH_Overrange := (INT_TO_REAL(DB10.Raw_Count) > (DB10.Counts_Max + 100));
DB10.pH_Underrange:= (INT_TO_REAL(DB10.Raw_Count) < 0);

For TDS on the on-board AI0, the SCALE_X block can be used directly because the TDS probe is factory-calibrated linear. A typical TIA Portal ladder rung is:

SCALE_X (EN := TRUE,
          MIN  := 0,
          MAX  := 6359,        // 0 – 2.3 V in counts
          VALUE:= IW64,        // AI0 raw count
          OUT  := "DB11".TDS_ppm);
// OUT is REAL scaled to (MIN/MAX) over (27648/27648) — i.e. 0 to 1000 ppm by default
// Adjust MIN/MAX to the calibrated span

The NORM_X block is the inverse direction. Use it when you have a percentage output (0.0 – 1.0) and want to push it back into the PLC's 0 – 27648 integer range, e.g. for a control-loop output. The TDS measurement does not need NORM_X; only SCALE_X.

Calibration Procedure for pH Probes

All Nernst-based pH probes drift with age, temperature, and reference-junction contamination. The PLC is the right place to put a calibration routine because it can store buffer pH values, capture the probe voltage at each buffer, and compute the slope and offset constants used in the linearization. A standard two-point calibration uses pH 4.01 and pH 6.86 (or pH 7.00) reference buffer solutions.

1. Rinse the probe in deionized water and blot — do not wipe, as wiping introduces static charge that biases the reference junction.
2. Place the probe in pH 6.86 buffer at 25 °C. Wait 2 minutes for thermal and chemical equilibrium. Read the on-board AI raw count and store it as V_pH686_raw.
3. Rinse and blot. Place the probe in pH 4.01 buffer at 25 °C. Wait 2 minutes. Read the raw count and store it as V_pH401_raw.
4. Compute the actual slope at the current temperature:r>

   Slope_V_per_pH = (V_pH686_V − V_pH401_V) / (6.86 − 4.01)
      = (V_pH686_V − V_pH401_V) / 2.85

5. Compute the offset (midpoint voltage):r>

   V_mid = V_pH401_V + Slope_V_per_pH × (7.00 − 4.01)
         = V_pH401_V + Slope_V_per_pH × 2.99

6. Persist Slope_V_per_pH and V_mid into the data block used by the linearization in OB1. The slope should land within 5 % of 0.05916 V/pH at 25 °C. If it deviates by more than 15 %, replace the probe.

TDS Conversion Formula (Voltage to ppm)

The factory calibration of the Gravity analog TDS sensor maps 0 – 2.3 V to 0 – 1000 ppm with a stated ±10 % FS accuracy. The probe is a conductive cell, so the output is strongly temperature-dependent. For higher accuracy, add a 2-point calibration against a 500 ppm and 1000 ppm standard solution and store the slope in the PLC:

Where V_OFFSET is typically 0 V (the sensor is biased to 0 V at 0 ppm) and SLOPE_PPM_PER_VOLT is computed from the two-point calibration. If the application allows factory accuracy, a single-point gain of 1000 / 2.3 = 434.78 ppm/V is sufficient.

Field Troubleshooting Matrix

Commissioning Verification Checklist

Before declaring the station operational, validate the chain end-to-end with a known reference:

1. Apply a precision voltage source (or a calibrated buffer) to the AI input and verify the PLC reading matches 0.1 % of the configured engineering range.
2. Place the pH probe in pH 6.86 buffer; the PLC display should read 6.86 ± 0.10.
3. Place the pH probe in pH 4.01 buffer; the PLC display should read 4.01 ± 0.10.
4. Verify the SB 1231 (if fitted) overrange flag clears when the probe is removed (raw count → 0).
5. Confirm the HMI / Webserver tag for pH and TDS is updating once per second (or per the configured update rate).
6. Force a known raw count into the input (simulate) and verify the scaled pH output matches the calculation by hand.

Frequently Asked Questions