Overview
The Siemens SIMATIC S7-200 EM235 analog expansion module is a mixed I/O block that provides four configurable analog inputs (one channel dedicated to voltage, three channels configurable for voltage or current depending on the wiring scheme) and one analog output. The module is commonly deployed with the CPU 212, CPU 214, CPU 215, CPU 216, and the CPU 22x series. When used for temperature acquisition, a bare resistance temperature detector (RTD) such as a PT100 cannot be wired directly to the EM235 because the module only accepts standardized voltage or current signals. The RTD output must first be conditioned by a transmitter that converts resistance to a 0–10 V, 0–20 mA, or 4–20 mA signal.
This article walks through the field-proven wiring of a three-wire PT100 sensor, a SITRANS TH100 two-wire head-mounted transmitter, and the EM235 analog input channel. It also covers DIP switch configuration, the analog output (I0/V0) terminal function, and STEP 7-Micro/WIN scaling.
EM235 Hardware Specifications
The EM235 is documented in the S7-200 Programmable Controller System Manual as Appendix A reference Figure A-14. The headline characteristics are:
| Parameter | Specification |
|---|---|
| Analog inputs | 4 single-ended (A, B, C, D) |
| Analog output | 1 (voltage or current, shared terminals) |
| Input resolution | 12 bits plus sign (effective 11-bit plus sign over most ranges) |
| Input ranges (unipolar) | 0–10 V, 0–5 V, 0–1 V, 0–20 mA, 4–20 mA |
| Input ranges (bipolar) | ±10 V, ±5 V, ±2.5 V, ±1 V, ±500 mV |
| Output ranges | 0–10 V or 0–20 mA (jumper-selected, software-scaled) |
| Module update time | Approx. 0.5 ms per enabled channel |
| Maximum input voltage | 30 V DC continuous; 24 V DC continuous on current inputs |
| 24 V DC supply current | 60 mA (from sensor supply or backplane) |
| Isolation | 500 V AC between field side and logic side |
Channel Function Reference (Figure A-14)
The reference figure in Appendix A of the S7-200 manual illustrates the four legal input wiring modes. Each channel is wired differently depending on the sensor type:
| Channel | Wiring Mode | Typical Use |
|---|---|---|
| A (A+, A−) | Differential voltage, 0–10 V | Direct voltage output sensor (active device) |
| B (B+, B−) | Voltage input, not used | If channel is disabled, short B+ to B− to avoid floating input |
| C (C+, C−) | 0–20 mA current loop, active source | Active 0–20 mA transmitter (sensor powers the loop) |
| D (D+, D−) | 4–20 mA current loop, passive sink | Passive 4–20 mA two-wire transmitter (loop powered from EM235) |
The terminal block also exposes M (common return), L+ and M (24 V DC user supply for field devices), and the analog output pair I0 / V0 / M0. I0 is the current output terminal and V0 is the voltage output terminal; only one is used at a time and is selected in STEP 7-Micro/WIN through the analog output configuration word.
Selecting a Temperature Sensor and Transmitter
A PT100 RTD changes resistance with temperature, nominally 100 Ω at 0 °C and approximately 138.5 Ω at 100 °C (3850 ppm/K DIN curve). The EM235 cannot read resistance directly. A transmitter is required to convert the PT100 resistance into a standard process signal.
For most water temperature applications, a 4–20 mA two-wire (loop-powered) transmitter is the field-proven choice because:
- The 4–20 mA signal is immune to lead resistance, so long cable runs do not introduce measurement error.
- 4 mA live-zero permits broken-wire detection (a 0 mA reading is unambiguously a fault).
- Loop-powered devices draw operating current from the same two wires that carry the signal, simplifying the wiring diagram.
The SITRANS TH100 from Siemens is a two-wire head-mounted Pt100/Pt1000 transmitter with a 4–20 mA output. It is specified for 12–35 V DC loop supply, has PC-configurable ranges via the SIPROM T software and a special interface, and supports three-wire and four-wire RTD connections. The factory default range is 0–100 °C = 4–20 mA unless the user has re-ranged it.
SITRANS TH100 Terminal Layout
| TH100 Terminal | Function | Connection |
|---|---|---|
| 1 (+) | Loop supply positive / 4–20 mA output positive | Goes to EM235 D+ |
| 2 (−) | Loop supply negative / 4–20 mA output return | Goes to EM235 D− |
| 3 | PT100 lead 1 (excitation +) | PT100 red lead (one end) |
| 4 | PT100 lead 2 (sense +) | PT100 red lead (other end on three-wire) |
| 5 | PT100 common (sense − / excitation −) | PT100 white lead (third wire) |
For a three-wire PT100, the two red leads of the same color connect to terminals 3 and 4; the third (white) lead connects to terminal 5. The transmitter performs lead-resistance compensation on the third wire.
EM235 DIP Switch Configuration
The EM235 has a 6-position DIP switch bank on the bottom of the module. Switch 1, 2, and 3 select the input range; switches 4, 5, and 6 are unused on the EM235 (the bipolar/unipolar mode is automatic). For 4–20 mA input on channel D, set switches 1–3 to OFF–ON–OFF (0 = OFF, 1 = ON, read left to right looking at the switch with the bank at the bottom):
| SW1 | SW2 | SW3 | Selected Range |
|---|---|---|---|
| ON | OFF | OFF | 0–5 V |
| OFF | ON | OFF | 0–20 mA (unipolar current) |
| OFF | ON | ON | 4–20 mA (unipolar current, live zero) |
| OFF | OFF | ON | ±5 V |
| ON | OFF | ON | ±2.5 V |
Power Supply and Loop Wiring
The EM235 exposes a 24 V DC sensor supply on terminals L+ and M. This supply is intended for sourcing the analog input current loops, not for powering the SITRANS TH100 alone (the TH100 needs at least 12 V across terminals 1 and 2 to operate, and the EM235 internal drop plus the sense resistor must leave at least 12 V at the transmitter).
The recommended topology is:
- Connect a clean 24 V DC supply (separate from any VFD bus) to the EM235 L+ and M terminals.
- From L+ and M, run a two-wire shielded cable to the SITRANS TH100 head transmitter mounted on the PT100 thermowell.
- From the TH100 terminals 1 (+) and 2 (−), run the 4–20 mA loop back to EM235 channel D terminals D+ and D−.
- Connect the cable shield at the EM235 end only, to the M terminal or to a dedicated ground bar bonded to the panel ground.
For multiple PT100 channels, repeat the loop with channels C, B, A as needed. Never mix voltage and current wiring on the same channel — the internal termination resistor is hardwired and a voltage source applied to a current-configured channel will be current-limited and read incorrectly.
Step-by-Step Connection Procedure
- Power down the S7-200 CPU and the 24 V DC sensor supply. Verify zero energy with a meter at the EM235 L+ and M terminals.
- Set the DIP switches on the EM235 to OFF–ON–ON (SW1=0, SW2=1, SW3=1) for 4–20 mA input on all current-capable channels. For a single PT100 on channel D only, the unused channels (A, B, C) should be disabled in STEP 7-Micro/WIN to reduce scan time.
- Wire the PT100 to the TH100. For three-wire PT100: red lead 1 to terminal 3, red lead 2 to terminal 4, white lead to terminal 5. Use a crimp tool calibrated for the terminal block; cold joints on RTD leads are a leading cause of drift.
- Power the TH100 loop. Connect 24 V DC positive to TH100 terminal 1, negative to TH100 terminal 2. Use a separate 24 V tap if the EM235 sensor supply is already heavily loaded.
- Bring the 4–20 mA signal back to the EM235. Connect TH100 terminal 1 (+) also to EM235 D+; TH100 terminal 2 (−) to EM235 D−. If using a four-wire RTD or an actively sourced transmitter, move the signal pair to channel C (0–20 mA) and follow Figure A-14 wiring C in the manual.
- Connect unused channels. Tie B+ to B− with a short jumper wire to prevent a floating input from producing random ADC codes.
- Apply 24 V to the EM235 and verify the SF (system fault) LED is off. The SF LED on the EM235 indicates loss of user-supply 24 V or a DIP switch / configuration mismatch.
- Apply CPU power and observe the EXTF LED on the CPU. If EXTF lights with no I/O connected, the EM235 is reporting a power or wiring fault; see the Siemens SiePortal knowledge base entry on EM235 not working for the canonical fault tree.
STEP 7-Micro/WIN Configuration
After wiring, configure the analog channel in STEP 7-Micro/WIN:
- Open the project and select System Block > Analog Input Filter. Set the filter to a value appropriate for the thermal time constant of the thermowell (typically 50–100 samples for water temperature).
- Enable only the channels physically wired. Disable A, B, and C if only D is used.
- For each enabled channel, set the range to 4–20 mA from the dropdown; the dropdown is keyed to the DIP switch selection, and a mismatch triggers a configuration error at run time.
- The EM235 maps 4 mA to a digital value of 0 and 20 mA to 32000 in unipolar current mode. Use the scaling formula:
Temperature °C = (AIW_value / 32000) × (Span_°C) + Min_°C - For the TH100 default range of 0–100 °C = 4–20 mA, AIW = 0 means 0 °C and AIW = 32000 means 100 °C. Convert with:
Temp_C = (AIWx * 100) / 32000
Verification
After the system is powered, perform the following checks before trusting the reading:
- Zero check: Disconnect the PT100 from the TH100 and substitute a precision 100 Ω resistor on terminals 3 and 4 (with terminal 5 left open or connected to either side for three-wire simulation). The PLC should read 0 °C ±0.5 °C.
- Span check: Substitute 138.5 Ω (Pt100 at 100 °C). The PLC should read 100 °C ±1 °C. If the TH100 has been re-ranged, substitute the equivalent resistance for the upper range point.
- Loop integrity: With a clamp meter on the loop wire, verify 4 mA at the lower range temperature and 20 mA at the upper range temperature. Any reading outside 3.8–20.5 mA indicates a wiring or supply problem.
- Noise check: Monitor AIW for at least 10 minutes. Water bath temperature should be stable to within ±2 counts at 12-bit resolution; greater wander indicates shield grounding issues or a noisy 24 V supply.
- Open RTD detection: Disconnect one PT100 lead. The TH100 should drive the loop to > 21 mA (over-range) and the PLC should see AIW saturate near 32767. If AIW reads 0, the TH100 is in fallback mode and the broken-wire alarm is not working.
Troubleshooting Matrix
| Symptom | Likely Cause | Corrective Action |
|---|---|---|
| EXTF LED lit on CPU at first power-up, no wiring connected | EM235 missing 24 V sensor supply | Apply 24 V to L+ and M; verify polarity |
| AIW reads 0 constantly | Channel wired as voltage but transmitter is current | Check DIP switch position matches wiring mode |
| AIW reads 32767 (saturation) | Loop open, transmitter unpowered, or RTD open | Measure loop current; check TH100 supply voltage > 12 V |
| Reading drifts with cable movement | Loose terminal screw on RTD leads | Re-torque; replace ferrule if deformed |
| Reading always 100 °C or full scale | TH100 PT100 terminals 3 and 4 reversed | Swap the two same-color red leads |
| Reading exactly 25 °C offset | TH100 still in factory Pt1000 mode or wrong range configured | Reconfigure TH100 with SIPROM T to 0–100 °C, Pt100 input |
| Channel A reads garbage, channel D reads correctly | Channel A is voltage-only; current source on A damages sense resistor | Re-wire to D; inspect A input for damage |
| SF LED on EM235 lit, all wiring correct | Configuration mismatch between DIP switch and STEP 7 range | Reconcile switch position with software configuration |
Frequently Asked Questions
Can I wire a bare PT100 directly to the EM235 without a transmitter?
No. The EM235 only accepts 0–10 V, 0–20 mA, or 4–20 mA signals. A PT100 produces a resistance change (100 Ω at 0 °C, ~138.5 Ω at 100 °C) that must be linearized and converted to a standard process signal by a transmitter such as the SITRANS TH100.
What is the correct EM235 DIP switch setting for 4–20 mA input?
For the 6ES7 235-0KD22-0XA0 and later, set SW1=OFF, SW2=ON, SW3=ON. The corresponding STEP 7-Micro/WIN range must also be set to 4–20 mA; otherwise the SF LED lights and the channel reports a configuration error.
Why does my EM235 read 32767 (positive full scale) right after startup?
An AIW value of 32767 in STEP 7-Micro/WIN indicates the input is above the configured range or the loop is open. Check that the SITRANS TH100 has at least 12 V across its terminals, that the loop wiring polarity is correct, and that the PT100 leads are properly seated in terminals 3, 4, and 5.
Can I use one EM235 with four separate PT100 transmitters?
Yes, but the EM235 has only one dedicated current channel (D) per the canonical Figure A-14 wiring. The other channels can be configured for current mode through the DIP switch, but consult the specific module revision's manual to confirm which channels support 4–20 mA and the maximum total loop current the 24 V sensor supply can source.
What does the I0 / V0 terminal on the EM235 do?
I0 and V0 are the analog output terminals (with M0 as common). I0 sources 0–20 mA and V0 sources 0–10 V; only one is active at a time and is selected in STEP 7-Micro/WIN. They are not used for temperature input from the SITRANS TH100 and should be left floating if unused, or wired to a chart recorder or VFD reference as needed.
How do I convert the AIW value to degrees Celsius in STEP 7-Micro/WIN?
For 4–20 mA mapped to 0–32000 with a SITRANS TH100 ranged 0–100 °C, the formula is Temp_C = (AIWx × 100) / 32000. Use a 32-bit integer divide to preserve resolution: Temp_C = (AIWx × 25) / 8000 produces identical results without overflow on a 16-bit word.