Configuring Siemens LOGO! 230RC with DM8 and AM2 PT100 Modules

1. System Overview

A single-phase control island built on the Siemens LOGO! 230RC logic module can deliver 8 × 230 V AC digital inputs, 4 × relay outputs, and two expansion slots of analog I/O without any external relay interposing. For an application that requires 10 AC digital inputs, 3 PT100 RTD inputs, and 4 relay outputs, the field-proven stack topology is:

LOGO! 230RC (base, slot 0)  ->  DM8 230R (slot 1)  ->  AM2 PT100 (slot 2)  ->  AM2 PT100 (slot 3)

This stack delivers the required I/O with one slot of headroom on the analog side and four unused relay outputs on the DM8 for future expansion. The 230RC base module alone already provides all four 10 A relay outputs needed, so the DM8 outputs can be reserved as spare or for auxiliary functions (alarms, lamps, auxiliary contactors).

2. Module Stack Architecture and I/O Map

Addressing in LOGO! Soft Comfort follows the slot position. Inputs I1..I8 belong to the 230RC, I9..I16 belong to the DM8, and AI1..AI2 plus AI3..AI4 belong to the two AM2 PT100 modules (channel count is module-relative in LOGO!). Outputs Q1..Q4 belong to the base, Q5..Q8 belong to the DM8.

2.1 Channel Assignment Example

The DM8 outputs Q5..Q8 and the unused RTD channel AI4 remain available for later scope creep.

3. AC Input Phase Isolation: The Critical Constraint

This is the single most overlooked rule in 230 V AC LOGO! wiring. The 230RC base module splits its eight digital inputs into two electrically isolated groups of four:

• Group A: I1, I2, I3, I4 (terminals share one internal common)
• Group B: I5, I6, I7, I8 (terminals share a separate internal common)

The two groups are separated on the terminal block by a physical gap — a missing terminal screw — that visually marks the isolation boundary. The reason is galvanic isolation: if you wire inputs from different phases (e.g., L1 and L2) into the same group, the internal commons are shorted together and the inputs cross-couple through the supply.

3.1 Rules by Module Family

3.2 Practical Wiring Strategies

Strategy 1 — single-phase supply (typical). If all digital inputs are fed from L1, no group rules apply. All four groups on the 230RC plus the DM8 can use L1 directly. This is the simplest layout and matches the application where the pushbuttons, switches, and sensors are all on the same lighting or control circuit.

Strategy 2 — mixed-phase supply (three-phase system). If signals come from different phases (e.g., motor contactor status feedback from L1 and L2), assign each input to the correct group. For example, I1..I4 on L1 and I5..I8 on L2. The DM8 cannot be mixed — if you need additional 230 V inputs on L2, use a DM16 (which has two groups) rather than a DM8.

Strategy 3 — reserve groups. If a single-phase UPS feeds the LOGO! power supply and the I/O distribution is single-phase, the isolation rules collapse into a single-phase sanity check: confirm the L wire feeding I1..I4 and I5..I8 is the same conductor all the way back to the breaker.

4. Digital Output Relay Ratings and Derating

The 230RC and DM8 230R use electromechanical relay outputs, not transistors. Two rating numbers must be respected: the published resistive rating and the much smaller inductive rating.

The 10 A and 5 A ratings on the LOGO! relays apply to resistive loads such as heating elements or incandescent lamps. For inductive loads (contactor coils, solenoid valves, motor windings), the inrush and stored energy reduce the permissible continuous current substantially — typically to AC-15 / DC-13 values published in the device manual.

4.1 Output Isolation

Relay contacts on the 230RC and DM8 are potential-free — each contact has its own pair of terminals and is electrically isolated from the LOGO! internal circuitry. This means different phases can be wired across different outputs on the same module. Q1 can switch L1, Q2 can switch L2, Q3 can switch L3, and so on, without cross-coupling. The phase-isolation rule that applies to inputs does not apply to outputs.

4.2 Practical Recommendation

For motor loads or large solenoid banks, drive a LOGO! Contact 230 contactor from a LOGO! relay output. The contactor coil draws < 50 mA at 230 V AC and is well within the LOGO! rating, while the contactor's main contacts handle the 4 kW load directly. This also adds galvanic isolation between the LOGO! and the field, reducing surge coupling back into the logic supply.

5. AM2 PT100 Analog Input Configuration

The AM2 PT100 expansion provides two 3-wire PT100 RTD inputs per module. The two modules together provide four channels — one more than the application requires. For a 3-sensor application, AI1, AI2, and AI3 are wired to PT100 probes; AI4 is left open.

5.1 Wiring

For each PT100 sensor, run three conductors to the AM2 terminals: two in one leg (to compensate for lead resistance) and one in the other. Cable length and gauge matter: long runs introduce lead-resistance error that is only partially compensated by the 3-wire scheme.

5.2 Programming the Analog Inputs

In LOGO! Soft Comfort, the analog input block exposes the value as a scaled integer (in tenths of °C, or in raw ohms). Use the Analog Threshold Trigger or Analog Comparator blocks to alarm on overtemperature. A common configuration is two thresholds: a warning at 80 °C and a trip at 95 °C for class B insulation motors.

6. Power Supply Topology with UPS

For process-critical applications where the LOGO! must ride through mains dips and brief outages, a small single-phase UPS feeds the LOGO! power supply terminals (L, N). The UPS should be sized for the LOGO! base plus expansion steady-state draw plus the relay coil current.

6.1 Power Budget

A 100 VA online UPS (e.g., a SITOP or generic 230 V AC line-interactive unit) provides roughly 10–15 minutes of ride-through at this load — sufficient for most process-grade applications.

6.2 Earthing and PE

Wire PE from the UPS output ground to the LOGO! PE terminal (where provided on the base module). The PE bond is required for EMC and surge immunity, even if no field device draws PE current. For the DM8, no separate PE terminal is required.

7. Field Wiring Topology

The diagram below shows a representative wiring topology with the 230RC base, DM8 expansion, two AM2 PT100 modules, and the LOGO! Contact 230 contactor as the motor-switching element.

8. Programming in LOGO! Soft Comfort

The LOGO! Soft Comfort programming environment is the standard tool for configuring the stack. It allows you to:

1. Detect the connected stack automatically via the Tools -> Detect LOGO! menu.
2. Assign symbolic names (e.g., I1 = START_PB) via the I/O name editor.
3. Configure AM2 analog inputs as PT100 mode and scale to engineering units (°C).
4. Program logic using FBD or LAD — for typical motor control, use a start/stop seal-in latch with an analog comparator trip from the RTD input.
5. Simulate the program before download.
6. Download to the LOGO! via Ethernet (LOGO! 8) or via the LOGO! cable (older 0BA6).

8.1 Sample Ladder Snippet — Seal-in with RTD Trip

Network 1: Seal-in (start permissive)
 |--[ I1 START_PB ]--+--[ I2 STOP_PB_NC ]--+--( )-- Q1 MOTOR_RUN
 |                   |                      |
 |--[ Q1 feedback ]--+                      |

Network 2: Overtemperature trip
 |--[ AI3 > 95 degC ]--+--[/ Q1 ]--  (forces Q1 off on overtemperature)

The analog comparator block AI3 thresholds the PT100 input against a fixed setpoint. If the bearing temperature exceeds 95 °C, the comparator output forces Q1 off regardless of the seal-in latch.

9. Commissioning Verification Checklist

Before energising the field wiring, walk the stack through this checklist:

1. Verify stack address assignment — confirm that I1..I16 and Q1..Q8 map correctly per slot.
2. Verify phase isolation — with a multimeter on ohms, confirm that L of group A (I1..I4) and L of group B (I5..I8) are the same phase if you intended a single-phase layout. If they should be different phases, confirm the wiring maps to the correct groups.
3. Verify input commons — confirm that N terminals of all input groups are tied to the same N bus.
4. Verify relay contact isolation — with the LOGO! unpowered, check that no two relay contact commons are bridged.
5. Verify PT100 wiring — for each sensor, measure resistance at the AM2 terminals with the sensor disconnected. PT100 should read ~ 107–110 Ω at room temperature (20–25 °C) including lead resistance.
6. Verify UPS voltage — confirm 230 V AC ± 10% on L and N at the LOGO! power terminals before power-up.
7. Power up — observe the LOGO! display / LEDs for normal run state.
8. Force each input — toggle each pushbutton or sensor and verify the corresponding I indicator on the LOGO! display changes.
9. Force each output — through LOGO! Soft Comfort online mode, force each Q output and verify the contactor or load responds.
10. RTD sanity check — compare LOGO! Soft Comfort's online AI value to a handheld RTD calibrator reading.

10. Troubleshooting Matrix

11. Related Module Sizing Notes

For applications where the LOGO! base + DM8 + AM2 stack exceeds the available slots or the budget, the following alternatives are field-proven:

• LOGO! 24RC — DC-powered variant with DC inputs (24 V DC sourcing instead of 230 V AC). Useful if the field signals are 24 V DC.
• LOGO! 12/24RCE with LOGO! AM2 (0–10 V analog) instead of AM2 PT100 — for non-RTD analog inputs.
• LOGO! 8 with Ethernet — supports remote I/O via the LOGO! CMR (communication module) for cellular telemetry.

Reference the Siemens LOGO! System Manual on the Siemens Industry Online Support portal for the full module catalog, derating curves, and wiring diagrams applicable to your specific firmware version.

12. Safety and Standards Notes

The configuration described here is built around 230 V AC mains-voltage signals and must comply with local electrical installation rules. The following points are non-negotiable:

• Isolation: when working with the module unpowered, confirm zero energy with a voltage tester on every input terminal, not just at the breaker.
• Overcurrent protection: each digital input group should be fed through a fused or breaker-protected branch sized for the field device count.
• Surge protection: if the inputs are routed outdoors or near inductive switching, fit surge arrestors at the panel entry.
• Relay contact protection: for inductive loads, fit an RC snubber (for AC) or flyback diode (for DC) across the load, or use the LOGO! Contact 230 contactor as an interposing element.
• Standards to verify: IEC 61131-2 for PLC environmental requirements, IEC 60947 for contactors, and IEC 60751 for PT100 sensor accuracy classes. Compliance with any of these is application-specific and must be verified by the integrator.

Disclaimer: this article documents the field-proven stack topology and the engineering rules of thumb that govern it. Always cross-check against the device manual for your specific module revision and firmware version. Electrical ratings, especially derated values for inductive loads, are subject to change between firmware versions and should be confirmed in the latest Siemens LOGO! manuals page.