HomeMaster MicroPLC — ESP32 DIN-Rail Controller, Modbus, ESPHome

Relay Outputs

The MicroPLC provides 1 mechanical relay (HF115F-005-1ZS3) for switching AC or DC loads. Only C and NC terminals are exposed (no NO) — functionally SPST-NC. The contact is closed when de-energised. Driven via optocoupler-isolated control circuitry.

System/board rating: 3 A MAX per output (system limit). The MicroPLC board/system can safely carry ONLY 3 A per relay output. This limit is determined by PCB trace width, terminal block rating, and thermal constraints, not the relay component rating. Using more than 3 A directly can overheat and permanently damage the board.

Relay contact (component) rating: 16 A @ 250 V AC (resistive). This value belongs ONLY to the relay component itself and is NOT usable as a system output rating. The MicroPLC must NOT be used above 3 A.

Loads above 3 A (or inductive/inrush loads) MUST be switched using an external contactor or power relay. The MicroPLC relay may be used as a control signal for the external contactor. Do not attempt to switch loads above 3 A directly through the MicroPLC relay outputs.

⚠️ External protection required: Every relay output MUST be protected by an external fuse or circuit breaker (max 3 A per channel). Relay output circuits are not internally fused. External overcurrent protection is mandatory for safe operation.

Relay Specifications

Relay Wiring Examples

Example wiring using C and NC only (no NO terminal). NC is closed when de-energised.

• Relay exposes C / NC only — there is no NO terminal
• There is no NO terminal — use DIO-430-R1 for failsafe-off loads
• NC is closed when de-energised — load is powered by default; do not use for failsafe-off loads
• ⚡ Loads may carry hazardous voltage
• Supports AC and DC switching (DC and inductive loads require derating)

⚠️ External protection rule: Every relay output MUST be protected by an external fuse or circuit breaker. The rated current of the protective device shall not exceed 3 A MAX per output (board/system limit).

⚠️ Common protection warning: If a single common fuse or circuit breaker is used to protect multiple relay outputs (as shown in the wiring example), the protective device shall NOT be sized by summing relay outputs. The rating of the common protective device shall not exceed 3 A MAX per output (board/system limit).

For inductive or DC loads (contactors, solenoids, motors), use appropriate suppression (RC snubbers, MOVs, or flyback diodes). Loads exceeding the recommended PCB current limit shall be switched using external contactors or wiring methods that bypass PCB copper paths.

Temperature Inputs

MicroPLC supports 1-Wire temperature sensors (DS18B20 or compatible) for reliable cabinet and process temperature monitoring.

Temperature Input Specifications

Wiring Examples

1-Wire wiring example.

Limitations / Notes

• Recommended topology: daisy-chain configuration (see wiring example above)
• Maximum number of sensors (recommended): ≤ 10 sensors per bus
• Typical maximum cable length (recommended): ≤ 50 m using shielded cable with proper topology
• Power mode: External power using +5V/GND is recommended. Parasitic power mode is not recommended for long cable runs or multiple sensors.

Communication & Protocols

• Modbus RTU (RS-485) – UART-based communication for expansion modules and field devices
• Wi-Fi – ESP32 integrated (Improv onboarding supported)

RS-485 / Modbus RTU

Terminal order differs across the HomeMaster range. Always read the silkscreen - do not wire by habit from another module. On this module the order is COM-B-A. Swapping A and B damages nothing but the node will not communicate. COM is required on every node.

All HomeMaster controllers and modules share the same RS-485 front end.

Bus wiring rules — apply to every device on the bus:

• One twisted pair for A/B, 120 Ω characteristic impedance.
• Run COM to every node. Required, not optional: the ports are not isolated, and COM is what bounds the common-mode voltage the transceivers see.
• Prefer one power supply for the whole bus, distributed in star topology. With separate supplies, additionally tie the 0 V references together at a single point.
• Bond the cable shield to cabinet PE at one end only. Never land a shield on A, B or COM.
• Where the bus crosses into a different electrical installation with its own earthing reference — a utility or billing meter, another building, another cabinet's PE system — fit an external galvanic RS-485 isolator at that boundary. The on-board components are transient protection, not isolation, and will not survive a sustained ground-potential difference.

Connect A, B, and COM to the bus.

Add 120Ω termination only at the two physical ends of the bus.

Tip: Cable recommendations, shield bonding, and routing rules are defined in the Cable Recommendations & Shield Grounding section below.

Cable Recommendations & Shield Grounding

This section applies to Analog (0–10V), Temperature (RTD / 1-Wire), and RS-485. Use shielded, twisted constructions and bond shields correctly to reduce EMI and ground-loop issues.

General Routing Rules

• Route low-level signal cables (Analog/RTD/1-Wire/RS-485) separately from mains, relay outputs, contactors, VFD motor cables, and power wiring.
• If crossing power cables is unavoidable, cross at 90°.
• Keep cable runs as short as practical and avoid parallel runs with high-current conductors.

1-Wire (DS18B20)

• Recommended: shielded 3-core ( +5V / DATA / GND ) for typical installations
• High-EMI / long runs: shielded pairs + overall shield (e.g. LI2YCY PIMF 2×2×0.50)
• Topology: daisy-chain (bus). Avoid star wiring.
• Stubs: keep sensor stubs ≤ 0.5 m.
• Pull-up (DATA): 4.7 kΩ typical; 2.2–3.3 kΩ for long/heavy loads.

RS-485 Cable

• Construction: twisted pair for A/B
• Characteristic impedance: 120 Ω (recommended)
• Shielding: overall shield is recommended in cabinets; use individually shielded pairs + overall shield in high-EMI
• Examples: J-Y(ST)Y 2×2×0.5 mm² or LI2YCY PiMF 2×2×0.50
• Use one twisted pair for A/B and a third conductor for COM. COM must be run to every node.

Shield Grounding

• Default recommendation: bond cable shield(s) to cabinet PE/EMC ground at the PLC end only.
• Do not connect shields to signal terminals (AI/AO/RTD/1-Wire/RS-485 A/B/COM).
• If both ends are in equipotential bonded cabinets, shields may be bonded at both ends using proper 360° clamps.