ESP32 MicroPLC

Relay Outputs

The MicroPLC provides 1 SPDT mechanical relay (HF115F-005-1ZS3) for switching AC or DC loads. The relay provides SPDT switching contacts (NO / NC / COM) available on the relay output terminals and is 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: 12 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 Normally Open (NO) and Normally Closed (NC) relay contacts.

• Each relay has COM / NO / NC terminals
• Use NO for default OFF loads
• Use NC for default ON 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 Communication (Modbus RTU)

The MicroPLC provides a half-duplex RS-485 interface with integrated protection and fail-safe biasing. The interface is available on the A / B / COM terminals.

Hardware Features:

• RS-485 transceiver: MAX485 (half-duplex)
• Fail-safe biasing network on A/B (idle state defined)
• Common-mode choke for EMI suppression
• TVS surge and ESD protection on A/B/COM
• Resettable PTC fuses on A and B lines
• Bidirectional logic level shifting between MCU and transceiver

RS-485 Wiring

Connect A, B, and COM to the bus.

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

Termination & Biasing

• Terminate with 120 Ω only at the two ends of the line.
• Do not terminate intermediate devices.
• Fail-safe biasing is already provided inside the MicroPLC.

COM / Reference Ground

• Connect COM between all RS-485 nodes.
• This limits common-mode voltage and prevents communication faults.

Power Supply for Extension Modules

• Recommended: Use the same power supply for MicroPLC and all RS-485 extension modules.
• Distribute power in a star topology from the PSU.
• If separate PSUs are used, connect 0V references together at one point (unless the extension module is galvanically isolated).

A common 0V reference prevents RS-485 common-mode voltage errors and communication faults.

Practical Limits (Recommended)

• Baud rates: 9,600–115,200 bps recommended
• Bus length: Up to 1200 m at low baud rates (RS-485 standard), but typical cabinet runs are much shorter
• Devices: Up to 32 standard unit loads; more devices require repeaters or low-load transceivers

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. Use the second pair for COM (0V reference) or spare.

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.