DIN-Rail AC Dimmer for Home Assistant — DIM-420-R1, Modbus RTU, 2 Channels

Tech Specs

Note: For absolute maximum ratings, detailed dimming load characteristics, and safety margins, always refer to the official DIM‑420‑R1 datasheet. Datasheet values are the authoritative reference.

Installation, Environmental & Mechanical

Home Assistant / Modbus / Web Config Integration

The DIM‑420‑R1 is a Modbus RTU slave on RS‑485 and can be controlled by:

• HomeMaster MicroPLC and MiniPLC controllers
• SCADA/PLC systems with Modbus RTU support
• Home Assistant using ESPHome Modbus integration package
• Any third‑party Modbus RTU master controller

Local behaviour (press logic, cut mode, thresholds, LED mapping) is configured using a browser‑based WebConfig tool over USB‑C (Web Serial). All parameters are stored in onboard flash and restored at power‑up.

Quick Setup Process (USB‑C WebConfig)

1. Mount & Power – Mount the module on a DIN rail and connect 24 V DC logic power.
2. Connect USB‑C – Connect a USB‑C cable from the module to a PC.
3. Open WebConfig – Open WebConfig in a Chromium‑based browser.
4. Connect Device – Click Connect and grant serial permissions.
5. Configure – Set Modbus address and baud, choose cut mode (Leading/Trailing) and load type per channel, adjust lower/upper thresholds, and map DI/Button/LED behaviour.
6. Save & Disconnect – Confirm that configuration is saved to flash, then disconnect USB‑C.

What WebConfig is and how it works: WebConfig is a browser-based configuration UI that runs entirely in your Chromium browser using the Web Serial API. Once connected, it continuously exchanges JSON messages with the DIM‑420‑R1 to read the current configuration (Modbus address, baud rate, cut mode, thresholds, DI/LED/button mapping) and apply any changes you make in the forms. Channel tiles show live AC presence and frequency badges, sliders let you test dimming levels in real time, and per‑input / per‑button tables control how wall switches and front buttons drive CH1/CH2. All edits are applied immediately and written to the module’s flash a short time after the last change, so your settings persist across power cycles.

Features:

• Driverless, browser‑based setup using Web Serial
• Live preview of mains frequency and channel status
• Per‑channel cut mode, preset, and threshold tuning
• Configurable DI press logic and LED sources
• Persistent flash storage and JSON diagnostics stream

uart:
  id: uart_modbus
  tx_pin: 17
  rx_pin: 16
  baud_rate: 19200
  parity: NONE
  stop_bits: 1

modbus:
  id: modbus_bus
  uart_id: uart_modbus
  turnaround_time: 100ms
  send_wait_time: 250ms

packages:
  dim1:
    url: https://github.com/isystemsautomation/homemaster-dev
    ref: main
    files:
      - path: DIM-420-R1/Firmware/v0.1.0/default_dim_420_r1_plc/default_dim_420_r1_plc.yaml
        vars:
          dim_prefix: "DIM#1"
          dim_id: dim_1
          dim_address: 5  # Modbus address set in WebConfig for this DIM
    refresh: 1d

The module needs a Modbus master on the bus. In a HomeMaster system that is the MiniPLC or the MicroPLC — both run ESPHome and connect to Home Assistant over the native API. To work out how many modules a panel needs and how wide it comes out, use the System Builder.

New to this kind of system? Two guides cover the ground: building a Home Assistant automation system and a wired ESPHome smart home.

Going deeper on this module: relay channels and loads in a DIN panel, what still works when Home Assistant is down and wiring a new house before the walls close.

Frequently asked questions

Does it work with Home Assistant?

Yes, through a Modbus master on the RS-485 bus. In a HomeMaster system that is the MiniPLC or the MicroPLC running ESPHome, which exposes both channels to Home Assistant over the native ESPHome API. There is no MQTT broker to run and no Modbus register map to write by hand.

Leading edge or trailing edge — which one do I need?

Trailing edge generally suits dimmable LED drivers and electronic transformers; leading edge suits resistive loads and iron-core transformers. The mode is selected per channel, so one module can drive two different load types. If a lamp flickers or buzzes on one mode, switch that channel to the other and re-test — and check what the driver manufacturer specifies, because a driver marked as non-dimmable will not behave on either setting.

How much load can a channel take?

The load ratings and the derating curves are in the datasheet in the Documentation section below — read them before specifying a circuit, because dimmer ratings depend on the load type and on the ambient temperature in the cabinet, not on a single headline figure. Fit an upstream fuse or breaker per channel. The module is IP20 and mixes SELV control electronics with hazardous mains on the dimming side, so it belongs in a closed control cabinet and the terminals must be covered.

Will the wall switch still dim if Home Assistant goes down?

Yes. The mapping between a digital input and a dimming channel, along with the press logic and the thresholds, is stored in the module's own flash and executed by the module. Loss of the network, of the controller or of the Home Assistant server does not stop local dimming. What stops is remote control and state reporting until the link returns.

What can I connect to the inputs?

Dry contacts — wall buttons, retractive switches, auxiliary contacts. The inputs are module-wetted 24 V with an ISO1212 IEC 61131-2 front end, so do not feed external voltage into them. Each input has its own return terminal, and mode (momentary or latching), debounce and inversion are configurable, as is what a short, long, double or short-then-long press does.

Do I have to use HomeMaster controllers?

No. The module is a standard Modbus RTU slave on RS-485 — address 1–247, default 3; baud 9600–115200, default 19200 with 8N1 framing. Any Modbus master reaches it: a third-party PLC, a SCADA system, or your own ESP32 with an RS-485 transceiver.

Does it measure the power it is switching?

No. There is no per-channel energy metering on this module. Where consumption has to be measured, that is a separate metering device on the same bus.

Is the RS-485 port isolated?

No. The transceiver shares the device's logic ground, and the ISO1212 input front end conditions the inputs but is not galvanic isolation either. What is present is transient protection — TVS diodes, resettable PTC fuses and fail-safe biasing. Run COM to every node on the bus, and fit an external RS-485 isolator where the bus crosses into a separate electrical installation. Note also that this is a mixed-voltage device: never bridge logic ground with the mains-side isolated rails.

How is it configured?

Over USB-C from a browser using WebConfig — Web Serial, no driver and no vendor application. Modbus address and baud rate, per-channel cut mode and load type, dimming thresholds and presets, input modes and press actions, front-button and LED mapping are all set there and stored in on-device flash (LittleFS), restored at power-up.

Connectors & terminal map

Top row (24Vdc | CHANNEL 2 | CHANNEL 1)

Bottom row (RS-485 | DIGITAL INPUT)

Ports & service interfaces

Housing notes

• CHANNEL 2 is printed LEFT of CHANNEL 1 on the housing.
• DI pairs read signal-then-Gnd, the opposite of DIO and ALM.
• MIXED VOLTAGE. Mains is present across the whole top row except POWER.
• No per-channel energy metering.

Cabling and Wirings

24 V DC Logic Power

Connect regulated 24 V DC SELV to the POWER terminals V+ and 0V. This powers the MCU, LEDs, RS‑485 interface, and USB‑C logic.

Connect + to V+ and − to 0V. Use a SELV 24 V DC supply with upstream protection.

Logic Power Checklist

• Use a regulated 24 V DC SELV power supply
• Install an external fuse or breaker (typ. 0.5 A) upstream of V+
• Observe polarity V+ / 0V
• Route 24 V DC wiring separately from mains AC dimmer outputs

Digital Inputs (DI1–DI4)

Connect potential-free (dry) contacts — wall switches, push buttons, or relay / open-collector / transistor outputs that simply close DI1–DI4 to their corresponding DIx_GND return. The module supplies the wetting current from its own 24 V rail; do not feed external voltage into the input. Three-wire sensors must be powered from your own supply, with their switching output wired to the input (the module provides no sensor-supply rail). Input mode (Momentary/Latching), debounce, invert, and press actions are configured in WebConfig.

Each input has its own DIx / DIx_GND pair. Use dry contacts only; do not apply mains to DI terminals.

• Keep DI wiring separate from AC dimmer wiring and other high‑current conductors.
• Use twisted pairs per DI channel where possible.
• Configure press logic (Short / Long / Double / Short‑then‑Long) and mapping in WebConfig.
• Inputs are wetted from the module 24 V supply through ISO1212 IEC 61131-2 front-end devices (not galvanic isolation); see field board schematic for details.

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 B-A-COM. 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 RS‑485 bus (COM on every node). Terminate at the two physical ends only.

• Use one twisted pair for A/B and a third conductor for COM. COM must be run to every node.
• Terminate the bus with ≈120 Ω resistors at the two physical ends only.
• Keep stubs short and avoid star wiring.
• Use a common 0 V reference (COM) between controller and modules unless galvanic isolation requires otherwise.

AC Load Wiring (Dimming Channels)

The dimming channels use mains domain terminals Lx_IN / Lx_OUT / Nx_IN / Nx_OUT (for CH1 and CH2). These carry hazardous voltage. Follow the wiring diagrams and load ratings in the official datasheet and field board schematic.

Typical CH1/CH2 connection using Lx_IN / Lx_OUT / Nx_IN / Nx_OUT terminals. Always follow the official datasheet wiring diagrams.

⚠️ Hazardous voltage: Lx/Nx terminals are on the mains side. Installation and wiring must be performed by qualified personnel only, with power isolated and in accordance with local electrical codes and the DIM‑420‑R1 datasheet.

Cable Recommendations & Shield Grounding

These recommendations apply to digital inputs, RS‑485, and mixed‑voltage installations where the DIM‑420‑R1 shares a cabinet with mains wiring and other control hardware. Detailed limits and examples are described in the DIM‑420‑R1 README and datasheet.

General Routing Rules

• Route low‑voltage signal wiring (24 V, DI, RS‑485, USB) separately from mains‑side dimmer wiring and other high‑current cables.
• Avoid long parallel runs between SELV wiring and mains bundles; cross at 90 ° where necessary.
• Use cable ducts and strain reliefs to prevent mechanical stress on terminals.
• Keep all conductors within their rated temperature and current limits as per the datasheet.

Digital Inputs (DI) Cable

• Use a twisted pair per DI channel (DIx + DIx_GND) for improved noise immunity.
• Recommended wire size: 0.2–0.75 mm² (AWG 24–18), compatible with 5.08 mm terminals.
• In noisy environments or long runs, use shielded twisted pair and follow shield grounding rules below.
• Keep DI runs physically separated from mains dimmer wiring and relay contact wiring.

RS‑485 (Modbus) Cable

• Use a twisted pair for A/B with characteristic impedance ≈120 Ω.
• Prefer shielded twisted pair in industrial or high‑EMI environments.
• Use a daisy‑chain bus (no star topology, minimal stubs).
• Route COM/0 V reference with the bus, using the second pair where available.

Shield Grounding

• Bond cable shields to the cabinet PE/EMC ground at the controller/module end using 360 ° EMC clamps where possible.
• By default, terminate shields at one end only to avoid ground loops; only bond at both ends if both cabinets share a reliable equipotential earth.
• Do not connect shields directly to signal terminals (DI, A/B/COM, V+/0V).
• Keep shield tails short and use dedicated grounding bars or clamps.

System Architecture & Pinout

System block diagram: RP2350A MCU, dimmer power stages, isolated rails, DI front‑end, RS‑485, and USB‑C.

MCU and connector pin assignments for DIM‑420‑R1; see README and schematics for detailed mapping.

Safety and Installation Notes

• The DIM‑420‑R1 is a mixed‑voltage device: it contains SELV 24 V control electronics and hazardous mains AC on dimmer channels.
• Installation, wiring, and maintenance must be performed by qualified personnel familiar with panel building and mains safety.
• Always isolate both 24 V DC and mains AC supplies before working on the device or its wiring.
• Mount inside a dry, clean, ventilated enclosure; protect all terminals against accidental contact.
• Respect isolation barriers between logic GND and mains‑side isolated rails; never bridge logic GND with GND_ISO rails.
• Follow the load type and cut‑mode recommendations in the datasheet to avoid flicker or damage to lamps/drivers.
• Provide appropriate upstream over‑current and residual current protection in accordance with local electrical codes.

Compliance & Certifications

The DIM-420-R1 module is CE marked. ISYSTEMS AUTOMATION S.R.L. (HomeMaster® brand) maintains the technical documentation and a signed EU Declaration of Conformity (DoC) available for download in the Documents and Resources section above.

• EMC Directive 2014/30/EU — EN 55032:2015 (Class B emissions), EN 55035:2017 (immunity), tested by Idvorsky Laboratories Ltd. (Job #1648)
• Low Voltage Directive 2014/35/EU — EN 62368-1:2020 + A11:2020, in-house dielectric and isolation testing by ISYSTEMS AUTOMATION compliance laboratory
• RoHS Directive 2011/65/EU — EN IEC 63000 technical documentation
• HomeMaster® — registered EU trademark (EUTM No. 019082911, EUIPO, registered 15 January 2025)

Licensing. Hardware design files — schematics, PCB layouts and bills of materials — are published under CERN-OHL-W v2. Firmware and the ESPHome integration packages are published under the MIT licence. Full terms are in the LICENSE files in the HOMEMASTER repository.