HomeMaster AIO-422-R1 — Analog I/O & RTD Module, 4 Inputs, Modbus

Tech Specs

Installation, Environmental & Mechanical

Home Assistant / Modbus / Web Config Integration

The AIO-422-R1 supports flexible integration with:

• HomeMaster MicroPLC / MiniPLC controllers as Modbus RTU masters
• PLC and SCADA systems via RS-485 Modbus RTU
• Home Assistant via ESPHome on the controller (entities from Modbus registers)
• Direct USB-C configuration using browser-based WebConfig

Configuration is saved persistently and restored at power-up.

Quick Setup Process (USB-C WebConfig)

1. Mount & Power — mount on DIN rail and connect 24 V DC.
2. Connect USB-C — attach USB-C to your PC.
3. Open WebConfig — open the WebConfig page in Chrome or Edge.
4. Connect Device — click Connect and select the serial device.
5. Configure Modbus — set address and baud rate.
6. Configure I/O behavior — analog input scaling, analog output setpoints, RTD wiring mode (2/3/4-wire).
7. Save & Disconnect — save settings and disconnect USB-C.

The WebConfig tool provides a complete view of the module configuration:

• Connection & setup – Modbus address and baud, status pills, Identify / Factory reset / Reboot, and the serial log.
• Analog I/O & RTD – live AI1–AI4 (0–10 V), AO1–AO2 DAC sliders with power-on policy, and RTD1/RTD2 temperature, diagnostics, wire mode and sensor type (Rref follows the type).
• Buttons & LEDs – map front-panel button actions and user-LED sources (AO thresholds, bus link, RTD fault, etc.).

Register-level and entity mapping details are available in the AIO-422-R1 README.

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:
  aio1:
    url: https://github.com/isystemsautomation/homemaster-dev
    ref: main
    files:
      - path: AIO-422-R1/Firmware/v0.2.0/default_aio_422_r1_plc/default_aio_422_r1_plc.yaml
        vars:
          aio_prefix: "AIO#1" # shown in Home Assistant entity names
          aio_id: aio_1      # internal unique id
          aio_address: 3     # Modbus address set in WebConfig
    refresh: 1d

Connectors & terminal map

Top row (24Vdc | AI 0-10V | AO 0-10V)

Bottom row (RS-485 | RTD/1 | RTD/2)

Ports & service interfaces

Housing notes

• Analog inputs are printed in DESCENDING order: AI4 AI3 AI2 AI1.
• Analog inputs share one common GND. Analog outputs have a GND each.
• RTD blocks are four terminals numbered 1-4 with jumper bridges printed between 1-2 and 3-4 for 2- and 3-wire modes.
• No field supply rail. Loop-powered sensors take 24 V from the panel.
• 4-20 mA is read via a 500 ohm 0.1 % shunt across an AI input.

Cabling and Wirings

24 V DC Supply (V+ / 0V)

Connect + to V+ and − to 0V.

• Use a regulated 24 V DC SELV supply
• Apply upstream fuse protection according to cabinet rules
• Keep power wiring separated from low-level analog/RTD cabling

Analog Inputs (AI1–AI4, 0–10 V)

AI channels expect 0–10 V signals and use AGND as analog ground.

Analog Outputs (AO1–AO2, 0–10 V)

Outputs provide 0–10 V (DAC MCP4725). Keep load current within recommended limits (max 10 mA per channel).

RTD Inputs (RTD1 / RTD2, PT100/PT1000)

Supports 2-, 3-, or 4-wire RTD wiring. Shielded RTD cables improve accuracy and fault detection stability.

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.

RS-485 Front End

Item	Value
Transceiver	MAX485CSA+T, half-duplex
Galvanic isolation	None — the transceiver shares the device's logic ground
Common-mode range	−7 V … +12 V referred to the device's own ground (MAX485 limit)
Terminals	A / B / COM
Surge protection	3 × SMAJ6.8CA TVS (A–COM, B–COM, A–B)
Overcurrent	2 × resettable PTC, 1.5 A hold, in series with A and B
EMI filtering	Common-mode choke on the A/B pair; COM referenced through 1 MΩ ∥ 4.7 nF
Idle state	Fail-safe biasing on board — do not add external bias resistors
Termination	120 Ω at the two physical ends of the bus only

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.

Wire A to A, B to B, and COM to every node (required).

RTD Field Board Jumpers (RTD1 / RTD2)

RTD configuration jumpers are placed on the field board for each RTD channel. Factory default is PT100 sensor and 2‑wire wiring mode. Use the table below as a reference when changing sensor type or wiring topology.

Cable Recommendations & Shield Grounding

General routing rules

• Route low-level signal cables (analog and RTD) separately from power and high-current conductors
• If crossing power cables is unavoidable, cross at 90°
• Keep cable runs as short as practical and avoid parallel runs with high-current wiring
• Label cables and provide strain relief at terminals

Analog (0–10 V) cable

• Use twisted pair (Signal + AGND) per analog channel
• Use overall shielding for typical installs, or individually shielded pairs for high-EMI environments
• Examples: J-Y(ST)Y (overall shield) or LI2YCY PiMF (shielded twisted pairs)

RTD temperature cable

• Recommended: shielded cable for RTD (PT100/PT1000), using 2/3/4-wire wiring as required
• Shielded pairs are recommended for best accuracy (especially for 4-wire)
• Example: J-Y(ST)Y (overall shield) or LI2YCY PiMF (pairs)

RS-485 (Modbus) cable

• Use twisted pair for A/B (characteristic impedance 120 Ω recommended)
• Prefer shielded cable in industrial/noisy panels
• Use one twisted pair for A/B and a third conductor for COM. COM must be run to every node.

Shield grounding

• Bond cable shield(s) to cabinet PE/EMC ground at the PLC/controller end only by default
• Do not connect shields directly to signal terminals (A/B/COM)

System Architecture & Pinout

System block diagram showing MCU, analog/RTD front-end, and RS-485 interfaces.

MCU GPIO and connector/pin reference (check revision notes in the datasheet).

Field board: analog inputs/outputs and RTD wiring interface.

MCU board: RS-485 transceiver, USB-C interface, and power conversion stages.

Safety and Installation Notes

• SELV circuits: the power input and signal terminals are SELV circuits; install only in an appropriate control cabinet
• Qualified personnel only: installation by trained technicians familiar with analog wiring and industrial control safety
• Protect all module wiring terminals against accidental contact using the required front plate/door
• Do not expose the module to outdoor conditions; the enclosure is IP20 (inside cabinet only)
• Use the recommended cable types (twisted/shielded) to reduce noise and improve analog/RTD accuracy
• Keep analog/RTD cables separated from power and high-current wiring to avoid interference

Compliance & Certifications

The AIO-422-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)
• RoHS Directive 2011/65/EU — EN IEC 63000 technical documentation
• LVD Directive 2014/35/EU — does not apply (SELV-only product powered from 24 V DC, below the 75 V DC LVD threshold; no mains-capable terminals)
• HomeMaster® — registered EU trademark (EUTM No. 019082911, EUIPO, registered 15 January 2025)