A compact industrial-grade 64-channel Modbus RTU output module featuring low-side drivers, per-channel LEDs, wide input voltage, DIN-rail support, and full configuration via Modbus holding registers. Designed for automation, distributed I/O, and custom control applications.

1. Product Overview

The Open Modbus OM-64DO is a 64-channel low-side output expansion module intended for industrial automation, machine control, and distributed I/O networks using Modbus RTU over RS-485. It switches DC loads such as solenoids, relays, indicator lamps, locks, and alarms up to 48 VDC. The module combines an STM32G031 microcontroller, four MCP23S17 SPI expanders, and eight TBD62083 Darlington driver arrays. This architecture enables reliable, flexible, and scalable output control with built-in protection and per-channel LED indication.

📦 Hardware Files

The full schematic and BOM are available at:

Schematic → https://github.com/OpenModbus/OM-64DO/blob/main/assets/schematics.pdf

BOM → https://github.com/OpenModbus/OM-64DO/blob/main/assets/bom.csv

Check the latest release for more!

2. Key Features

• 64 open-collector low-side outputs
• Up to 500 mA per output, 2 A max per driver package
• 5–48 VDC recommended load voltage (50 V absolute maximum)
• Wide supply input: 7–28 VDC
• RS-485 Modbus RTU communication (THVD1450 transceiver)
• Fully configurable serial parameters
• Per-channel LED indicators
• DEGSON 15EDG pluggable screw terminals, or KEFA screw terminals
• DIN-rail mount using UM72S profile
• Compact PCB (163 × 72 mm)

3. Applications

Ideal for:

• PLC expansion
• Building automation
• Access control
• Alarm and signaling systems
• Relay switching
• Industrial prototyping

4. Hardware Architecture

Microcontroller

The STM32G031F8P6 MCU handles Modbus RTU processing, configuration storage, SPI communication, and output refresh logic.

I/O Expanders

Four MCP23S17 SPI-based expanders provide the 64 output control signals in groups of 16.

Output Drivers

The TBD62083 Darlington arrays switch the actual loads. Their key characteristics include:

• 8 channels per IC
• 500 mA per channel
• 2 A total per IC
• Integrated flyback diodes
• LED indication per channel

RS-485 Communication

A robust THVD1450 transceiver provides differential RS-485 communication with:

• Fail-safe features
• High ESD immunity
• Suitable for long cable runs

Power

The board accepts 7–28 VDC and includes a high-efficiency R-78E3.3-0.5 regulator for its 3.3 V logic domain.

5. Electrical Specifications

Supply

• 7–28 VDC input
• Internal regulated 3.3 V logic
• Low power MCU operation

Outputs

• 64 open-collector low-side channels
• 5–48 VDC recommended load
• 50 VDC absolute maximum
• 500 mA max per output
• 2 A max per driver IC
• Built-in flyback protection
• LED indicator on each output

Communication

• RS-485, 2-wire
• Modbus RTU protocol
• Default settings: 9600 baud, 8N1, address 1
• All communication parameters can be reconfigured via holding registers

6. Mechanical Information

PCB: 160 × 72 mm

DIN-rail enclosure compatibility: UM72S profile

DEGSON 15EDG pluggable screw terminals or KEFA screw terminals

Mounting suitable for industrial control cabinets

7. Functional Overview

Each output activates when its Modbus coil value is 1, causing the driver to sink current to ground.

When the coil value is 0, the channel turns off, leaving the load unpowered.

A dedicated LED mirrors the state of each output channel.

8. Modbus Interface

Supported Function Codes

0x01 — Read Coils

0x05 — Write Single Coil

0x0F — Write Multiple Coils

0x03 — Read Holding Registers

0x06 — Write Single Register

0x10 — Write Multiple Registers

Coils (Outputs 1–64)

Coils map directly to output channels. Addressing begins at 0x00.

Check out the GitHub repository for more information.

9. Wiring

The OM-64DO uses low-side (open-collector) switching. That means the module connects the load to ground when ON, and the load must be powered from your external power supply.

The 64 outputs are grouped into four banks of 16 (Bank A–D). This allows one board to drive devices powered from different voltages simultaneously.

Examples:

Bank A → 12 V devices

Bank B → 24 V relays

Bank C → 48 V solenoids

Bank D → 5 V indicators

Low-Side Output Principle

Each output contains a protected Darlington transistor that sinks current:

Load +V ─────── Load ─────── OUTx (SINK)

│

└── GND (OM-64DO)

Recommended Wiring Steps

Connect the load voltage (5–48 VDC) to the Vx terminal of the bank.

Connect the output pin (OUTx) to the negative side of the load.

Tie power supply GND to OM-64DO GND.

Ensure the supply can support the total current of that bank.

⚠️ Important Grounding Requirement: All load power supplies must share a common ground with the OM-64DO. This is required for the low-side drivers and internal logic to reference the same electrical 0 V.

Wiring Example

The example below shows how to drive 24 V and 12 V relays at the same time using separate banks.

Bank A (VA) → 24 V relays

Bank B (VB) → 12 V relays

Both supplies share a common ground

Loads return their negative side to the selected OUTx channel

This wiring approach can be extended to all four banks, each with its own voltage.

10. Safety Notes

Do not exceed 50 VDC load voltage

Ensure load currents stay within limits

For inductive loads, external suppression can further improve reliability

Maintain proper RS-485 termination and shielding

11. Contact & Support

For questions, feedback, or technical assistance regarding the Open Modbus OM-64DO, the following support channels are available:

📧 Email Support

For general inquiries, firmware questions, or troubleshooting: contact@sszczep.dev

🛠 GitHub Repository

Issue tracker, source code, hardware design files, and community contributions: https://github.com/OpenModbus/OM-64DO

💬 Open Modbus Discussions

Share projects, ask for help, or suggest features: https://github.com/orgs/OpenModbus/discussions

📝 Bug Reports & Feature Requests

Submit issues directly via GitHub Issues: https://github.com/OpenModbus/OM-64DO/issues