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.
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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.
The full schematic, PCB layout, and BOM are available at:
- Schematic → https://github.com/OpenModbus/OM-64DO/blob/main/assets/schematics.pdf
- PCB Design → https://github.com/OpenModbus/OM-64DO/blob/main/assets/board_design.pdf
- BOM → https://github.com/OpenModbus/OM-64DO/blob/main/assets/bom.csv
- 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
- DIN-rail mount using UM72S profile
- Compact PCB (163.1 × 72 mm)
Ideal for:
- PLC expansion
- Building automation
- Access control
- Alarm and signaling systems
- Relay switching
- Industrial prototyping
The STM32G031F8P6 MCU handles Modbus RTU processing, configuration storage, SPI communication, and output refresh logic.
Four MCP23S17 SPI-based expanders provide the 64 output control signals in groups of 16.
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
A robust THVD1450 transceiver provides differential RS-485 communication with:
- Fail-safe features
- High ESD immunity
- Suitable for long cable runs
The board accepts 7–28 VDC and includes a high-efficiency R-78E3.3-1.0 regulator for its 3.3 V logic domain.
- 7–28 VDC input
- Internal regulated 3.3 V logic
- Low power MCU operation
- 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
- RS-485, 2-wire
- Modbus RTU protocol
- Default settings: 9600 baud, 8N1, address 1
- All communication parameters can be reconfigured via holding registers
- PCB: 163.1 × 72 mm
- DIN-rail enclosure compatibility: UM72S profile
- DEGSON 15EDG pluggable screw terminals
- Mounting suitable for industrial control cabinets
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.
- 0x01 — Read Coils
- 0x05 — Write Single Coil
- 0x0F — Write Multiple Coils
- 0x03 — Read Holding Registers
- 0x06 — Write Single Register
- 0x10 — Write Multiple Registers
Coils map directly to output channels.
Addressing begins at 0x00.
| Coil Address (Hex) | Function |
|---|---|
| 0 (0x00) | OUTPUT 1 |
| 1 (0x01) | OUTPUT 2 |
| 2 (0x02) | OUTPUT 3 |
| 3 (0x03) | OUTPUT 4 |
| 4 (0x04) | OUTPUT 5 |
| … | … |
| 63 (0x3F) | OUTPUT 64 |
Configuration registers start at 0x00 and are stored in non-volatile memory.
| Register Address (Hex) | Name | Description | Allowed Values | Default |
|---|---|---|---|---|
| 0 (0x00) | Slave Address | Modbus device ID | 1–247 | 1 |
| 1 (0x01) | Baud Rate Code | Serial speed selector: 0=1200, 1=2400, 2=4800, 3=9600, 4=19200, 5=38400, 6=57600, 7=115200 | 0–7 | 3 |
| 2 (0x02) | Parity | Communication parity | 0=None, 1=Even, 2=Odd | 0 |
| 3 (0x03) | Data Bits | Serial data width | 5–9 (typ. 8) | 8 |
| 4 (0x04) | Stop Bits | Serial stop bits | 1–2 | 1 |
Below are typical Modbus RTU frames for controlling and configuring the OM-64DO.
Function: Write Single Coil (0x05)
Coil address: 0x0000
Value: ON → 0xFF00
Frame: 01 05 00 00 FF 00 8C 3A
Function: Write Multiple Coils (0x0F)
Start address: 0x0004
Quantity: 0x0008 (8 coils)
Data: FF (all eight bits ON)
Frame: 01 0F 00 04 00 08 01 FF 4F 15
Function: Write Multiple Coils (0x0F)
Start address: 0x0000
Quantity: 0x0040 (64 coils)
Data: FF FF FF FF FF FF FF FF
Frame: 01 0F 00 00 00 40 08 FF FF FF FF FF FF FF FF AA 20
Function: Write Multiple Coils (0x0F)
Start address: 0x0000
Quantity: 0x0040
Data: 00 00 00 00 00 00 00 00
Frame: 01 0F 00 00 00 40 08 00 00 00 00 00 00 00 00 EB A4
Function: Read Coils (0x01)
Start address: 0x0000
Quantity: 0x0040
Frame: 01 01 00 00 00 40 3D FA
Function: Write Single Register (0x06)
Register: 0x0000 (Slave Address)
Value: 0x0005
Frame: 01 06 00 00 00 05 49 C9
Function: Write Single Register (0x06)
Register: 0x0001 (Baud Rate Code)
Value: 0x0007 → 115200 baud
Frame: 01 06 00 01 00 07 99 C8
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
Each output contains a protected Darlington transistor that sinks current:
Load +V ─────── Load ─────── OUTx (SINK)
│
└── GND (OM-64DO)
- 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.
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.
- 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
For questions, feedback, or technical assistance regarding the Open Modbus OM-64DO, the following support channels are available:
For general inquiries, firmware questions, or troubleshooting: contact@sszczep.dev
Issue tracker, source code, hardware design files, and community contributions: https://github.com/OpenModbus/OM-64DO
Share projects, ask for help, or suggest features: https://github.com/orgs/OpenModbus/discussions
Submit issues directly via GitHub Issues: https://github.com/OpenModbus/OM-64DO/issues