The primary design goals were:
- Low cost
- DIN-rail mountable form factor
- Full ESPHome compatibility
- 120/80 digital I/Os, with delay not noticeable by human
- Reasonable safety and isolation
- Modular and expandable architecture
- Powered by 24VDC

Each module is built around shift registers for both inputs and outputs.

A single module includes two pairs of registers, providing:

  • 16 digital inputs

  • 16 digital outputs

Outputs are driven through ULN2803 transistor arrays, allowing direct relay control.
Inputs are protected by TVS diodes and digital isolators, making the system robust against electrical noise and suitable for long cable runs in harsh environments.

Modules can be plugged together to scale up the number of I/Os.
A special “new line” module makes it possible to connect additional sets of modules on a different DIN rail, enabling virtually unlimited expansion.

Controller

The main controller currently uses an ESP32-S3, but the design is flexible — any MCU capable of driving shift registers can be used.