MQTT-Modbus RTU Data Diode

I need a compact gateway that listens to my MQTT broker and pushes commands one-way to a Modbus RTU bus over RS-485. The direction is fixed—MQTT to Modbus devices only—so no bytes must ever travel back toward the broker. Each received MQTT message simply instructs the gateway to read a specified register or coil on the serial line; the read result may be discarded or logged locally but must never be published upstream.

To make this work I’m looking to have both hardware and firmware produced. A small microcontroller board (ESP32, STM32, or a part you prefer) with an RS-485 transceiver will do the job. The firmware should:

• Subscribe to configurable MQTT topics, parse address/function parameters, and issue the matching Modbus RTU read commands.
• Enforce the data-diode rule at firmware and interface level, guaranteeing that no Modbus response is forwarded back to MQTT.
• Handle typical Modbus read functions (01/02/03/04) with timeouts and basic error trapping.

Deliverables
• Schematic, PCB layout, and BOM ready for prototype fabrication.
• Well-commented firmware source and compiled binary.
• README covering topic syntax, flashing instructions, and a short test procedure that proves one-way flow.

If you already have reusable libraries or reference hardware, mention them so we can move fast toward the first working board.