Modbus RTU - Questions to mbedtls and Read Holding Registers (0x03)

[TK5090]

🔐 1. Secure Modbus RTU with mbedTLS
Is there any native integration of mbedTLS with a Modbus stack in Zephyr?
I couldn’t find any out-of-the-box support for “Modbus Secure” (especially for RTU), but I’m wondering:

Is there any recommended way to use mbedTLS (e.g., for encryption/authentication) with a Modbus RTU stack in Zephyr?

Or would I have to implement a custom wrapper (e.g., AES/HMAC over Modbus frames manually)?

My idea is to use application-level encryption/authentication over RTU (e.g., AES-CBC + HMAC-SHA256), not TCP/TLS.

📥 2. Read Holding Registers (0x03) with extended data callback
Zephyr Modbus RTU stack seem to call a read callback like:

uint16_t read_callback(uint16_t reg, uint16_t *dest);
This is very limited — you only get a single register and a 16-bit pointer.

Is there a way (or has someone extended the stack) to support a callback like this:

uint16_t read_callback(uint16_t reg, uint8_t *buffer, uint16_t nreg);
So that you can handle multiple registers at once with a full buffer pointer?
That would make things easier, especially when working with structured data or mapping to a central data model.

[TK5090]

Hello, anybody there for help?