Modbus-Impl (RTU) for rp-usb-serial & rp-pio-serial

A small no_std Modbus RTU helper library designed to run on embedded Rust targets (e.g. RP2040/RP2350) and work with rp-usb-serial USB CDC link & rp-pio-serial PIO-based software serial for RP2040 & RP2350 using arbitrary GPIO pins. It currently focuses on Modbus function codes 01, 02, 03, and 04 (read operations) and builds valid Modbus RTU frames including CRC16.

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What it does

At runtime the library processes fixed-length Modbus requests carried over a byte-stream transport (USB CDC). For each incoming request frame it:

1. Validates CRC16 (Modbus polynomial 0xA001, init 0xFFFF)
2. Parses the request fields:
   • Unit ID
   • Function code (0x01/0x02/0x03/0x04/0x05/0x06)
   • Start address
   • Quantity
3. Checks address range using is_valid(addr)
4. Builds one of:
   • A normal response frame for the requested function, or
   • An exception response frame:
     • Function | 0x80
     • Exception code (ILLEGAL_FUNCTION / ILLEGAL_DATA_ADDRESS / ILLEGAL_DATA_VALUE)
     • CRC16

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Supported Modbus Functions

• 0x01 Read Coils
• 0x02 Read Discrete Inputs
• 0x03 Read Holding Registers (16-bit registers, big-endian in the payload)
• 0x04 Read Input Registers (16-bit registers, big-endian in the payload)
• 0x05 Write Single Coil
• 0x06 Write Single Register
• 0x0F Write Multiple Coils
• 0x10 Write Multiple Registers

Coils/inputs are packed into bytes using Modbus rules (LSB-first bit packing).

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Data Model

The library defines interfaces so you can plug in your own memory map:

RegisterRead

Used for 16-bit register based functions (FC03/FC04):

• get(addr: u16) -> u16
• is_valid(addr: u16) -> bool

RegisterWrite

Used for 16-bit register based functions (FC06/FC16)

• set_reg(addr: u16, val: u16)
• set_qty(qty: u16)
• get_qty() -> usize

BitRead

Used for bit based functions (FC01/FC02):

• get(addr: u16) -> bool
• is_valid(addr: u16) -> bool

BitWrite

Used for 16 bit based functions (FC05/FC15)

• set_bit(addr: u16, val: bool)
• set_qty(qty: u16)
• get_qty() -> usize

It also provides basic storage types that implement these traits:

• Hreg<N> for Holding Registers (FC03/FC06/FC16)
• Ireg<N> for Input Registers (FC04)
• Coil<N> for Coils (FC01/FC05/FC15)
• Ists<N> for Discrete Inputs (FC02)

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Implementation Overview

Key components:

• crc16_modbus(data: &[u8]) -> u16
  Implements Modbus RTU CRC16.

• Frame parsing

  • Requests are assumed to be 8 bytes long (standard RTU frame for function 01/02/03/04 read requests, 05/06/15/16 write requests).
  • parse_pdu() | parse_frame dispatch supports multiple function codes.

• Response builders

  • build_resp_bit_reads() builds FC01/FC02 responses.
  • build_resp_regs() builds FC03/FC04 responses.
  • build_exception_resp() builds exception responses.

• ModbusCtx::pharse_frame() The main entry that takes a request frame and outputs either a response or an exception.

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Typical Usage Flow

In your main loop you typically:

1. Receive bytes from rp-usb-serial into an 8-byte or more byte depend on func FrameLen4Func(func, &rx_accum, rx_len); buffer.
2. Call:
   • ctx.pharse_frame::<MAX_QTY>(frame, &mut resp_buf, &mut exc_buf);
3. Send the resulting frame back with:
   • RpUsbConsole::write(&resp_or_exc[..len])

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Notes / Limitations

• This library is RTU-focused but transport-agnostic: it assumes requests arrive as a byte stream and are accumulated into exact 8-byte frames.
• Read functions are implemented (01/02/03/04).
• Write functions are implemented (05/06/15/16).
• Bit packing follows Modbus LSB-first conventions.

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