Oliver Walter
5b9c09df3b
ESPHome only has built-in support for Modbus RTU over RS485, so the application layer moves to it. The transport already conformed: the CRC was CRC-16/MODBUS, the line rate 19200 8N1, the delimiter a 3.5-character idle gap pinned at 1750 us above 19200, and the address space 1..247 with broadcast never answered. Only the bytes between the address and the CRC change, and no external crate is needed for a read-only server. frame.rs drops the explicit LEN byte, which RTU does not have — a request's length is implied by its function code. The parser gets simpler and MAX_FRAME grows to the RTU limit of 256. The CRC is now the only integrity check there is, so a truncated frame fails it rather than a length comparison; a test asserts every single-bit corruption is still caught, and another pins our encoding against the published example 01 03 00 6B 00 03 74 17. proto.rs becomes a Modbus server: function codes 0x03 and 0x04 over one 19-register table, 0x08 sub-function 0x0000 as the link test that replaces PING, and the four standard exception codes. Registers are big-endian with 32-bit values high word first, which is what masters and ESPHome's S_DWORD assume. Snapshot::registers renders the whole map and reads slice it, so a range read cannot disagree with a single-register read of the same address — the alternative, assembling only the requested registers per read, has no such guarantee. Both read function codes serve the same table. That is not only for masters that implement just 0x03: a read overlapping the measurement block is refused with SERVER_DEVICE_FAILURE when the sensor has never produced a reading, and ESPHome coalesces adjacent registers of one type into a single command, so diagnostic entities merged into that command would go unavailable along with the measurement they were meant to explain. Requesting them under the other function code puts them in their own command. The generated ESPHome config does this. The firmware barely changes, because dispatch's signature does not: flags widen to u16, buffers follow MAX_FRAME, and comments name registers rather than commands. Now ~35 KiB flash and ~2.6 KiB RAM. tools/wiredsensor.py is rewritten and grows from 15 checks to 21, adding agreement between the two function codes, sub-range consistency, and a per-code assertion for every exception. It stays a hand-written Modbus implementation rather than moving to pymodbus: half these checks inject deliberately malformed frames, and a conforming client library exists precisely to make those unconstructable. It also keeps the wire format independent of wiredsensor-core, so a shared bug cannot cancel itself out. Not yet exercised on hardware — the host tests and register-map cross-checks pass, but the timing-dependent behaviour needs `wiredsensor.py test` against a real bus. This replaces the old protocol rather than joining it; command codes 0x01..0x04 are all valid Modbus function codes, so the two cannot share a segment. PROTOCOL_VERSION is 2. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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