Rust-port behavior differences

The original C++ code used several push-style callbacks and duplicate type names that do not map cleanly into a Rust crate. These are the intentional differences in the Rust port.

ISO 11783-6 VT object-pool codec conformance

The VT object-pool codec in src/isobus/vt/objects.rs was audited against ISO 11783-6 (Virtual Terminal) and brought into conformance on the major divergences. This section records what changed and what remains.

Resolved divergences (now conformant)

• Number display math: (value + offset) * scale (was a division).
• Scale field type: IEEE-754 f32 (was i32).
• Child positions: signed i16 X/Y (was u16); each child record is the standard 6 bytes [oid:u16][x:i16][y:i16] (was 2 bytes, OID only).
• Child counts: u8 per the standard (was u16).
• Macro reference lists: [num_macros:u8][num × (event:u8, macro:u8)] round-trip through the codec (were absent).
• Per-type child record size: 6-byte positional records for WorkingSet/DataMask/AlarmMask/Container/Key/Button/WindowMask; 2-byte OID-only records for SoftKeyMask/KeyGroup.
• Body field layouts corrected for: WorkingSet (gained background/selectable/active_mask), AlarmMask (dropped non-standard options byte), InputBoolean (background, width, foreground as a Font Attributes ref, variable_ref, value, enabled — no height/options), InputString (Length is u8; justification precedes length), InputNumber (gained value/justification/standard Options 2; dropped non-standard input_attributes), OutputNumber (gained value/justification), OutputLine/OutputRectangle/OutputEllipse (line_attributes first), Meter/LinearBarGraph/ArchedBarGraph (u16 min/max + value; ArchedBarGraph bar_width not number_of_ticks), PictureGraphic (actual_width/actual_height/transparency/u32 raw-data length), StringVariable (gained Length field).
• No per-object length prefix: the codec now serializes each object as the ISO 11783-6 wire layout [id:u16][type:u8][body…] (the earlier [len:u16] field is gone). Object boundaries are recovered by a parse-by-type walker (object_body_total_len) that knows every body’s length, so the same ObjectPool::deserialize path consumes machbus-produced pools, real .iop files, and pools uploaded by third-party VT clients. net::iop_parser delegates to this codec — one source of truth.
• Self-delimiting bodies: to support the prefix-free format, InputAttributes gained its standard length:u8 field ([type][len][string]), Macro gained its standard num_bytes:u16 prefix ([num_bytes][commands…]), and StringVariable emits the actual value length on the wire — all three now match ISO 11783-6.

Remaining (intentional) gaps

• Hosted VT rendering is still partial evidence, not certification: Working Set language codes and Working Set Special Controls language/country pairs are modelled for hosted selection policy, but text-resource selection and profile-specific editor behaviour still need more evidence. Standard GraphicContext surfaces, PictureGraphic-backed ScaledGraphic, Animation, and the bounded PNG GraphicData subset now have backend-neutral render coverage where their referenced image/frame payloads are valid. Unsupported PNG format variants and unsupported graphics-context canvas operations remain explicit placeholders rather than certification-quality rendering.
• Protocol-state object bodies: Aux* objects remain outside the retained data-mask renderer, even though they are modelled by the VT object pool/protocol layers. Compatibility extension rows remain compatibility claims, not ISO standard-completeness claims.
• Palette / font metrics: the render layer’s colour palette RGB values and font-size→pixel mapping are repo-owned approximations (the standard’s exact values are licensed material, not reproduced here).

Wire codecs return values instead of pushing frames

Most ISOBUS subsystem codecs return Vec<Outbound> or typed values instead of directly pushing into a global network manager. The session facade is responsible for routing those outbound frames onto the transport.

Why:

• unit tests can assert exact frame sequences without a live bus;
• no hidden global state is required;
• C/Python bindings can expose fallible calls with a stable error channel.

Duplicate C++ names were disambiguated

Some C++ headers used the same public name for different layouts. Rust requires one canonical item per module path, so the port uses explicit names:

• classic FS FileServerProperties lives in src/isobus/fs/types.rs;
• v2 FS properties use FileServerPropertiesV2 in src/isobus/fs/properties.rs;
• v2 volume state uses VolumeStateV2.

Section Control is split by role

The Section Control master and client logic live under src/isobus/sc/master.rs and src/isobus/sc/client.rs. This keeps role-specific timing and state separate while allowing a facade to compose them later.

DTC memory is explicit

Diagnostic occurrence/history handling is centralized in DmMemory. The diagnostics plugins expose active and previous DTC lists; it does not hide occurrence-count changes behind unrelated message helpers.

Implement pump bridge is explicit

The Implement plugin has concrete inbound PGN callbacks for the supported hitch, PTO, and auxiliary-valve command PGNs. Unsupported implement-message families remain codec-only until wired through an explicit facade method; no no-op event bridge is kept to imply hidden behavior.

C ABI is an explicit facade

The C header is generated from src/ffi.rs only. Internal Rust constants, helper modules, and protocol tables are not part of the C ABI unless they are wrapped by an Machbus* type or machbus_* function in that file.