Tongs — modular fieldbus bridges with a fault model

Affects: ForgeIEC Studio 🔧tongs-* Anvil server (anvild)

What it’s about

A PLC platform is only as good as its fieldbus connection. But every bus is different: Modbus TCP has no real diagnostics, EtherCAT has a fully clocked slave state machine, Profibus has its own fault classes, EtherNet/IP has CIP connections.

Tongs solves this with a plug-in family: one daemon per protocol, controlled by anvild, with a uniform fault layer and structured diagnostic bits from the device FDDs. Instead of a mega driver with all protocols inside, a modular component family.

One daemon per protocol

Status updated in October 2026.

All run under the same anvild subprocess manager. Lifecycle, logging, auto-restart — shared, not re-invented per protocol.

Uniform fault model

Spec tongs-platform-v1.md §5 defines a shared fault grading across all bridges:

Reported uniformly in Studio + anvild — regardless of whether underneath is Modbus, EtherCAT or Profibus. Operator doesn’t have to learn a separate error table per bus.

Call via MCP: bellows.protocol_status returns the current fault state per segment including diagnostic bits.

Diagnostic bits — structured, not free text

For vendor devices (e.g. Weidmueller bus terminals) there are concrete diagnostic bits in the status bytes:

• Bit 0: module missing
• Bit 1: short circuit at output
• Bit 2: wire break
• Bit 3: supply voltage out of spec
• …

These bits live in the manufacturer’s FDD (Field Device Description). ForgeIEC Studio:

1. Imports FDDs (including embedded PDF data sheets)
2. Parses the <DiagnosticDecode> sections
3. Shows per-bit meaning + severity in Studio’s diagnostics tab
4. Makes resolved bits accessible via the catalog.diag_bits MCP tool

AI diagnostic loop: “Why doesn’t Motor_Ready switch?” → AI calls bellows.protocol_status + catalog.diag_bits and can answer in plain text: “Module Stachelbeere reports wire break at output 3.”

Per-segment own daemon process

Architectural benefit: a crashing bridge does not take down the whole master. If the EtherCAT bridge hits a fresh slave bug and dies, Modbus + Profibus + the PLC logic continue undisturbed. anvild restarts the crashed bridge daemon automatically.

This differs from monolithic runtimes where a driver fault brings the whole machine down.

Bus configuration in .forge

In the .forge project format, bus segments are stored structured:

[[bus_segments]]
name = "Modbus_HMI"
protocol = "modbustcp"
settings.host = "192.168.1.50"
settings.port = 502

[[bus_segments.devices]]
name = "Stachelbeere"
fdd_path = "catalog/weidmueller/UR20-FBC-PN-IRT-V2.fdd"

Studio renders this as a bus tree, generates codegen + auto-rolls the matching tongs bridge daemon at save.

IEC access in ST code with a clear 5-level namespace: anvil.<segment>.<device>.Status.<var> — e.g. anvil.Modbus_HMI.Stachelbeere.Status.WireBreak.

Plus: Anvil ABI probe — no more version roulette

Tongs bridges talk to anvild via Anvil shared memory. With mixed versions between anvild + bridge, silently inconsistent data used to be possible — bridge writes ABI variant A, anvild reads variant B, nobody notices.

With the ABI probe (commits 7653cc5 + 9b17adc) the type hash of SHM topics is verified at connect. A mismatch immediately throws a Studio warning with concrete version comparison — instead of silent data corruption.

What the commits delivered

• 83fe7d4 — anvil.* tool family (4 tools)
• 3c61445 — bellows.protocol_status + tasks.cycle_overview
• 9cc6dcd — force-gate layers 1-3 (also affects bus I/O)
• 5dc087c — FDD-driven diagnostic-bit resolver
• 5fe1e69 — module-picker filter via <ModuleProtocol> in the coupler FDD
• 66923df — Weidmueller FDDs self-contained with CDATA PDFs + Git-LFS
• 6774397 — FDD editor: diagnostics tab + documents tab
• 7653cc5, 9b17adc — Anvil ABI probe layers 3a/3b
• 21759e7 — MCP-4a remote bind enables bus diagnostics over distance

Spec: documentation/architecture/tongs-platform-v1.md

• bus-system-v1.md + anvil-platform-v2.md §13.

Where you use it

Where to read more

• Bus configuration
• MCP tool families: anvil + bellows + bus system
• Spec: documentation/architecture/tongs-platform-v1.md (source tree)