Phase 3 PR 21 — Modbus TCP driver: first native-protocol greenfield for v2. New src/Driver.Modbus project with ModbusDriver implementing IDriver + ITagDiscovery + IReadable + IWritable. Validates the driver-agnostic abstractions (IAddressSpaceBuilder, DriverAttributeInfo, DataValueSnapshot, WriteRequest/WriteResult) generalize beyond Galaxy — nothing Galaxy-specific is used here. ModbusDriverOptions carries Host/Port/UnitId/Timeout + a pre-declared tag list (Modbus has no discovery protocol — tags are configuration). IModbusTransport abstracts the socket layer so tests swap in-memory fakes; concrete ModbusTcpTransport speaks the MBAP ADU (TxId + Protocol=0 + Length + UnitId + PDU) over TcpClient, serializes requests through a semaphore for single-flight in-order responses, validates the response TxId matches, surfaces server exception PDUs as ModbusException with function code + exception code. DiscoverAsync streams one folder per driver with a BaseDataVariable per tag + DriverAttributeInfo that flags writable tags as SecurityClassification.Operate vs ViewOnly for read-only regions. ReadAsync routes per-tag by ModbusRegion: FC01 for Coils, FC02 for DiscreteInputs, FC03 for HoldingRegisters, FC04 for InputRegisters; register values decoded through System.Buffers.Binary.BinaryPrimitives (BigEndian for single-register Int16/UInt16 + two-register Int32/UInt32/Float32 per standard modbus word-swap conventions). WriteAsync uses FC05 (Write Single Coil with 0xFF00/0x0000 encoding) for booleans, FC06 (Write Single Register) for 16-bit types, FC16 (Write Multiple Registers) for 32-bit types. Unknown tag → BadNodeIdUnknown; write to InputRegister or DiscreteInput or Writable=false tag → BadNotWritable; exception during transport → BadInternalError + driver health Degraded. Subscriptions + Historian + Alarms deliberately out of scope — Modbus has no push model (subscribe would be a polling overlay, additive PR) and no history/alarm semantics at the protocol level. Tests (9 new ModbusDriverTests): InitializeAsync connects + populates the tag map + sets health=Healthy; Read Int16 from HoldingRegister returns BigEndian value; Read Float32 spans two registers BigEndian (IEEE 754 single for 25.5f round-trips exactly); Read Coil returns boolean from the bit-packed response; unknown tag name returns BadNodeIdUnknown without an exception; Write UInt16 round-trips via FC06; Write Float32 uses FC16 (two-register write verified by decoding back through the fake register bank); Write to InputRegister returns BadNotWritable; Discover streams one folder + one variable per tag with correct DriverDataType mapping (Int16/Int32→Int32, UInt16/UInt32→Int32, Float32→Float32, Bool→Boolean). FakeTransport simulates a 256-register/256-coil bank + implements the 7 function codes the driver uses. slnx updated with the new src + tests entries. Full solution post-add: 0 errors, 189 tests pass (9 Modbus + 180 pre-existing). IDriver abstraction validated against a fundamentally different protocol — Modbus TCP has no AlarmExtension, no ScanState, no IPC boundary, no historian, no LDAP — and the same builder/reader/writer contract plugged straight in. Future PRs on this driver: ISubscribable via a polling loop, IHostConnectivityProbe for dead-device detection, PLC-specific data-type extensions (Int64/BCD/string-in-registers).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriverOptions.cs

@@ -0,0 +1,39 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+/// <summary>
+///     Modbus TCP driver configuration. Bound from the driver's <c>DriverConfig</c> JSON at
+///     <c>DriverHost.RegisterAsync</c>. Every register the driver exposes appears in
+///     <see cref="Tags"/>; names become the OPC UA browse name + full reference.
+/// </summary>
+public sealed class ModbusDriverOptions
+{
+    public string Host { get; init; } = "127.0.0.1";
+    public int Port { get; init; } = 502;
+    public byte UnitId { get; init; } = 1;
+    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(2);
+
+    /// <summary>Pre-declared tag map. Modbus has no discovery protocol — the driver returns exactly these.</summary>
+    public IReadOnlyList<ModbusTagDefinition> Tags { get; init; } = [];
+}
+
+/// <summary>
+///     One Modbus-backed OPC UA variable. Address is zero-based (Modbus spec numbering, not
+///     the documentation's 1-based coil/register conventions).
+/// </summary>
+/// <param name="Name">
+///     Tag name, used for both the OPC UA browse name and the driver's full reference. Must be
+///     unique within the driver.
+/// </param>
+/// <param name="Region">Coils / DiscreteInputs / InputRegisters / HoldingRegisters.</param>
+/// <param name="Address">Zero-based address within the region.</param>
+/// <param name="DataType">Logical data type. Int16/UInt16 = single register; Int32/UInt32/Float32 = two registers big-endian.</param>
+/// <param name="Writable">When true and Region supports writes (Coils / HoldingRegisters), IWritable routes writes here.</param>
+public sealed record ModbusTagDefinition(
+    string Name,
+    ModbusRegion Region,
+    ushort Address,
+    ModbusDataType DataType,
+    bool Writable = true);
+
+public enum ModbusRegion { Coils, DiscreteInputs, InputRegisters, HoldingRegisters }
+public enum ModbusDataType { Bool, Int16, UInt16, Int32, UInt32, Float32 }