Phase 3 PR 46 -- DL205 BCD decoder (binary-coded-decimal numeric encoding). Adds ModbusDataType.Bcd16 and Bcd32 to the driver. Bcd16 is 1 register wide, Bcd32 is 2 registers wide; Bcd32 respects ModbusByteOrder (BigEndian/WordSwap) the same way Int32 does so the CDAB-style families (including DL205/DL260 themselves) can be configured. DecodeRegister uses the new internal DecodeBcd helper: walks each nibble from MSB to LSB, multiplies the running result by 10, adds the nibble as a decimal digit. Explicitly rejects nibbles > 9 with InvalidDataException -- hardware sometimes produces garbage during write-in-progress transitions and silently returning wrong numeric values would quietly corrupt the caller's data. EncodeRegister's new EncodeBcd inverts the operation (mod/div by 10 nibble-by-nibble) with an up-front overflow check against 10^nibbles-1. Why this matters for DL205/DL260: AutomationDirect DirectLOGIC uses BCD as the default numeric encoding for timers, counters, and operator-display numerics (not binary). A plain Int16 read of register 0x1234 returns 4660; the BCD path returns 1234. The two differ enough that silently defaulting to Int16 would give wildly wrong HMI values -- the caller must opt in to Bcd16/Bcd32 per tag. Unit tests: DecodeBcd (theory: 0,1,9,10,1234,9999), DecodeBcd_rejects_nibbles_above_nine, EncodeBcd (theory), Bcd16_decodes_DL205_register_1234_as_decimal_1234 (control: same bytes as Int16 decode to 4660), Bcd16_encode_round_trips_with_decode, Bcd16_encode_rejects_out_of_range_values, Bcd32_decodes_8_digits_big_endian, Bcd32_word_swap_handles_CDAB_layout, Bcd32_encode_round_trips_with_decode, Bcd_RegisterCount_matches_underlying_width. 66/66 Modbus.Tests pass. Integration test: DL205BcdQuirkTests.DL205_BCD16_decodes_HR1072_as_decimal_1234 against dl205.json pymodbus profile (HR[1072]=0x1234). Asserts Bcd16 decode=1234 AND Int16 decode=0x1234 on the same wire bytes to prove the paths are distinct. 3/3 DL205 integration tests pass with MODBUS_SIM_PROFILE=dl205.

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

@@ -404,8 +404,8 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
     /// </summary>
     internal static ushort RegisterCount(ModbusTagDefinition tag) => tag.DataType switch
     {
-        ModbusDataType.Int16 or ModbusDataType.UInt16 or ModbusDataType.BitInRegister => 1,
-        ModbusDataType.Int32 or ModbusDataType.UInt32 or ModbusDataType.Float32 => 2,
+        ModbusDataType.Int16 or ModbusDataType.UInt16 or ModbusDataType.BitInRegister or ModbusDataType.Bcd16 => 1,
+        ModbusDataType.Int32 or ModbusDataType.UInt32 or ModbusDataType.Float32 or ModbusDataType.Bcd32 => 2,
         ModbusDataType.Int64 or ModbusDataType.UInt64 or ModbusDataType.Float64 => 4,
         ModbusDataType.String => (ushort)((tag.StringLength + 1) / 2), // 2 chars per register
         _ => throw new InvalidOperationException($"Non-register data type {tag.DataType}"),
@@ -435,6 +435,17 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
         {
             case ModbusDataType.Int16: return BinaryPrimitives.ReadInt16BigEndian(data);
             case ModbusDataType.UInt16: return BinaryPrimitives.ReadUInt16BigEndian(data);
+            case ModbusDataType.Bcd16:
+            {
+                var raw = BinaryPrimitives.ReadUInt16BigEndian(data);
+                return (int)DecodeBcd(raw, nibbles: 4);
+            }
+            case ModbusDataType.Bcd32:
+            {
+                var b = NormalizeWordOrder(data, tag.ByteOrder);
+                var raw = BinaryPrimitives.ReadUInt32BigEndian(b);
+                return (int)DecodeBcd(raw, nibbles: 8);
+            }
             case ModbusDataType.BitInRegister:
             {
                 var raw = BinaryPrimitives.ReadUInt16BigEndian(data);
@@ -510,6 +521,21 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
                 var v = Convert.ToUInt16(value);
                 var b = new byte[2]; BinaryPrimitives.WriteUInt16BigEndian(b, v); return b;
             }
+            case ModbusDataType.Bcd16:
+            {
+                var v = Convert.ToUInt32(value);
+                if (v > 9999) throw new OverflowException($"BCD16 value {v} exceeds 4 decimal digits");
+                var raw = (ushort)EncodeBcd(v, nibbles: 4);
+                var b = new byte[2]; BinaryPrimitives.WriteUInt16BigEndian(b, raw); return b;
+            }
+            case ModbusDataType.Bcd32:
+            {
+                var v = Convert.ToUInt32(value);
+                if (v > 99_999_999u) throw new OverflowException($"BCD32 value {v} exceeds 8 decimal digits");
+                var raw = EncodeBcd(v, nibbles: 8);
+                var b = new byte[4]; BinaryPrimitives.WriteUInt32BigEndian(b, raw);
+                return NormalizeWordOrder(b, tag.ByteOrder);
+            }
             case ModbusDataType.Int32:
             {
                 var v = Convert.ToInt32(value);
@@ -579,9 +605,46 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
         ModbusDataType.Float32 => DriverDataType.Float32,
         ModbusDataType.Float64 => DriverDataType.Float64,
         ModbusDataType.String => DriverDataType.String,
+        ModbusDataType.Bcd16 or ModbusDataType.Bcd32 => DriverDataType.Int32,
         _ => DriverDataType.Int32,
     };
 
+    /// <summary>
+    ///     Decode an N-nibble binary-coded-decimal value. Each nibble of <paramref name="raw"/>
+    ///     encodes one decimal digit (most-significant nibble first). Rejects nibbles &gt; 9 —
+    ///     the hardware sometimes produces garbage during transitions and silent non-BCD reads
+    ///     would quietly corrupt the caller's data.
+    /// </summary>
+    internal static uint DecodeBcd(uint raw, int nibbles)
+    {
+        uint result = 0;
+        for (var i = nibbles - 1; i >= 0; i--)
+        {
+            var digit = (raw >> (i * 4)) & 0xF;
+            if (digit > 9)
+                throw new InvalidDataException(
+                    $"Non-BCD nibble 0x{digit:X} at position {i} of raw=0x{raw:X}");
+            result = result * 10 + digit;
+        }
+        return result;
+    }
+
+    /// <summary>
+    ///     Encode a decimal value as N-nibble BCD. Caller is responsible for range-checking
+    ///     against the nibble capacity (10^nibbles - 1).
+    /// </summary>
+    internal static uint EncodeBcd(uint value, int nibbles)
+    {
+        uint result = 0;
+        for (var i = 0; i < nibbles; i++)
+        {
+            var digit = value % 10;
+            result |= digit << (i * 4);
+            value /= 10;
+        }
+        return result;
+    }
+
     private IModbusTransport RequireTransport() =>
         _transport ?? throw new InvalidOperationException("ModbusDriver not initialized");
 

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

@@ -89,6 +89,18 @@ public enum ModbusDataType
     BitInRegister,
     /// <summary>ASCII string packed 2 chars per register, <see cref="ModbusTagDefinition.StringLength"/> characters long.</summary>
     String,
+    /// <summary>
+    ///     16-bit binary-coded decimal. Each nibble encodes one decimal digit (0-9). Register
+    ///     value <c>0x1234</c> decodes as decimal <c>1234</c> — NOT binary <c>0x04D2 = 4660</c>.
+    ///     DL205/DL260 and several Mitsubishi / Omron families store timers, counters, and
+    ///     operator-facing numerics as BCD by default.
+    /// </summary>
+    Bcd16,
+    /// <summary>
+    ///     32-bit (two-register) BCD. Decodes 8 decimal digits. Word ordering follows
+    ///     <see cref="ModbusTagDefinition.ByteOrder"/> the same way <see cref="Int32"/> does.
+    /// </summary>
+    Bcd32,
 }
 
 /// <summary>

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205BcdQuirkTests.cs

@@ -0,0 +1,56 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.DL205;
+
+/// <summary>
+///     Verifies DL205/DL260 binary-coded-decimal register handling against the
+///     <c>dl205.json</c> pymodbus profile. HR[1072] = 0x1234 on the profile represents
+///     decimal 1234 (BCD nibbles). Reading it as <see cref="ModbusDataType.Int16"/> would
+///     return 0x1234 = 4660; the <see cref="ModbusDataType.Bcd16"/> path decodes 1234.
+/// </summary>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "DL205")]
+public sealed class DL205BcdQuirkTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task DL205_BCD16_decodes_HR1072_as_decimal_1234()
+    {
+        if (sim.SkipReason is not null) Assert.Skip(sim.SkipReason);
+        if (!string.Equals(Environment.GetEnvironmentVariable("MODBUS_SIM_PROFILE"), "dl205",
+                StringComparison.OrdinalIgnoreCase))
+        {
+            Assert.Skip("MODBUS_SIM_PROFILE != dl205 — skipping (standard profile does not seed HR[1072]).");
+        }
+
+        var options = new ModbusDriverOptions
+        {
+            Host = sim.Host,
+            Port = sim.Port,
+            UnitId = 1,
+            Timeout = TimeSpan.FromSeconds(2),
+            Tags =
+            [
+                new ModbusTagDefinition("DL205_Count_Bcd",
+                    ModbusRegion.HoldingRegisters, Address: 1072,
+                    DataType: ModbusDataType.Bcd16, Writable: false),
+                new ModbusTagDefinition("DL205_Count_Int16",
+                    ModbusRegion.HoldingRegisters, Address: 1072,
+                    DataType: ModbusDataType.Int16, Writable: false),
+            ],
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+        await using var driver = new ModbusDriver(options, driverInstanceId: "dl205-bcd");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(["DL205_Count_Bcd", "DL205_Count_Int16"],
+            TestContext.Current.CancellationToken);
+
+        results[0].StatusCode.ShouldBe(0u);
+        results[0].Value.ShouldBe(1234, "DL205 BCD register 0x1234 represents decimal 1234 per the DirectLOGIC convention");
+
+        results[1].StatusCode.ShouldBe(0u);
+        results[1].Value.ShouldBe((short)0x1234, "same register read as Int16 returns the raw 0x1234 = 4660 value — proves BCD path is distinct");
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusDataTypeTests.cs

@@ -219,4 +219,97 @@ public sealed class ModbusDataTypeTests
         ModbusDriver.DecodeRegister(wire, hi).ShouldBe("He");
         ModbusDriver.DecodeRegister(wire, lo).ShouldBe("eH");
     }
+
+    // --- BCD (binary-coded decimal, DL205/DL260 default numeric encoding) ---
+
+    [Theory]
+    [InlineData(0x0000u, 0u)]
+    [InlineData(0x0001u, 1u)]
+    [InlineData(0x0009u, 9u)]
+    [InlineData(0x0010u, 10u)]
+    [InlineData(0x1234u, 1234u)]
+    [InlineData(0x9999u, 9999u)]
+    public void DecodeBcd_16_bit_decodes_expected_decimal(uint raw, uint expected)
+        => ModbusDriver.DecodeBcd(raw, nibbles: 4).ShouldBe(expected);
+
+    [Fact]
+    public void DecodeBcd_rejects_nibbles_above_nine()
+    {
+        Should.Throw<InvalidDataException>(() => ModbusDriver.DecodeBcd(0x00A5u, nibbles: 4))
+            .Message.ShouldContain("Non-BCD nibble");
+    }
+
+    [Theory]
+    [InlineData(0u, 0x0000u)]
+    [InlineData(5u, 0x0005u)]
+    [InlineData(42u, 0x0042u)]
+    [InlineData(1234u, 0x1234u)]
+    [InlineData(9999u, 0x9999u)]
+    public void EncodeBcd_16_bit_encodes_expected_nibbles(uint value, uint expected)
+        => ModbusDriver.EncodeBcd(value, nibbles: 4).ShouldBe(expected);
+
+    [Fact]
+    public void Bcd16_decodes_DL205_register_1234_as_decimal_1234()
+    {
+        // HR[1072] = 0x1234 on the DL205 profile represents decimal 1234. A plain Int16 decode
+        // would return 0x04D2 = 4660 — proof the BCD path is different.
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Bcd16);
+        ModbusDriver.DecodeRegister(new byte[] { 0x12, 0x34 }, tag).ShouldBe(1234);
+
+        var int16Tag = tag with { DataType = ModbusDataType.Int16 };
+        ModbusDriver.DecodeRegister(new byte[] { 0x12, 0x34 }, int16Tag).ShouldBe((short)0x1234);
+    }
+
+    [Fact]
+    public void Bcd16_encode_round_trips_with_decode()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Bcd16);
+        var wire = ModbusDriver.EncodeRegister(4321, tag);
+        wire.ShouldBe(new byte[] { 0x43, 0x21 });
+        ModbusDriver.DecodeRegister(wire, tag).ShouldBe(4321);
+    }
+
+    [Fact]
+    public void Bcd16_encode_rejects_out_of_range_values()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Bcd16);
+        Should.Throw<OverflowException>(() => ModbusDriver.EncodeRegister(10000, tag))
+            .Message.ShouldContain("4 decimal digits");
+    }
+
+    [Fact]
+    public void Bcd32_decodes_8_digits_big_endian()
+    {
+        // 0x12345678 as BCD = decimal 12_345_678.
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Bcd32);
+        ModbusDriver.DecodeRegister(new byte[] { 0x12, 0x34, 0x56, 0x78 }, tag).ShouldBe(12_345_678);
+    }
+
+    [Fact]
+    public void Bcd32_word_swap_handles_CDAB_layout()
+    {
+        // PLC stored 12_345_678 with word swap: low-word 0x5678 first, high-word 0x1234 second.
+        // Wire bytes [0x56, 0x78, 0x12, 0x34] + WordSwap → decode to decimal 12_345_678.
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Bcd32,
+            ByteOrder: ModbusByteOrder.WordSwap);
+        ModbusDriver.DecodeRegister(new byte[] { 0x56, 0x78, 0x12, 0x34 }, tag).ShouldBe(12_345_678);
+    }
+
+    [Fact]
+    public void Bcd32_encode_round_trips_with_decode()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Bcd32);
+        var wire = ModbusDriver.EncodeRegister(87_654_321u, tag);
+        wire.ShouldBe(new byte[] { 0x87, 0x65, 0x43, 0x21 });
+        ModbusDriver.DecodeRegister(wire, tag).ShouldBe(87_654_321);
+    }
+
+    [Fact]
+    public void Bcd_RegisterCount_matches_underlying_width()
+    {
+        var b16 = new ModbusTagDefinition("A", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Bcd16);
+        var b32 = new ModbusTagDefinition("B", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Bcd32);
+        ModbusDriver.RegisterCount(b16).ShouldBe((ushort)1);
+        ModbusDriver.RegisterCount(b32).ShouldBe((ushort)2);
+    }
 }