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.

Three substantive issues caught + fixed during the validation pass:
1. pymodbus rejects unknown keys at device-list / setup level. My PR 43 commit had `_layout_note`, `_uint16_layout`, `_bits_layout`, `_write_note` device-level JSON-comment fields that crashed pymodbus startup with `INVALID key in setup`. Removed all device-level _* fields. Inline `_quirk` keys WITHIN individual register entries are tolerated by pymodbus 3.13.0 — kept those in dl205.json since they document the byte math per quirk and the README + git history aren't enough context for a hand-author reading raw integer values. Documented the constraint in the top-level _comment of each profile.
2. pymodbus rejects sweeping `write` ranges that include any cell not assigned a type. My initial standard.json had `write: [[0, 2047]]` but only seeded HR[0..31] + HR[100] + HR[200..209] + bits[1024..1109] — pymodbus blew up on cell 32 (gap between HR[31] and HR[100]). Fixed by listing per-block write ranges that exactly mirror the seeded ranges. Same fix in dl205.json (was `[[0, 16383]]`).
3. pymodbus simulator stores all 4 standard Modbus tables in ONE underlying cell array — each cell can only be typed once (BITS or UINT16, not both). My initial standard.json had `bits[0..31]` AND `uint16[0..31]` overlapping at the same addresses; pymodbus crashed with `ERROR "uint16" <Cell> used`. Fixed by relocating coils to address 1024+, well clear of the uint16 entries at 0..209. Documented the layout constraint in the standard.json top-level _comment.
Substantive driver bug fixed: ModbusTcpTransport.ConnectAsync was using `new TcpClient()` (default constructor — dual-stack, IPv6 first) then `ConnectAsync(host, port)` with the user's hostname. .NET's TcpClient default-resolves "localhost" to ::1 first, fails to connect to pymodbus (which binds 0.0.0.0 IPv4-only), and only then retries IPv4 — the failure surfaces as the entire ConnectAsync timeout (2s by default) before the IPv4 attempt even starts. PR 30's smoke test silently SKIPPED because the fixture's TCP probe hit the same dual-stack ordering and timed out. Both fixed: ModbusSimulatorFixture probe now resolves Dns.GetHostAddresses, prefers AddressFamily.InterNetwork, dials IPv4 explicitly. ModbusTcpTransport does the same — resolves first, prefers IPv4, falls back to whatever Dns returns (handles IPv6-only hosts in the future). This is a real production-readiness fix because most Modbus PLCs are IPv4-only — a generic dual-stack TcpClient would burn the entire connect timeout against any IPv4-only PLC, masquerading as a connection failure when the PLC is actually fine.
Smoke-test address shifted HR[100] -> HR[200]. Standard.json's HR[100] is the auto-incrementing register that drives subscribe-and-receive tests, so write-then-read against it would race the increment. HR[200] is the first cell of a writable scratch range present in BOTH simulator profiles. DL205Profile.cs xml-doc updated to explain the shift; tag name "DL205_Smoke_HReg100" -> "Smoke_HReg200" + smoke test references updated. dl205.json gains a matching scratch HR[200..209] range so the smoke test runs identically against either profile.
Validation matrix:
- standard.json boot: clean (TCP 5020 listening within ~3s of pymodbus.simulator launch).
- dl205.json boot: clean.
- pymodbus client direct FC06 to HR[200]=1234 + FC03 read: round-trip OK.
- raw-bytes PowerShell TcpClient FC06 + 12-byte response: matches FC06 spec (echo of address + value).
- DL205SmokeTest against standard.json: 1/1 pass (was failing as 'BadInternalError' due to the dual-stack timeout + tag-name typo — both fixed).
- DL205SmokeTest against dl205.json: 1/1 pass.
- Modbus.Tests Unit suite: 52/52 pass — dual-stack transport fix is non-breaking.
- Solution build clean.
Memory + future-PR setup: pymodbus install + activation pattern is now bullet-pointed at the top of Pymodbus/README.md so future PRs (the per-quirk DL205_<behavior> tests in PR 44+) don't have to repeat the trial-and-error of getting the simulator + integration tests cooperating. The three bugs above are documented inline in the JSON profiles + ModbusTcpTransport so they don't bite again.

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

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);
@@ -472,13 +483,21 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
             }
             case ModbusDataType.String:
             {
-                // ASCII, 2 chars per register, packed high byte = first char.
-                // Respect the caller's StringLength (truncate nul-padded regions).
+                // ASCII, 2 chars per register. HighByteFirst (standard) packs the first char in
+                // the high byte of each register; LowByteFirst (DL205/DL260) packs the first char
+                // in the low byte. Respect StringLength (truncate nul-padded regions).
                 var chars = new char[tag.StringLength];
                 for (var i = 0; i < tag.StringLength; i++)
                 {
-                    var b = data[i];
-                    if (b == 0) { return new string(chars, 0, i); }
+                    var regIdx = i / 2;
+                    var highByte = data[regIdx * 2];
+                    var lowByte  = data[regIdx * 2 + 1];
+                    byte b;
+                    if (tag.StringByteOrder == ModbusStringByteOrder.HighByteFirst)
+                        b = (i % 2 == 0) ? highByte : lowByte;
+                    else
+                        b = (i % 2 == 0) ? lowByte : highByte;
+                    if (b == 0) return new string(chars, 0, i);
                     chars[i] = (char)b;
                 }
                 return new string(chars);
@@ -502,6 +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);
@@ -543,7 +577,14 @@ public sealed class ModbusDriver(ModbusDriverOptions options, string driverInsta
                 var s = Convert.ToString(value) ?? string.Empty;
                 var regs = (tag.StringLength + 1) / 2;
                 var b = new byte[regs * 2];
-                for (var i = 0; i < tag.StringLength && i < s.Length; i++) b[i] = (byte)s[i];
+                for (var i = 0; i < tag.StringLength && i < s.Length; i++)
+                {
+                    var regIdx = i / 2;
+                    var destIdx = tag.StringByteOrder == ModbusStringByteOrder.HighByteFirst
+                        ? (i % 2 == 0 ? regIdx * 2 : regIdx * 2 + 1)
+                        : (i % 2 == 0 ? regIdx * 2 + 1 : regIdx * 2);
+                    b[destIdx] = (byte)s[i];
+                }
                 // remaining bytes stay 0 — nul-padded per PLC convention
                 return b;
             }
@@ -564,9 +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

@@ -55,6 +55,12 @@ public sealed class ModbusProbeOptions
 /// <param name="ByteOrder">Word ordering for multi-register types. Ignored for Bool / Int16 / UInt16 / BitInRegister / String.</param>
 /// <param name="BitIndex">For <c>DataType = BitInRegister</c>: which bit of the holding register (0-15, LSB-first).</param>
 /// <param name="StringLength">For <c>DataType = String</c>: number of ASCII characters (2 per register, rounded up).</param>
+/// <param name="StringByteOrder">
+///     Per-register byte order for <c>DataType = String</c>. Standard Modbus packs the first
+///     character in the high byte (<see cref="ModbusStringByteOrder.HighByteFirst"/>).
+///     AutomationDirect DirectLOGIC (DL205/DL260) and a few legacy families pack the first
+///     character in the low byte instead — see <c>docs/v2/dl205.md</c> §strings.
+/// </param>
 public sealed record ModbusTagDefinition(
     string Name,
     ModbusRegion Region,
@@ -63,7 +69,8 @@ public sealed record ModbusTagDefinition(
     bool Writable = true,
     ModbusByteOrder ByteOrder = ModbusByteOrder.BigEndian,
     byte BitIndex = 0,
-    ushort StringLength = 0);
+    ushort StringLength = 0,
+    ModbusStringByteOrder StringByteOrder = ModbusStringByteOrder.HighByteFirst);
 
 public enum ModbusRegion { Coils, DiscreteInputs, InputRegisters, HoldingRegisters }
 
@@ -82,6 +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>
@@ -95,3 +114,17 @@ public enum ModbusByteOrder
     BigEndian,
     WordSwap,
 }
+
+/// <summary>
+///     Per-register byte order for ASCII strings packed 2 chars per register. Standard Modbus
+///     convention is <see cref="HighByteFirst"/> — the first character of each pair occupies
+///     the high byte of the register. AutomationDirect DirectLOGIC (DL205, DL260, DL350) and a
+///     handful of legacy controllers pack <see cref="LowByteFirst"/>, which inverts that within
+///     each register. Word ordering across multiple registers is always ascending address for
+///     strings — only the byte order inside each register flips.
+/// </summary>
+public enum ModbusStringByteOrder
+{
+    HighByteFirst,
+    LowByteFirst,
+}

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

@@ -28,10 +28,20 @@ public sealed class ModbusTcpTransport : IModbusTransport
 
     public async Task ConnectAsync(CancellationToken ct)
     {
-        _client = new TcpClient();
+        // Resolve the host explicitly + prefer IPv4. .NET's TcpClient default-constructor is
+        // dual-stack (IPv6 first, fallback to IPv4) — but most Modbus TCP devices (PLCs and
+        // simulators like pymodbus) bind 0.0.0.0 only, so the IPv6 attempt times out and we
+        // burn the entire ConnectAsync budget before even trying IPv4. Resolving first +
+        // dialing the IPv4 address directly sidesteps that.
+        var addresses = await System.Net.Dns.GetHostAddressesAsync(_host, ct).ConfigureAwait(false);
+        var ipv4 = System.Linq.Enumerable.FirstOrDefault(addresses,
+            a => a.AddressFamily == System.Net.Sockets.AddressFamily.InterNetwork);
+        var target = ipv4 ?? (addresses.Length > 0 ? addresses[0] : System.Net.IPAddress.Loopback);
+
+        _client = new TcpClient(target.AddressFamily);
         using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
         cts.CancelAfter(_timeout);
-        await _client.ConnectAsync(_host, _port, cts.Token).ConfigureAwait(false);
+        await _client.ConnectAsync(target, _port, cts.Token).ConfigureAwait(false);
         _stream = _client.GetStream();
     }
 

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.IntegrationTests/DL205/DL205Profile.cs

@@ -14,13 +14,17 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.DL205;
 /// </remarks>
 public static class DL205Profile
 {
-    /// <summary>Holding register the smoke test reads. Address 100 sidesteps the DL205
-    /// register-zero quirk (pending confirmation) — see modbus-test-plan.md.</summary>
-    public const ushort SmokeHoldingRegister = 100;
+    /// <summary>
+    ///     Holding register the smoke test writes + reads. Address 200 is the first cell of the
+    ///     scratch HR range in both <c>Pymodbus/standard.json</c> (HR[200..209] = 0) and
+    ///     <c>Pymodbus/dl205.json</c> (HR[4096..4103] added in PR 43 for the same purpose), so
+    ///     the smoke test runs identically against either simulator profile. Originally
+    ///     targeted HR[100] — moved to HR[200] when the standard profile claimed HR[100] as
+    ///     the auto-incrementing register that drives subscribe-and-receive tests.
+    /// </summary>
+    public const ushort SmokeHoldingRegister = 200;
 
-    /// <summary>Expected value the pymodbus profile seeds into register 100. When running
-    /// against a real DL205 (or a pymodbus profile where this register is writable), the smoke
-    /// test seeds this value first, then reads it back.</summary>
+    /// <summary>Value the smoke test writes then reads back to assert round-trip integrity.</summary>
     public const short SmokeHoldingValue = 1234;
 
     public static ModbusDriverOptions BuildOptions(string host, int port) => new()
@@ -32,7 +36,7 @@ public static class DL205Profile
         Tags =
         [
             new ModbusTagDefinition(
-                Name: "DL205_Smoke_HReg100",
+                Name: "Smoke_HReg200",
                 Region: ModbusRegion.HoldingRegisters,
                 Address: SmokeHoldingRegister,
                 DataType: ModbusDataType.Int16,

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205SmokeTests.cs

@@ -38,13 +38,13 @@ public sealed class DL205SmokeTests(ModbusSimulatorFixture sim)
         // zeroed at simulator start, and tests must not depend on prior-test state per the
         // test-plan conventions.
         var writeResults = await driver.WriteAsync(
-            [new(FullReference: "DL205_Smoke_HReg100", Value: (short)DL205Profile.SmokeHoldingValue)],
+            [new(FullReference: "Smoke_HReg200", Value: (short)DL205Profile.SmokeHoldingValue)],
             TestContext.Current.CancellationToken);
         writeResults.Count.ShouldBe(1);
         writeResults[0].StatusCode.ShouldBe(0u, "write must succeed against the ModbusPal DL205 profile");
 
         var readResults = await driver.ReadAsync(
-            ["DL205_Smoke_HReg100"],
+            ["Smoke_HReg200"],
             TestContext.Current.CancellationToken);
         readResults.Count.ShouldBe(1);
         readResults[0].StatusCode.ShouldBe(0u);

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205StringQuirkTests.cs

@@ -0,0 +1,81 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.DL205;
+
+/// <summary>
+///     Verifies the DL205/DL260 low-byte-first ASCII string packing quirk against the
+///     <c>dl205.json</c> pymodbus profile. Standard Modbus packs the first char of each pair
+///     in the high byte of the register; DirectLOGIC packs it in the low byte instead. Without
+///     <see cref="ModbusStringByteOrder.LowByteFirst"/> the driver decodes "eHllo" garbage
+///     even though the bytes on the wire are identical.
+/// </summary>
+/// <remarks>
+///     <para>
+///         Requires the dl205 profile (<c>Pymodbus\serve.ps1 -Profile dl205</c>). The standard
+///         profile does not seed HR[1040..1042] with string bytes, so running this against the
+///         standard profile returns <c>"\0\0\0\0\0"</c> and the test fails. Skip when the env
+///         var <c>MODBUS_SIM_PROFILE</c> is not set to <c>dl205</c>.
+///     </para>
+/// </remarks>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "DL205")]
+public sealed class DL205StringQuirkTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task DL205_string_low_byte_first_decodes_Hello_from_HR1040()
+    {
+        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[1040..1042]).");
+        }
+
+        var options = new ModbusDriverOptions
+        {
+            Host = sim.Host,
+            Port = sim.Port,
+            UnitId = 1,
+            Timeout = TimeSpan.FromSeconds(2),
+            Tags =
+            [
+                new ModbusTagDefinition(
+                    Name: "DL205_Hello_Low",
+                    Region: ModbusRegion.HoldingRegisters,
+                    Address: 1040,
+                    DataType: ModbusDataType.String,
+                    Writable: false,
+                    StringLength: 5,
+                    StringByteOrder: ModbusStringByteOrder.LowByteFirst),
+                // Control: same address, HighByteFirst, to prove the driver would have decoded
+                // garbage without the quirk flag.
+                new ModbusTagDefinition(
+                    Name: "DL205_Hello_High",
+                    Region: ModbusRegion.HoldingRegisters,
+                    Address: 1040,
+                    DataType: ModbusDataType.String,
+                    Writable: false,
+                    StringLength: 5,
+                    StringByteOrder: ModbusStringByteOrder.HighByteFirst),
+            ],
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+        await using var driver = new ModbusDriver(options, driverInstanceId: "dl205-string");
+        await driver.InitializeAsync(driverConfigJson: "{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(["DL205_Hello_Low", "DL205_Hello_High"],
+            TestContext.Current.CancellationToken);
+
+        results.Count.ShouldBe(2);
+        results[0].StatusCode.ShouldBe(0u);
+        results[0].Value.ShouldBe("Hello", "DL205 low-byte-first ordering must produce 'Hello' from HR[1040..1042]");
+
+        // The high-byte-first read of the same wire bytes should differ — not asserting the
+        // exact garbage string (that would couple the test to the ASCII byte math) but the two
+        // decodes MUST disagree, otherwise the quirk flag is a no-op.
+        results[1].StatusCode.ShouldBe(0u);
+        results[1].Value.ShouldNotBe("Hello");
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/ModbusSimulatorFixture.cs

@@ -46,8 +46,18 @@ public sealed class ModbusSimulatorFixture : IAsyncDisposable
 
         try
         {
-            using var client = new TcpClient();
-            var task = client.ConnectAsync(Host, Port);
+            // Force IPv4 family on the probe — pymodbus's TCP server binds 0.0.0.0 (IPv4 only)
+            // while .NET's TcpClient default-resolves "localhost" → IPv6 ::1 first, fails to
+            // connect, and only then tries IPv4. Under .NET 10 the IPv6 fail surfaces as a
+            // 2s timeout (no graceful fallback by default), so the C# probe times out even
+            // though a PowerShell probe of the same endpoint succeeds. Resolving + dialing
+            // explicit IPv4 sidesteps the dual-stack ordering.
+            using var client = new TcpClient(System.Net.Sockets.AddressFamily.InterNetwork);
+            var task = client.ConnectAsync(
+                System.Net.Dns.GetHostAddresses(Host)
+                    .FirstOrDefault(a => a.AddressFamily == System.Net.Sockets.AddressFamily.InterNetwork)
+                    ?? System.Net.IPAddress.Loopback,
+                Port);
             if (!task.Wait(TimeSpan.FromSeconds(2)) || !client.Connected)
             {
                 SkipReason = $"Modbus simulator at {Host}:{Port} did not accept a TCP connection within 2s. " +

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Pymodbus/dl205.json

@@ -1,5 +1,5 @@
 {
-  "_comment": "DL205.json — AutomationDirect DirectLOGIC DL205/DL260 quirk simulator. Models each behavior in docs/v2/dl205.md as concrete register values so DL205_<behavior> integration tests can assert against this profile WITHOUT a live PLC. Loaded by `pymodbus.simulator`. See ../README.md. Per-quirk address layout matches the table in dl205.md exactly. `shared blocks: true` matches DL series behavior — coils/HR overlay the same word address space (a Y-output is both a discrete bit AND part of a system V-memory register).",
+  "_comment": "DL205.json — DirectLOGIC DL205/DL260 quirk simulator. Models docs/v2/dl205.md as concrete register values. NOTE: pymodbus rejects unknown keys at device-list / setup level; explanatory comments live at top-level _comment + in README + git. Inline _quirk keys WITHIN individual register entries are accepted by pymodbus 3.13.0 (it only validates addr / value / action / parameters per entry). Each quirky uint16 is a pre-computed raw 16-bit value; pymodbus serves it verbatim. shared blocks=true matches DL series memory model. write list mirrors each seeded block — pymodbus rejects sweeping write ranges that include undefined cells.",
 
   "server_list": {
     "srv": {
@@ -27,15 +27,37 @@
       },
       "invalid": [],
       "write": [
-        [0, 16383]
+        [0, 0],
+        [200, 209],
+        [1024, 1024],
+        [1040, 1042],
+        [1056, 1057],
+        [1072, 1072],
+        [1280, 1282],
+        [1343, 1343],
+        [1407, 1407],
+        [2048, 2050],
+        [3072, 3074],
+        [4000, 4007],
+        [8448, 8448]
       ],
 
-      "_comment_uint16": "Holding-register seeds. Every quirky value is a raw uint16 with the byte math worked out in dl205.md so the simulator serves it verbatim — pymodbus does NOT decode strings, BCD, or float-CDAB on its own; that's the driver's job.",
-
       "uint16": [
-        {"_quirk": "V0 marker. HR[0] = 0xCAFE proves register 0 is valid on DL205/DL260 (rejects-register-0 was a DL05/DL06 relative-mode artefact). 0xCAFE = 51966.",
+        {"_quirk": "V0 marker. HR[0]=0xCAFE proves register 0 is valid on DL205/DL260 (rejects-register-0 was a DL05/DL06 relative-mode artefact). 0xCAFE = 51966.",
          "addr": 0,    "value": 51966},
 
+        {"_quirk": "Scratch HR range 200..209 — mirrors the standard.json scratch range so the smoke test (DL205Profile.SmokeHoldingRegister=200) round-trips identically against either profile.",
+         "addr": 200, "value": 0},
+        {"addr": 201, "value": 0},
+        {"addr": 202, "value": 0},
+        {"addr": 203, "value": 0},
+        {"addr": 204, "value": 0},
+        {"addr": 205, "value": 0},
+        {"addr": 206, "value": 0},
+        {"addr": 207, "value": 0},
+        {"addr": 208, "value": 0},
+        {"addr": 209, "value": 0},
+
         {"_quirk": "V2000 marker. V2000 octal = decimal 1024 = PDU 0x0400. Marker 0x2000 = 8192.",
          "addr": 1024, "value": 8192},
 
@@ -57,16 +79,14 @@
         {"_quirk": "BCD register. Decimal 1234 stored as BCD nibbles 0x1234 = 4660. NOT binary 1234 (= 0x04D2).",
          "addr": 1072, "value": 4660},
 
-        {"_quirk": "FC03 cap test. Real DL205/DL260 FC03 caps at 128 registers (above spec's 125). HR[1280..1407] is 128 contiguous registers; rest of block defaults to 0.",
+        {"_quirk": "FC03 cap test marker — first cell of a 128-register span the FC03 cap test reads. Other cells in the span aren't seeded explicitly, so reads of HR[1283..1342] / 1344..1406 return the default 0; the seeded markers at 1280, 1281, 1282, 1343, 1407 prove the span boundaries.",
          "addr": 1280, "value": 0},
         {"addr": 1281, "value": 1},
         {"addr": 1282, "value": 2},
-        {"addr": 1343, "value": 63,  "_marker": "FC03Block_mid"},
-        {"addr": 1407, "value": 127, "_marker": "FC03Block_last"}
+        {"addr": 1343, "value": 63},
+        {"addr": 1407, "value": 127}
       ],
 
-      "_comment_bits": "Coils — Y outputs at 2048+, C relays at 3072+, scratch C at 4000-4007 for write tests. DL260 X inputs would be at discrete-input addresses 0..511 but pymodbus's shared-blocks mode + same-table-as-coils means those would conflict with HR seeds; FC02 tests against this profile use a separate discrete-input block instead — that's why `di size` is large but the X-input markers live in `bits` only when `shared blocks=false`. Document trade-off in README.",
-
       "bits": [
         {"_quirk": "Y0 marker. DL260 maps Y0 to coil 2048 (0-based). Coil 2048 = ON proves the mapping.",
          "addr": 2048, "value": 1},

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/Pymodbus/standard.json

@@ -1,5 +1,5 @@
 {
-  "_comment": "Standard.json — generic Modbus TCP server for the integration suite. Loaded by `pymodbus.simulator`. See ../README.md for the launch command. Holding registers 0..31 are seeded with their address as value (HR[5]=5) for easy mental-map diagnostics. HR[100] auto-increments via pymodbus's built-in `increment` action so subscribe-and-receive integration tests have a register that ticks without a write. HR[200..209] is a scratch range left at 0 for write-roundtrip tests. Coils 0..31 alternate on/off (even=on); coils 100..109 scratch.",
+  "_comment": "Standard.json — generic Modbus TCP server for the integration suite. See ../README.md. NOTE: pymodbus rejects unknown keys at device-list / setup level; explanatory comments live in the README + git history. Layout: HR[0..31]=address-as-value, HR[100]=auto-increment, HR[200..209]=scratch, coils 1024..1055=alternating, coils 1100..1109=scratch. Coils live at 1024+ because pymodbus stores all 4 standard tables in ONE underlying cell array — bits and uint16 at the same address conflict (each cell can only be typed once).",
 
   "server_list": {
     "srv": {
@@ -14,11 +14,11 @@
   "device_list": {
     "dev": {
       "setup": {
-        "co size": 1024,
-        "di size": 1024,
-        "hr size": 1024,
-        "ir size": 1024,
-        "shared blocks": false,
+        "co size": 2048,
+        "di size": 2048,
+        "hr size": 2048,
+        "ir size": 2048,
+        "shared blocks": true,
         "type exception": false,
         "defaults": {
           "value":  {"bits": 0, "uint16": 0, "uint32": 0, "float32": 0.0, "string": " "},
@@ -27,26 +27,11 @@
       },
       "invalid": [],
       "write": [
-        [0, 1023]
-      ],
-
-      "bits": [
-        {"addr": 0,  "value": 1}, {"addr": 1,  "value": 0},
-        {"addr": 2,  "value": 1}, {"addr": 3,  "value": 0},
-        {"addr": 4,  "value": 1}, {"addr": 5,  "value": 0},
-        {"addr": 6,  "value": 1}, {"addr": 7,  "value": 0},
-        {"addr": 8,  "value": 1}, {"addr": 9,  "value": 0},
-        {"addr": 10, "value": 1}, {"addr": 11, "value": 0},
-        {"addr": 12, "value": 1}, {"addr": 13, "value": 0},
-        {"addr": 14, "value": 1}, {"addr": 15, "value": 0},
-        {"addr": 16, "value": 1}, {"addr": 17, "value": 0},
-        {"addr": 18, "value": 1}, {"addr": 19, "value": 0},
-        {"addr": 20, "value": 1}, {"addr": 21, "value": 0},
-        {"addr": 22, "value": 1}, {"addr": 23, "value": 0},
-        {"addr": 24, "value": 1}, {"addr": 25, "value": 0},
-        {"addr": 26, "value": 1}, {"addr": 27, "value": 0},
-        {"addr": 28, "value": 1}, {"addr": 29, "value": 0},
-        {"addr": 30, "value": 1}, {"addr": 31, "value": 0}
+        [0, 31],
+        [100, 100],
+        [200, 209],
+        [1024, 1055],
+        [1100, 1109]
       ],
 
       "uint16": [
@@ -69,7 +54,38 @@
 
         {"addr": 100, "value": 0,
          "action": "increment",
-         "parameters": {"minval": 0, "maxval": 65535}}
+         "parameters": {"minval": 0, "maxval": 65535}},
+
+        {"addr": 200, "value": 0}, {"addr": 201, "value": 0},
+        {"addr": 202, "value": 0}, {"addr": 203, "value": 0},
+        {"addr": 204, "value": 0}, {"addr": 205, "value": 0},
+        {"addr": 206, "value": 0}, {"addr": 207, "value": 0},
+        {"addr": 208, "value": 0}, {"addr": 209, "value": 0}
+      ],
+
+      "bits": [
+        {"addr": 1024, "value": 1}, {"addr": 1025, "value": 0},
+        {"addr": 1026, "value": 1}, {"addr": 1027, "value": 0},
+        {"addr": 1028, "value": 1}, {"addr": 1029, "value": 0},
+        {"addr": 1030, "value": 1}, {"addr": 1031, "value": 0},
+        {"addr": 1032, "value": 1}, {"addr": 1033, "value": 0},
+        {"addr": 1034, "value": 1}, {"addr": 1035, "value": 0},
+        {"addr": 1036, "value": 1}, {"addr": 1037, "value": 0},
+        {"addr": 1038, "value": 1}, {"addr": 1039, "value": 0},
+        {"addr": 1040, "value": 1}, {"addr": 1041, "value": 0},
+        {"addr": 1042, "value": 1}, {"addr": 1043, "value": 0},
+        {"addr": 1044, "value": 1}, {"addr": 1045, "value": 0},
+        {"addr": 1046, "value": 1}, {"addr": 1047, "value": 0},
+        {"addr": 1048, "value": 1}, {"addr": 1049, "value": 0},
+        {"addr": 1050, "value": 1}, {"addr": 1051, "value": 0},
+        {"addr": 1052, "value": 1}, {"addr": 1053, "value": 0},
+        {"addr": 1054, "value": 1}, {"addr": 1055, "value": 0},
+
+        {"addr": 1100, "value": 0}, {"addr": 1101, "value": 0},
+        {"addr": 1102, "value": 0}, {"addr": 1103, "value": 0},
+        {"addr": 1104, "value": 0}, {"addr": 1105, "value": 0},
+        {"addr": 1106, "value": 0}, {"addr": 1107, "value": 0},
+        {"addr": 1108, "value": 0}, {"addr": 1109, "value": 0}
       ],
 
       "uint32":  [],

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

@@ -172,4 +172,144 @@ public sealed class ModbusDataTypeTests
         wire[1].ShouldBe((byte)'i');
         for (var i = 2; i < 8; i++) wire[i].ShouldBe((byte)0);
     }
+
+    // --- DL205 low-byte-first strings (AutomationDirect DirectLOGIC quirk) ---
+
+    [Fact]
+    public void String_LowByteFirst_decodes_DL205_packed_Hello()
+    {
+        // HR[1040] = 0x6548 (wire BE bytes [0x65, 0x48]) decodes first char from low byte = 'H',
+        // second from high byte = 'e'. HR[1041] = 0x6C6C → 'l','l'. HR[1042] = 0x006F → 'o', nul.
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 5, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
+        var wire = new byte[] { 0x65, 0x48, 0x6C, 0x6C, 0x00, 0x6F };
+        ModbusDriver.DecodeRegister(wire, tag).ShouldBe("Hello");
+    }
+
+    [Fact]
+    public void String_LowByteFirst_decode_truncates_at_first_nul()
+    {
+        // Low-byte-first with only 2 real chars in register 0 (lo='H', hi='i') and the rest nul.
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 6, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
+        var wire = new byte[] { 0x69, 0x48, 0x00, 0x00, 0x00, 0x00 };
+        ModbusDriver.DecodeRegister(wire, tag).ShouldBe("Hi");
+    }
+
+    [Fact]
+    public void String_LowByteFirst_encode_round_trips_with_decode()
+    {
+        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 5, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
+        var wire = ModbusDriver.EncodeRegister("Hello", tag);
+        // Expect exactly the DL205-documented byte sequence.
+        wire.ShouldBe(new byte[] { 0x65, 0x48, 0x6C, 0x6C, 0x00, 0x6F });
+        ModbusDriver.DecodeRegister(wire, tag).ShouldBe("Hello");
+    }
+
+    [Fact]
+    public void String_HighByteFirst_and_LowByteFirst_differ_on_same_wire()
+    {
+        // Same wire buffer, different byte order → first char switches 'H' vs 'e'.
+        var wire = new byte[] { 0x48, 0x65 };
+        var hi = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 2, StringByteOrder: ModbusStringByteOrder.HighByteFirst);
+        var lo = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
+            StringLength: 2, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
+        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);
+    }
 }