From 463c5a43201889546aac7debae177e087d8b47c1 Mon Sep 17 00:00:00 2001
From: Joseph Doherty <dohejw01@gmail.com>
Date: Sat, 18 Apr 2026 21:51:15 -0400
Subject: [PATCH] Phase 3 PR 48 -- DL205 CDAB word order for Float32 end-to-end
 test. The driver has supported ModbusByteOrder.WordSwap (CDAB) since PR 24
 for all multi-register types -- the underlying word-swap code path was
 already there. PR 48 closes the loop with an integration test that validates
 it end-to-end against the dl205 pymodbus profile: HR[1056..1057] stores
 IEEE-754 1.5f with the low word at the lower address (0x0000 at HR[1056],
 0x3FC0 at HR[1057]). Reading with WordSwap returns 1.5f; reading with
 BigEndian returns a tiny denormal (~5.74e-41) -- a silent "value is 0" bug
 that typically surfaces in the field only when an operator notices a setpoint
 readout stuck at 0 while the PLC display shows the real value. Test asserts
 both: WordSwap==1.5f AND BigEndian!=1.5f, proving the flag is not a no-op. No
 driver code changes -- the word-swap normalization at NormalizeWordOrder()
 has handled Float32/Int32/UInt32 correctly since PR 24 and the unit test
 suite already covers it (Int32_WordSwap_decodes_CDAB_layout + Float32
 equivalent). This PR exists primarily to lock in the integration-level
 validation so future refactors of the codec don't silently break DL205/DL260
 floats. 6/6 DL205 integration tests pass with MODBUS_SIM_PROFILE=dl205.

---
 .../DL205/DL205FloatCdabQuirkTests.cs         | 64 +++++++++++++++++++
 1 file changed, 64 insertions(+)
 create mode 100644 tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205FloatCdabQuirkTests.cs

diff --git a/tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205FloatCdabQuirkTests.cs b/tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205FloatCdabQuirkTests.cs
new file mode 100644
index 0000000..0d04148
--- /dev/null
+++ b/tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests/DL205/DL205FloatCdabQuirkTests.cs
@@ -0,0 +1,64 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.IntegrationTests.DL205;
+
+/// <summary>
+///     Verifies DL205/DL260 CDAB word ordering for 32-bit floats against the
+///     <c>dl205.json</c> pymodbus profile. DirectLOGIC stores IEEE-754 singles with the low
+///     word at the lower register address (CDAB) rather than the high word (ABCD). Reading
+///     <c>HR[1056..1057]</c> with <see cref="ModbusByteOrder.BigEndian"/> produces a tiny
+///     denormal (~5.74e-41) instead of the intended 1.5f — a silent "value is 0" bug in the
+///     field unless the caller opts into <see cref="ModbusByteOrder.WordSwap"/>.
+/// </summary>
+[Collection(ModbusSimulatorCollection.Name)]
+[Trait("Category", "Integration")]
+[Trait("Device", "DL205")]
+public sealed class DL205FloatCdabQuirkTests(ModbusSimulatorFixture sim)
+{
+    [Fact]
+    public async Task DL205_Float32_CDAB_decodes_1_5f_from_HR1056()
+    {
+        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[1056..1057]).");
+        }
+
+        var options = new ModbusDriverOptions
+        {
+            Host = sim.Host,
+            Port = sim.Port,
+            UnitId = 1,
+            Timeout = TimeSpan.FromSeconds(2),
+            Tags =
+            [
+                new ModbusTagDefinition("DL205_Float_CDAB",
+                    ModbusRegion.HoldingRegisters, Address: 1056,
+                    DataType: ModbusDataType.Float32, Writable: false,
+                    ByteOrder: ModbusByteOrder.WordSwap),
+                // Control: same address, BigEndian — proves the default decode produces garbage.
+                new ModbusTagDefinition("DL205_Float_ABCD",
+                    ModbusRegion.HoldingRegisters, Address: 1056,
+                    DataType: ModbusDataType.Float32, Writable: false,
+                    ByteOrder: ModbusByteOrder.BigEndian),
+            ],
+            Probe = new ModbusProbeOptions { Enabled = false },
+        };
+        await using var driver = new ModbusDriver(options, driverInstanceId: "dl205-cdab");
+        await driver.InitializeAsync("{}", TestContext.Current.CancellationToken);
+
+        var results = await driver.ReadAsync(["DL205_Float_CDAB", "DL205_Float_ABCD"],
+            TestContext.Current.CancellationToken);
+
+        results[0].StatusCode.ShouldBe(0u);
+        results[0].Value.ShouldBe(1.5f, "DL205 Float32 with WordSwap (CDAB) must decode HR[1056..1057] as 1.5f");
+
+        // The BigEndian read of the same wire bytes should differ — not asserting the exact
+        // denormal value (that couples the test to IEEE-754 bit math) but the two decodes MUST
+        // disagree, otherwise the word-order flag is a no-op.
+        results[1].StatusCode.ShouldBe(0u);
+        results[1].Value.ShouldNotBe(1.5f);
+    }
+}
-- 
2.49.1