diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
index b219d86a..430fbedb 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
@@ -360,12 +360,14 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
             {
                 var parsed = AbLegacyAddress.TryParse(def.Address);
 
-                // PCCC bit-within-word writes — task #181 pass 2. RMW against a parallel
-                // parent-word runtime (strip the /N bit suffix). Per-parent-word lock serialises
-                // concurrent bit writers. Applies to N-file bit-in-word (N7:0/3) + B-file bits
-                // (B3:0/0). T/C/R sub-elements don't hit this path because they're not Bit typed.
-                if (def.DataType == AbLegacyDataType.Bit && parsed?.BitIndex is int bit
-                    && parsed.FileLetter is not "B" and not "I" and not "O")
+                // PCCC bit-within-word writes — RMW against a parallel parent-word runtime (strip the /N
+                // bit suffix). The per-parent-word lock serialises concurrent bit writers. Applies to every
+                // bit-addressable PCCC file: N-file (N7:0/3), B-file (B3:0/0), and the I/O image files
+                // (I:0/0, O:1/2); L-file bits RMW a 32-bit parent, the rest a 16-bit word. T/C/R sub-elements
+                // don't reach this path because they're not Bit typed. NOTE: an Input-image (I) write is sent
+                // to the PLC like any other write — the device drives the input image from physical inputs and
+                // may reject it; we surface that real PCCC status rather than pre-rejecting at the driver.
+                if (def.DataType == AbLegacyDataType.Bit && parsed?.BitIndex is int bit)
                 {
                     results[i] = new WriteResult(
                         await WriteBitInWordAsync(device, parsed, bit, w.Value, cancellationToken).ConfigureAwait(false));
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyBitRmwTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyBitRmwTests.cs
index b1ba8804..8f0b07a0 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyBitRmwTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyBitRmwTests.cs
@@ -105,4 +105,76 @@ public sealed class AbLegacyBitRmwTests
         factory.Tags["N7:0"].WriteCount.ShouldBe(2);
         Convert.ToInt32(factory.Tags["N7:0"].Value).ShouldBe(0x21); // bits 0 + 5
     }
+
+    /// <summary>B/I/O-file bit set reads the parent word, ORs the bit, writes it back (was BadNotSupported).</summary>
+    [Theory]
+    [InlineData("B3:0/3", "B3:0")]
+    [InlineData("I:0/3", "I:0")]
+    [InlineData("O:1/3", "O:1")]
+    public async Task Bit_set_on_B_I_O_file_RMWs_parent_word(string bitAddr, string parentName)
+    {
+        var factory = new FakeAbLegacyTagFactory
+        {
+            Customise = p => new FakeAbLegacyTag(p) { Value = (short)0b0001 },
+        };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbLegacyTagDefinition("F", "ab://10.0.0.5/1,0", bitAddr, AbLegacyDataType.Bit)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        var results = await drv.WriteAsync([new WriteRequest("F", true)], CancellationToken.None);
+
+        results.Single().StatusCode.ShouldBe(AbLegacyStatusMapper.Good);
+        factory.Tags.ShouldContainKey(parentName);
+        Convert.ToInt32(factory.Tags[parentName].Value).ShouldBe(0b1001);
+    }
+
+    /// <summary>B-file bit clear preserves the other bits in the parent word.</summary>
+    [Fact]
+    public async Task Bit_clear_preserves_other_bits_in_B_file_word()
+    {
+        var factory = new FakeAbLegacyTagFactory
+        {
+            Customise = p => new FakeAbLegacyTag(p) { Value = unchecked((short)0xFFFF) },
+        };
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = [new AbLegacyTagDefinition("F", "ab://10.0.0.5/1,0", "B3:0/3", AbLegacyDataType.Bit)],
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await drv.WriteAsync([new WriteRequest("F", false)], CancellationToken.None);
+
+        Convert.ToInt32(factory.Tags["B3:0"].Value).ShouldBe(unchecked((short)0xFFF7));
+    }
+
+    /// <summary>Concurrent bit writes to the same B-file word compose correctly (per-parent lock).</summary>
+    [Fact]
+    public async Task Concurrent_bit_writes_to_same_B_file_word_compose_correctly()
+    {
+        var factory = new FakeAbLegacyTagFactory
+        {
+            Customise = p => new FakeAbLegacyTag(p) { Value = (short)0 },
+        };
+        var tags = Enumerable.Range(0, 8)
+            .Select(b => new AbLegacyTagDefinition($"Bit{b}", "ab://10.0.0.5/1,0", $"B3:0/{b}", AbLegacyDataType.Bit))
+            .ToArray();
+        var drv = new AbLegacyDriver(new AbLegacyDriverOptions
+        {
+            Devices = [new AbLegacyDeviceOptions("ab://10.0.0.5/1,0")],
+            Tags = tags,
+            Probe = new AbLegacyProbeOptions { Enabled = false },
+        }, "drv-1", factory);
+        await drv.InitializeAsync("{}", CancellationToken.None);
+
+        await Task.WhenAll(Enumerable.Range(0, 8).Select(b =>
+            drv.WriteAsync([new WriteRequest($"Bit{b}", true)], CancellationToken.None)));
+
+        Convert.ToInt32(factory.Tags["B3:0"].Value).ShouldBe(0xFF);
+    }
 }