diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs
index f255362..54c67f7 100644
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs
@@ -107,7 +107,11 @@ public sealed record AbLegacyAddress(
         }
 
         // Reject unknown file letters — these cover SLC/ML/PLC-5 canonical families.
-        if (!IsKnownFileLetter(letter)) return null;
+        // Function-file letters (RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI) are MicroLogix-only.
+        if (!IsKnownFileLetter(letter))
+        {
+            if (!IsFunctionFileLetter(letter) || profile?.SupportsFunctionFiles != true) return null;
+        }
 
         var octalForIo = profile?.OctalIoAddressing == true && (letter == "I" || letter == "O");
 
@@ -151,4 +155,14 @@ public sealed record AbLegacyAddress(
         "N" or "F" or "B" or "L" or "ST" or "T" or "C" or "R" or "I" or "O" or "S" or "A" => true,
         _ => false,
     };
+
+    /// <summary>
+    ///     MicroLogix 1100/1400 function-file prefixes. Each maps to a single fixed instance with a
+    ///     known sub-element catalogue (see <see cref="AbLegacyDataType"/>).
+    /// </summary>
+    internal static bool IsFunctionFileLetter(string letter) => letter switch
+    {
+        "RTC" or "HSC" or "DLS" or "MMI" or "PTO" or "PWM" or "STI" or "EII" or "IOS" or "BHI" => true,
+        _ => false,
+    };
 }
diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDataType.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDataType.cs
index 8e5bfad..48e253d 100644
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDataType.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDataType.cs
@@ -26,6 +26,72 @@ public enum AbLegacyDataType
     CounterElement,
     /// <summary>Control sub-element — caller addresses <c>.LEN</c>, <c>.POS</c>, <c>.EN</c>, <c>.DN</c>, <c>.ER</c>.</summary>
     ControlElement,
+    /// <summary>
+    ///     MicroLogix 1100/1400 function-file sub-element (RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI).
+    ///     Sub-element catalogue lives in <see cref="AbLegacyFunctionFile.SubElementType"/>.
+    /// </summary>
+    MicroLogixFunctionFile,
+}
+
+/// <summary>
+///     MicroLogix function-file sub-element catalogue. Covers the most-commonly-addressed members
+///     per file — not exhaustive (Rockwell defines 30+ on RTC alone). Unknown sub-elements fall
+///     back to <see cref="DriverDataType.Int32"/> at the <see cref="AbLegacyDataTypeExtensions"/>
+///     boundary so the driver never refuses a tag the customer happens to know about.
+/// </summary>
+public static class AbLegacyFunctionFile
+{
+    /// <summary>
+    ///     Driver-surface type for <paramref name="fileLetter"/>.<paramref name="subElement"/>.
+    ///     Returns <see cref="DriverDataType.Int32"/> if the sub-element is unrecognised — keeps
+    ///     the driver permissive without forcing every quirk into the catalogue.
+    /// </summary>
+    public static DriverDataType SubElementType(string fileLetter, string? subElement)
+    {
+        if (subElement is null) return DriverDataType.Int32;
+        var key = (fileLetter.ToUpperInvariant(), subElement.ToUpperInvariant());
+        return key switch
+        {
+            // Real-time clock — all stored as Int16 (year is 4-digit Int16).
+            ("RTC", "HR") or ("RTC", "MIN") or ("RTC", "SEC") or
+            ("RTC", "MON") or ("RTC", "DAY") or ("RTC", "YR") or ("RTC", "DOW") => DriverDataType.Int32,
+            ("RTC", "DS") or ("RTC", "BL") or ("RTC", "EN") => DriverDataType.Boolean,
+
+            // High-speed counter — accumulator/preset are Int32, status flags are bits.
+            ("HSC", "ACC") or ("HSC", "PRE") or ("HSC", "OVF") or ("HSC", "UNF") => DriverDataType.Int32,
+            ("HSC", "EN") or ("HSC", "UF") or ("HSC", "IF") or
+            ("HSC", "IN") or ("HSC", "IH") or ("HSC", "IL") or
+            ("HSC", "DN") or ("HSC", "CD") or ("HSC", "CU") => DriverDataType.Boolean,
+
+            // Daylight saving + memory module info.
+            ("DLS", "STR") or ("DLS", "STD") => DriverDataType.Int32,
+            ("DLS", "EN") => DriverDataType.Boolean,
+            ("MMI", "FT") or ("MMI", "LBN") => DriverDataType.Int32,
+            ("MMI", "MP") or ("MMI", "MCP") => DriverDataType.Boolean,
+
+            // Pulse-train / PWM output blocks.
+            ("PTO", "ACC") or ("PTO", "OF") or ("PTO", "IDA") or ("PTO", "ODA") => DriverDataType.Int32,
+            ("PTO", "EN") or ("PTO", "DN") or ("PTO", "EH") or ("PTO", "ED") or
+            ("PTO", "RP") or ("PTO", "OUT") => DriverDataType.Boolean,
+            ("PWM", "ACC") or ("PWM", "OF") or ("PWM", "PE") or ("PWM", "PD") => DriverDataType.Int32,
+            ("PWM", "EN") or ("PWM", "DN") or ("PWM", "EH") or ("PWM", "ED") or
+            ("PWM", "RP") or ("PWM", "OUT") => DriverDataType.Boolean,
+
+            // Selectable timed interrupt + event input interrupt.
+            ("STI", "SPM") or ("STI", "ER") or ("STI", "PFN") => DriverDataType.Int32,
+            ("STI", "EN") or ("STI", "TIE") or ("STI", "DN") or
+            ("STI", "PS") or ("STI", "ED") => DriverDataType.Boolean,
+            ("EII", "PFN") or ("EII", "ER") => DriverDataType.Int32,
+            ("EII", "EN") or ("EII", "TIE") or ("EII", "PE") or
+            ("EII", "ES") or ("EII", "ED") => DriverDataType.Boolean,
+
+            // I/O status + base hardware info — mostly status flags + a few counters.
+            ("IOS", "ID") or ("IOS", "TYP") => DriverDataType.Int32,
+            ("BHI", "OS") or ("BHI", "FRN") or ("BHI", "BSN") or ("BHI", "CC") => DriverDataType.Int32,
+
+            _ => DriverDataType.Int32,
+        };
+    }
 }
 
 /// <summary>Map a PCCC data type to the driver-surface <see cref="DriverDataType"/>.</summary>
@@ -40,6 +106,7 @@ public static class AbLegacyDataTypeExtensions
         AbLegacyDataType.String => DriverDataType.String,
         AbLegacyDataType.TimerElement or AbLegacyDataType.CounterElement
             or AbLegacyDataType.ControlElement => DriverDataType.Int32,
+        AbLegacyDataType.MicroLogixFunctionFile => DriverDataType.Int32,
         _ => DriverDataType.Int32,
     };
 }
diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/PlcFamilies/AbLegacyPlcFamilyProfile.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/PlcFamilies/AbLegacyPlcFamilyProfile.cs
index cb81574..7cdea61 100644
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/PlcFamilies/AbLegacyPlcFamilyProfile.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/PlcFamilies/AbLegacyPlcFamilyProfile.cs
@@ -10,7 +10,8 @@ public sealed record AbLegacyPlcFamilyProfile(
     int MaxTagBytes,
     bool SupportsStringFile,
     bool SupportsLongFile,
-    bool OctalIoAddressing)
+    bool OctalIoAddressing,
+    bool SupportsFunctionFiles)
 {
     public static AbLegacyPlcFamilyProfile ForFamily(AbLegacyPlcFamily family) => family switch
     {
@@ -27,7 +28,8 @@ public sealed record AbLegacyPlcFamilyProfile(
         MaxTagBytes: 240,           // SLC 5/05 PCCC max packet data
         SupportsStringFile: true,   // ST file available SLC 5/04+
         SupportsLongFile: true,     // L file available SLC 5/05+
-        OctalIoAddressing: false);  // SLC500 I:/O: indices are decimal in RSLogix 500
+        OctalIoAddressing: false,   // SLC500 I:/O: indices are decimal in RSLogix 500
+        SupportsFunctionFiles: false); // SLC500 has no function files
 
     public static readonly AbLegacyPlcFamilyProfile MicroLogix = new(
         LibplctagPlcAttribute: "micrologix",
@@ -35,7 +37,8 @@ public sealed record AbLegacyPlcFamilyProfile(
         MaxTagBytes: 232,
         SupportsStringFile: true,
         SupportsLongFile: false,    // ML 1100/1200/1400 don't ship L files
-        OctalIoAddressing: false);  // MicroLogix follows SLC-style decimal I/O addressing
+        OctalIoAddressing: false,   // MicroLogix follows SLC-style decimal I/O addressing
+        SupportsFunctionFiles: true); // ML 1100/1400 expose RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI
 
     public static readonly AbLegacyPlcFamilyProfile Plc5 = new(
         LibplctagPlcAttribute: "plc5",
@@ -43,7 +46,8 @@ public sealed record AbLegacyPlcFamilyProfile(
         MaxTagBytes: 240,           // DF1 full-duplex packet limit at 264 bytes, PCCC-over-EIP caps lower
         SupportsStringFile: true,
         SupportsLongFile: false,    // PLC-5 predates L files
-        OctalIoAddressing: true);   // RSLogix 5 displays I:/O: word + bit indices as octal
+        OctalIoAddressing: true,    // RSLogix 5 displays I:/O: word + bit indices as octal
+        SupportsFunctionFiles: false);
 
     /// <summary>
     ///     Logix ControlLogix / CompactLogix accessed through the legacy PCCC compatibility layer.
@@ -56,7 +60,8 @@ public sealed record AbLegacyPlcFamilyProfile(
         MaxTagBytes: 240,
         SupportsStringFile: true,
         SupportsLongFile: true,
-        OctalIoAddressing: false);  // Logix natively uses decimal arrays even via the PCCC bridge
+        OctalIoAddressing: false,   // Logix natively uses decimal arrays even via the PCCC bridge
+        SupportsFunctionFiles: false);
 }
 
 /// <summary>Which PCCC PLC family the device is.</summary>
diff --git a/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs b/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs
index 8039a0f..a3fcc5b 100644
--- a/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs
+++ b/tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs
@@ -139,4 +139,83 @@ public sealed class AbLegacyAddressTests
         AbLegacyPlcFamilyProfile.MicroLogix.OctalIoAddressing.ShouldBeFalse();
         AbLegacyPlcFamilyProfile.LogixPccc.OctalIoAddressing.ShouldBeFalse();
     }
+
+    // ---- MicroLogix function-file letters (Issue #245) ----
+    //
+    // MicroLogix 1100/1400 expose RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI function files. Other
+    // PCCC families (SLC500 / PLC-5 / LogixPccc) reject those file letters.
+
+    [Theory]
+    [InlineData("RTC:0.HR", "RTC", "HR")]
+    [InlineData("RTC:0.MIN", "RTC", "MIN")]
+    [InlineData("RTC:0.YR", "RTC", "YR")]
+    [InlineData("HSC:0.ACC", "HSC", "ACC")]
+    [InlineData("HSC:0.PRE", "HSC", "PRE")]
+    [InlineData("HSC:0.EN", "HSC", "EN")]
+    [InlineData("DLS:0.STR", "DLS", "STR")]
+    [InlineData("PTO:0.OF", "PTO", "OF")]
+    [InlineData("PWM:0.EN", "PWM", "EN")]
+    [InlineData("STI:0.SPM", "STI", "SPM")]
+    [InlineData("EII:0.PFN", "EII", "PFN")]
+    [InlineData("MMI:0.FT", "MMI", "FT")]
+    [InlineData("BHI:0.OS", "BHI", "OS")]
+    [InlineData("IOS:0.ID", "IOS", "ID")]
+    public void TryParse_MicroLogix_accepts_function_files(string input, string expectedLetter, string expectedSub)
+    {
+        var a = AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.MicroLogix);
+        a.ShouldNotBeNull();
+        a.FileLetter.ShouldBe(expectedLetter);
+        a.SubElement.ShouldBe(expectedSub);
+    }
+
+    [Theory]
+    [InlineData("RTC:0.HR")]
+    [InlineData("HSC:0.ACC")]
+    [InlineData("PTO:0.OF")]
+    [InlineData("BHI:0.OS")]
+    public void TryParse_Slc500_rejects_function_files(string input)
+    {
+        AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.Slc500).ShouldBeNull();
+    }
+
+    [Theory]
+    [InlineData("RTC:0.HR")]
+    [InlineData("HSC:0.ACC")]
+    public void TryParse_Plc5_and_LogixPccc_reject_function_files(string input)
+    {
+        AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.Plc5).ShouldBeNull();
+        AbLegacyAddress.TryParse(input, AbLegacyPlcFamily.LogixPccc).ShouldBeNull();
+    }
+
+    [Fact]
+    public void TryParse_Default_overload_rejects_function_files()
+    {
+        // Without a family the parser cannot allow MicroLogix-only letters — back-compat with
+        // the family-less overload from before #244.
+        AbLegacyAddress.TryParse("RTC:0.HR").ShouldBeNull();
+        AbLegacyAddress.TryParse("HSC:0.ACC").ShouldBeNull();
+    }
+
+    [Fact]
+    public void MicroLogixProfile_advertises_function_file_support()
+    {
+        AbLegacyPlcFamilyProfile.MicroLogix.SupportsFunctionFiles.ShouldBeTrue();
+        AbLegacyPlcFamilyProfile.Slc500.SupportsFunctionFiles.ShouldBeFalse();
+        AbLegacyPlcFamilyProfile.Plc5.SupportsFunctionFiles.ShouldBeFalse();
+        AbLegacyPlcFamilyProfile.LogixPccc.SupportsFunctionFiles.ShouldBeFalse();
+    }
+
+    [Theory]
+    [InlineData("RTC", "HR", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Int32)]
+    [InlineData("RTC", "EN", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Boolean)]
+    [InlineData("HSC", "ACC", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Int32)]
+    [InlineData("HSC", "EN", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Boolean)]
+    [InlineData("DLS", "STR", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Int32)]
+    [InlineData("DLS", "EN", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Boolean)]
+    [InlineData("PWM", "OUT", ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType.Boolean)]
+    public void FunctionFile_subelement_catalogue_maps_to_expected_driver_type(
+        string letter, string sub, ZB.MOM.WW.OtOpcUa.Core.Abstractions.DriverDataType expected)
+    {
+        AbLegacyFunctionFile.SubElementType(letter, sub).ShouldBe(expected);
+    }
 }