diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs
index 2cce003..d65eafe 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs
@@ -102,6 +102,19 @@ public sealed record AbLegacyAddress(
             if (maxBit < 0 || b > maxBit) return null;
         }
 
+        // I/O/S are single-letter system files — they carry no file number in the PCCC spec.
+        // Accepting I3:0 or S2:1 would pass a malformed address straight to libplctag; reject early.
+        if (fileNumber is not null && IsNoFileNumberLetter(letter)) return null;
+
+        // A PCCC address cannot have both a sub-element and a bit index: the word is either
+        // structured (T4:0.ACC) or bit-addressed (N7:0/3), never both.
+        if (subElement is not null && bitIndex is not null) return null;
+
+        // Sub-elements are only meaningful on Timer (T), Counter (C), and Control (R) files —
+        // those are the only structured-element file types in the PCCC spec. Accepting B3:0.DN
+        // or N7:0.FOO would produce an address libplctag silently misinterprets.
+        if (subElement is not null && !IsSubElementFileLetter(letter)) return null;
+
         return new AbLegacyAddress(letter, fileNumber, word, bitIndex, subElement);
     }
 
@@ -122,4 +135,18 @@ 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>
+    ///     Returns <see langword="true"/> for file letters that carry no explicit file number in the
+    ///     PCCC spec. <c>I</c> (input), <c>O</c> (output), and <c>S</c> (status) are single-letter
+    ///     system files; a digit after the letter (e.g. <c>I3</c>) is a malformed address.
+    /// </summary>
+    private static bool IsNoFileNumberLetter(string letter) => letter is "I" or "O" or "S";
+
+    /// <summary>
+    ///     Returns <see langword="true"/> for file letters that may carry a sub-element suffix
+    ///     (<c>.ACC</c>, <c>.PRE</c>, etc.). Only Timer (<c>T</c>), Counter (<c>C</c>), and
+    ///     Control (<c>R</c>) files have structured elements in the PCCC spec.
+    /// </summary>
+    private static bool IsSubElementFileLetter(string letter) => letter is "T" or "C" or "R";
 }
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs
index d032357..9609e67 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyAddressTests.cs
@@ -65,4 +65,41 @@ public sealed class AbLegacyAddressTests
         a.ShouldNotBeNull();
         a.ToLibplctagName().ShouldBe(input);
     }
+
+    // ---- Driver.AbLegacy-003: Parser tightening ----
+
+    [Theory]
+    [InlineData("T4:0.ACC/2")]      // sub-element + bit index — never valid in PCCC
+    [InlineData("C5:0.PRE/3")]
+    public void TryParse_rejects_subelement_plus_bitindex(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+
+    [Theory]
+    [InlineData("I3:0")]            // I is a system file — no file number allowed
+    [InlineData("O2:1")]
+    [InlineData("S2:1")]
+    public void TryParse_rejects_file_number_on_IOS_files(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+
+    [Theory]
+    [InlineData("B3:0.DN")]         // B (bit) file has no structured elements
+    [InlineData("N7:0.FOO")]        // N (integer) file has no structured elements
+    [InlineData("F8:0.ACC")]        // F (float) file has no structured elements
+    [InlineData("L9:0.PRE")]        // L (long) file has no structured elements
+    public void TryParse_rejects_subelement_on_non_structured_file(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldBeNull();
+
+    [Theory]
+    [InlineData("T4:0.ACC")]        // T, C, R are the only structured-element files
+    [InlineData("C5:0.PRE")]
+    [InlineData("R6:0.LEN")]
+    public void TryParse_accepts_subelement_only_on_TCR_files(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldNotBeNull();
+
+    [Theory]
+    [InlineData("I:0/0")]           // I/O/S without file number are valid
+    [InlineData("O:1/2")]
+    [InlineData("S:1")]
+    public void TryParse_accepts_IOS_without_file_number(string input) =>
+        AbLegacyAddress.TryParse(input).ShouldNotBeNull();
 }