diff --git a/docs/drivers/AbLegacy.md b/docs/drivers/AbLegacy.md
index c445a270..88ae97a6 100644
--- a/docs/drivers/AbLegacy.md
+++ b/docs/drivers/AbLegacy.md
@@ -100,7 +100,7 @@ physical inputs and may reject the write.
   re-parses carry identical numbers. This is not a double conversion: libplctag's `pccc.c`
   (`parse_pccc_elem_num` / `parse_pccc_bit_num`) parses decimal numerals only (a FIXME there says octal is
   unsupported), so the driver must convert. Pinned by `Plc5_io_indices_are_octal` (`I:017/17` -> `I:15/15`).
-  The connectivity-probe address is normalised the same way.
+  The connectivity-probe address is normalised the same way — by both the driver's probe loop and the Test Connect probe, through one helper (`AbLegacyProbeAddress.Normalise`, #660); an unparseable Test Connect probe address fails cleanly ("not a valid AbLegacy address") instead of reaching the transport. Normalisation yields the canonical name; the libplctag `S:0` → `S2:0` translation (#659) happens downstream in the runtime, so the two compose.
   - ⚠️ **Behaviour change / migration.** Before this change a PLC-5 `I:`/`O:` address was passed to libplctag as
     written, i.e. read as **decimal**. An address copied from RSLogix in octal (e.g. `I:017`) therefore read the
     wrong word, and an authored decimal such as `I:1/12` now means bit 10 (octal 12). A previously "working"
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 4afe4fe7..a352a803 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs
@@ -693,7 +693,7 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
             LibplctagPlcAttribute: state.Profile.LibplctagPlcAttribute,
             // Normalised exactly like tag paths (PLC-5 octal I/O -> decimal); an unparseable probe address is passed through
             // so the transport reports the failure as it always did.
-            TagName: AbLegacyAddress.TryParse(_options.Probe.ProbeAddress, state.Options.PlcFamily)?.ToLibplctagName()
+            TagName: AbLegacyProbeAddress.Normalise(_options.Probe.ProbeAddress, state.Options.PlcFamily)
                      ?? _options.Probe.ProbeAddress!,
             Timeout: _options.Probe.Timeout);
 
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverProbe.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverProbe.cs
index d1b05167..6d5eed4b 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverProbe.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriverProbe.cs
@@ -98,12 +98,18 @@ public sealed class AbLegacyDriverProbe : IDriverProbe
             ?? FirstProbeableTagAddress(opts, firstDevice)
             ?? "S:0";
 
+        // Normalise through the same helper as the driver's probe loop (#660): PLC-5 octal I/O -> decimal.
+        // An unparseable address is a clean failure here — the factory would otherwise throw outside the try.
+        var normalisedName = AbLegacyProbeAddress.Normalise(tagName, firstDevice?.PlcFamily ?? AbLegacyPlcFamily.Slc500);
+        if (normalisedName is null)
+            return new(false, $"Probe address '{tagName}' is not a valid AbLegacy address for this PLC family.", null);
+
         var p = new AbLegacyTagCreateParams(
             Gateway: parsed.Gateway,
             Port: parsed.Port,
             CipPath: parsed.CipPath,
             LibplctagPlcAttribute: profile.LibplctagPlcAttribute,
-            TagName: tagName,
+            TagName: normalisedName,
             Timeout: timeout);
 
         var factory = _tagFactory ?? (AbLegacyTransportResolver.Resolve(opts) == AbLegacyTransport.Native
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyProbeAddress.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyProbeAddress.cs
new file mode 100644
index 00000000..93f192de
--- /dev/null
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyProbeAddress.cs
@@ -0,0 +1,16 @@
+using ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.PlcFamilies;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy;
+
+/// <summary>
+///     The single authority that turns an operator-authored probe address into the name handed to the tag
+///     factory: family-aware parse (PLC-5 <c>I:</c>/<c>O:</c> octal indices become decimal) then
+///     <see cref="AbLegacyAddress.ToLibplctagName"/> — the same normalisation tag paths get. Shared by the
+///     driver's probe loop and the Test Connect probe (#660).
+/// </summary>
+internal static class AbLegacyProbeAddress
+{
+    /// <summary>Returns the normalised name, or null when <paramref name="address"/> does not parse for the family.</summary>
+    public static string? Normalise(string? address, AbLegacyPlcFamily family) =>
+        AbLegacyAddress.TryParse(address, family)?.ToLibplctagName();
+}
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyDriverProbeTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyDriverProbeTests.cs
index 1cf1591b..f58536d7 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyDriverProbeTests.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/AbLegacyDriverProbeTests.cs
@@ -5,6 +5,7 @@
 using System.Net;
 using System.Net.Sockets;
 using Shouldly;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 using Xunit;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests;
@@ -147,4 +148,56 @@ public sealed class AbLegacyDriverProbeTests
             $"Expected 'timed out' or 'Connect failed' but got: {result.Message}");
         result.Latency.ShouldBeNull();
     }
+
+    // ---- #660: Test Connect normalises the probe address exactly like the driver's probe loop ----
+
+    private static async Task<(DriverProbeResult Result, string? SeenName)> ProbeWithFakeAsync(string family, string? probeAddress)
+    {
+        var listener = new TcpListener(IPAddress.Loopback, 0);
+        listener.Start();
+        try
+        {
+            var port = ((IPEndPoint)listener.LocalEndpoint).Port;
+            var factory = new FakeAbLegacyTagFactory();
+            var probe = new AbLegacyDriverProbe(factory);
+            var addr = probeAddress is null ? "" : $",\"probe\":{{\"probeAddress\":\"{probeAddress}\"}}";
+            var json = $"{{\"devices\":[{{\"hostAddress\":\"ab://127.0.0.1:{port}/1,0\",\"plcFamily\":\"{family}\"}}]{addr}}}";
+            var result = await probe.ProbeAsync(json, TimeSpan.FromSeconds(3), TestContext.Current.CancellationToken);
+            return (result, factory.Tags.Keys.SingleOrDefault());
+        }
+        finally { listener.Stop(); }
+    }
+
+    [Theory]
+    [InlineData("Plc5", "I:017", "I:15")]
+    [InlineData("Plc5", "O:010/07", "O:8/7")]
+    [InlineData("LogixPccc", "I:017", "I:17")]
+    [InlineData("Slc500", "I:017", "I:17")]
+    [InlineData("MicroLogix", "I:017", "I:17")]
+    [InlineData("Slc500", "N7:0", "N7:0")]
+    [InlineData("Slc500", null, "S:0")]
+    public async Task Probe_address_reaches_the_factory_family_normalised(string family, string? configured, string expected)
+    {
+        var (result, seen) = await ProbeWithFakeAsync(family, configured);
+        result.Ok.ShouldBeTrue(result.Message);
+        seen.ShouldBe(expected);
+    }
+
+    [Fact]
+    public async Task Normalised_default_probe_then_composes_with_the_libplctag_translation()
+    {
+        var (_, seen) = await ProbeWithFakeAsync("Plc5", "I:017");
+        LibplctagLegacyName.Translate(seen!).ShouldBe("I1:15");
+        var (_, status) = await ProbeWithFakeAsync("Slc500", null);
+        LibplctagLegacyName.Translate(status!).ShouldBe("S2:0");
+    }
+
+    [Fact]
+    public async Task Unparseable_probe_address_is_a_clean_failure_not_an_exception()
+    {
+        var (result, seen) = await ProbeWithFakeAsync("Slc500", "not-an-address");
+        result.Ok.ShouldBeFalse();
+        result.Message!.ShouldContain("not-an-address");
+        seen.ShouldBeNull();
+    }
 }