Phase 3 PR 47 -- DL205 V-memory octal-address helper. Adds DirectLogicAddress static class with two entry points: UserVMemoryToPdu(string) parses a DirectLOGIC V-address (V-prefixed or bare, whitespace tolerated) as OCTAL and returns the 0-based Modbus PDU address. V2000 octal = decimal 1024 = PDU 0x0400, which is the canonical start of the user V-memory bank on DL205/DL260. SystemVMemoryBasePdu + SystemVMemoryToPdu(ushort offset) handle the system bank (V40400 and up) which does NOT follow the simple octal-to-decimal formula -- the CPU relocates the system bank to PDU 0x2100 in H2-ECOM100 absolute mode. A naive caller converting 40400 octal would land at PDU 0x4100 (decimal 16640) and miss the system registers entirely; the helper routes the correct 0x2100 base. Why this matters: DirectLOGIC operators think in OCTAL (the ladder-logic editor, the Productivity/Do-more UI, every AutomationDirect manual addresses V-memory octally) while the Modbus wire is DECIMAL. Integrators routinely copy V-addresses from the PLC documentation into client configs and read garbage because they treated V2000 as decimal 2000 (HR[2000] = 0 in the dl205 sim, zero in most PLCs). The helper makes the translation explicit per the D2-USER-M appendix + H2-ECOM-M \u00A76.5 references cited in docs/v2/dl205.md. Unit tests: UserVMemoryToPdu_converts_octal_V_prefix (V0, V1, V7, V10, V2000, V7777, V10000, V17777 -- the exact sweep documented in dl205.md), UserVMemoryToPdu_accepts_bare_or_prefixed_or_padded (case + whitespace tolerance), UserVMemoryToPdu_rejects_non_octal_digits (V8/V19/V2009 must throw ArgumentException with 'octal' in the message -- .NET has no base-8 int.Parse so we hand-walk digits to catch 8/9 instead of silently accepting them), UserVMemoryToPdu_rejects_empty_input, UserVMemoryToPdu_overflow_rejected (200000 octal = 0x10000 overflows ushort), SystemVMemoryBasePdu_is_0x2100_for_V40400, SystemVMemoryToPdu_offsets_within_bank, SystemVMemoryToPdu_rejects_overflow. 23/23 Modbus.Tests pass. Integration tests against dl205.json pymodbus profile: DL205_V2000_user_memory_resolves_to_PDU_0x0400_marker (reads HR[0x0400]=0x2000), DL205_V40400_system_memory_resolves_to_PDU_0x2100_marker (reads HR[0x2100]=0x4040). 5/5 DL205 integration tests pass. Caller opts into the helper per tag by calling DirectLogicAddress.UserVMemoryToPdu("V2000") as the ModbusTagDefinition Address -- no driver-wide "DL205 mode" flag needed, because users mix DL and non-DL tags in a single driver instance all the time.

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/DirectLogicAddress.cs

@@ -0,0 +1,74 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+/// <summary>
+///     AutomationDirect DirectLOGIC address-translation helpers. DL205 / DL260 / DL350 CPUs
+///     address V-memory in OCTAL while the Modbus wire uses DECIMAL PDU addresses — operators
+///     see "V2000" in the PLC ladder-logic editor but the Modbus client must write PDU 0x0400.
+///     The formulas differ between user V-memory (simple octal-to-decimal) and system V-memory
+///     (fixed bank mappings), so the two cases are separate methods rather than one overloaded
+///     "ToPdu" call.
+/// </summary>
+/// <remarks>
+///     See <c>docs/v2/dl205.md</c> §V-memory for the full CPU-family matrix + rationale.
+///     References: D2-USER-M appendix (DL205/D2-260), H2-ECOM-M §6.5 (absolute vs relative
+///     addressing), AutomationDirect forum guidance on V40400 system-base.
+/// </remarks>
+public static class DirectLogicAddress
+{
+    /// <summary>
+    ///     Convert a DirectLOGIC user V-memory address (octal) to a 0-based Modbus PDU address.
+    ///     Accepts bare octal (<c>"2000"</c>) or <c>V</c>-prefixed (<c>"V2000"</c>). Range
+    ///     depends on CPU model — DL205 D2-260 user memory is V1400-V7377 + V10000-V17777
+    ///     octal, DL260 extends to V77777 octal.
+    /// </summary>
+    /// <exception cref="ArgumentException">Input is null / empty / contains non-octal digits (8,9).</exception>
+    /// <exception cref="OverflowException">Parsed value exceeds ushort.MaxValue (0xFFFF).</exception>
+    public static ushort UserVMemoryToPdu(string vAddress)
+    {
+        if (string.IsNullOrWhiteSpace(vAddress))
+            throw new ArgumentException("V-memory address must not be empty", nameof(vAddress));
+        var s = vAddress.Trim();
+        if (s[0] == 'V' || s[0] == 'v') s = s.Substring(1);
+        if (s.Length == 0)
+            throw new ArgumentException($"V-memory address '{vAddress}' has no digits", nameof(vAddress));
+
+        // Octal conversion. Reject 8/9 digits up-front — int.Parse in the obvious base would
+        // accept them silently because .NET has no built-in base-8 parser.
+        uint result = 0;
+        foreach (var ch in s)
+        {
+            if (ch < '0' || ch > '7')
+                throw new ArgumentException(
+                    $"V-memory address '{vAddress}' contains non-octal digit '{ch}' — DirectLOGIC V-addresses are octal (0-7)",
+                    nameof(vAddress));
+            result = result * 8 + (uint)(ch - '0');
+            if (result > ushort.MaxValue)
+                throw new OverflowException(
+                    $"V-memory address '{vAddress}' exceeds the 16-bit Modbus PDU address range");
+        }
+        return (ushort)result;
+    }
+
+    /// <summary>
+    ///     DirectLOGIC system V-memory starts at octal V40400 on DL260 / H2-ECOM100 in factory
+    ///     "absolute" addressing mode. Unlike user V-memory, the mapping is NOT a simple
+    ///     octal-to-decimal conversion — the CPU relocates the system bank to Modbus PDU 0x2100
+    ///     (decimal 8448). This helper returns the CPU-family base plus a user-supplied offset
+    ///     within the system bank.
+    /// </summary>
+    public const ushort SystemVMemoryBasePdu = 0x2100;
+
+    /// <param name="offsetWithinSystemBank">
+    ///     0-based register offset within the system bank. Pass 0 for V40400 itself; pass 1 for
+    ///     V40401 (octal), and so on. NOT an octal-decoded value — the system bank lives at
+    ///     consecutive PDU addresses, so the offset is plain decimal.
+    /// </param>
+    public static ushort SystemVMemoryToPdu(ushort offsetWithinSystemBank)
+    {
+        var pdu = SystemVMemoryBasePdu + offsetWithinSystemBank;
+        if (pdu > ushort.MaxValue)
+            throw new OverflowException(
+                $"System V-memory offset {offsetWithinSystemBank} maps past 0xFFFF");
+        return (ushort)pdu;
+    }
+}