feat: Implement clock reading

S7.Net/Plc.Clock.cs

@@ -0,0 +1,53 @@
+using System;
+using System.IO;
+using System.Linq;
+using S7.Net.Helper;
+using S7.Net.Types;
+using DateTime = System.DateTime;
+
+namespace S7.Net;
+
+partial class Plc
+{
+    private const byte SzlFunctionGroupTimers = 0x07;
+    private const byte SzlSubFunctionReadClock = 0x01;
+
+    private static byte[] BuildClockReadRequest()
+    {
+        var stream = new MemoryStream();
+
+        WriteSzlRequestHeader(stream, SzlFunctionGroupTimers, SzlSubFunctionReadClock, 4);
+        stream.Write(new byte[] { 0x0a, 0x00, 0x00, 0x00 });
+
+        stream.SetLength(stream.Position);
+        return stream.ToArray();
+    }
+
+    private static DateTime ParseClockReadResponse(byte[] message)
+    {
+        const int pduErrOffset = 20;
+        const int dtResultOffset = pduErrOffset + 2;
+        const int dtLenOffset = dtResultOffset + 2;
+        const int dtValueOffset = dtLenOffset + 4;
+
+        var pduErr = Word.FromByteArray(message.Skip(pduErrOffset).Take(2).ToArray());
+        if (pduErr != 0)
+        {
+            throw new Exception($"Response from PLC indicates error 0x{pduErr:X4}.");
+        }
+
+        var dtResult = message[dtResultOffset];
+        if (dtResult != 0xff)
+        {
+            throw new Exception($"Response from PLC indicates error 0x{dtResult:X2}.");
+        }
+
+        var len = Word.FromByteArray(message.Skip(dtLenOffset).Take(2).ToArray());
+        if (len != Types.DateTime.Length)
+        {
+            throw new Exception($"Unexpected response length {len}, expected {Types.DateTime.Length}.");
+        }
+
+        return Types.DateTime.FromByteArray(message.Skip(dtValueOffset).Take(Types.DateTime.Length).ToArray());
+    }
+}

S7.Net/PlcAsynchronous.cs

@@ -320,7 +320,10 @@ public async Task<List<DataItem>> ReadMultipleVarsAsync(List<DataItem> dataItems
         /// <returns>A task that represents the asynchronous operation, with it's result set to the current PLC time on completion.</returns>
         public async Task<System.DateTime> ReadClockAsync(CancellationToken cancellationToken = default)
         {
-            throw new NotImplementedException();
+            var request = BuildClockReadRequest();
+            var response = await RequestTsduAsync(request, cancellationToken);
+
+            return ParseClockReadResponse(response);
         }
 
         /// <summary>

S7.Net/PlcSynchronous.cs

@@ -498,7 +498,10 @@ public void ReadMultipleVars(List<DataItem> dataItems)
         /// <returns>The current PLC time.</returns>
         public System.DateTime ReadClock()
         {
-            throw new NotImplementedException();
+            var request = BuildClockReadRequest();
+            var response = RequestTsdu(request);
+
+            return ParseClockReadResponse(response);
         }
 
         /// <summary>

S7.Net/Types/DateTime.cs

@@ -8,6 +8,11 @@ namespace S7.Net.Types
     /// </summary>
     public static class DateTime
     {
+        /// <summary>
+        /// The length in bytes of DateTime stored in the PLC.
+        /// </summary>
+        public const int Length = 10;
+
         /// <summary>
         /// The minimum <see cref="T:System.DateTime"/> value supported by the specification.
         /// </summary>