diff --git a/S7.Net/PLC.cs b/S7.Net/PLC.cs
index d62e783..d997b7a 100644
--- a/S7.Net/PLC.cs
+++ b/S7.Net/PLC.cs
@@ -1,5 +1,8 @@
 using System;
+using System.Collections.Generic;
+using System.Linq;
 using System.Net.Sockets;
+using S7.Net.Types;
 
 
 namespace S7.Net
@@ -178,6 +181,25 @@ namespace S7.Net
             }
         }
 
+        private void AssertPduSizeForRead(ICollection<DataItem> dataItems)
+        {
+            // 12 bytes of header data, 12 bytes of parameter data for each dataItem
+            if ((dataItems.Count + 1) * 12 > MaxPDUSize) throw new Exception("Too many vars requested for read");
+            
+            // 14 bytes of header data, 4 bytes of result data for each dataItem and the actual data
+            if (GetDataLength(dataItems) + dataItems.Count * 4 + 14 > MaxPDUSize) throw new Exception("Too much data requested for read");
+        }
+
+        private void AssertPduSizeForWrite(ICollection<DataItem> dataItems)
+        {
+            // 12 bytes of header data, 18 bytes of parameter data for each dataItem
+            if (dataItems.Count * 18 + 12 > MaxPDUSize) throw new Exception("Too many vars supplied for write");
+
+            // 12 bytes of header data, 16 bytes of data for each dataItem and the actual data
+            if (GetDataLength(dataItems) + dataItems.Count * 16 + 12 > MaxPDUSize)
+                throw new Exception("Too much data supplied for write");
+        }
+
         private void ConfigureConnection()
         {
             if (tcpClient == null)
@@ -189,6 +211,13 @@ namespace S7.Net
             tcpClient.SendTimeout = WriteTimeout;
         }
 
+        private int GetDataLength(IEnumerable<DataItem> dataItems)
+        {
+            // Odd length variables are 0-padded
+            return dataItems.Select(di => VarTypeToByteLength(di.VarType, di.Count))
+                .Sum(len => (len & 1) == 1 ? len + 1 : len);
+        }
+
         #region IDisposable Support
         private bool disposedValue = false; // To detect redundant calls
 
diff --git a/S7.Net/PlcAsynchronous.cs b/S7.Net/PlcAsynchronous.cs
index eb44f87..2a2de3e 100644
--- a/S7.Net/PlcAsynchronous.cs
+++ b/S7.Net/PlcAsynchronous.cs
@@ -205,15 +205,10 @@ namespace S7.Net
         /// <param name="dataItems">List of dataitems that contains the list of variables that must be read. Maximum 20 dataitems are accepted.</param>
         public async Task<List<DataItem>> ReadMultipleVarsAsync(List<DataItem> dataItems)
         {
-            int cntBytes = dataItems.Sum(dataItem => VarTypeToByteLength(dataItem.VarType, dataItem.Count));
-
-            //TODO: Figure out how to use MaxPDUSize here
             //Snap7 seems to choke on PDU sizes above 256 even if snap7 
             //replies with bigger PDU size in connection setup.
-            if (dataItems.Count > 20)
-                throw new Exception("Too many vars requested");
-            if (cntBytes > 222)
-                throw new Exception("Too many bytes requested"); // TODO: proper TDU check + split in multiple requests
+            AssertPduSizeForRead(dataItems);
+            
             try
             {
                 // first create the header
@@ -410,6 +405,8 @@ namespace S7.Net
         /// <returns>Task that completes when response from PLC is parsed.</returns>
         public async Task WriteAsync(params DataItem[] dataItems)
         {
+            AssertPduSizeForWrite(dataItems);
+
             var message = new ByteArray();
             var length = S7WriteMultiple.CreateRequest(message, dataItems);
             await stream.WriteAsync(message.Array, 0, length).ConfigureAwait(false);
diff --git a/S7.Net/PlcSynchronous.cs b/S7.Net/PlcSynchronous.cs
index 4d432d3..7d2ef01 100644
--- a/S7.Net/PlcSynchronous.cs
+++ b/S7.Net/PlcSynchronous.cs
@@ -374,6 +374,8 @@ namespace S7.Net
         /// <param name="dataItems">The DataItem(s) to write to the PLC.</param>
         public void Write(params DataItem[] dataItems)
         {
+            AssertPduSizeForWrite(dataItems);
+
             var message = new ByteArray();
             var length = S7WriteMultiple.CreateRequest(message, dataItems);
             stream.Write(message.Array, 0, length);
@@ -479,15 +481,10 @@ namespace S7.Net
         /// <param name="dataItems">List of dataitems that contains the list of variables that must be read. Maximum 20 dataitems are accepted.</param>
         public void ReadMultipleVars(List<DataItem> dataItems)
         {
-            int cntBytes = dataItems.Sum(dataItem => VarTypeToByteLength(dataItem.VarType, dataItem.Count));
-
-            //TODO: Figure out how to use MaxPDUSize here
             //Snap7 seems to choke on PDU sizes above 256 even if snap7 
             //replies with bigger PDU size in connection setup.
-            if (dataItems.Count > 20)
-                throw new Exception("Too many vars requested");
-            if (cntBytes > 222)
-                throw new Exception("Too many bytes requested"); // TODO: proper TDU check + split in multiple requests
+            AssertPduSizeForRead(dataItems);
+
             try
             {
                 // first create the header