diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/CPUMonitor.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/CPUMonitor.cs
index 5890279f31..9708236f5d 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/CPUMonitor.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/CPUMonitor.cs
@@ -237,10 +237,66 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             switch (machineType)
             {
                 case MachineType.ZXSpectrum16:
-                case MachineType.ZXSpectrum48:
+                case MachineType.ZXSpectrum48:                
+
+                    if ((lastPortAddr & 0xc000) == 0x4000)
+                        highByte407f = true;
+
+                    if (highByte407f)
+                    {
+                        // high byte 40-7f
+                        if (lowBitSet)
+                        {
+                            // high byte 40-7f
+                            // low bit set
+                            // C:1, C:1, C:1, C:1
+                            switch (T)
+                            {
+                                case 1:
+                                case 2:
+                                case 3:
+                                case 4:
+                                    return true;
+                            }
+                        }
+                        else
+                        {
+                            // high byte 40-7f
+                            // low bit reset
+                            // C:1, C:3
+                            switch (T)
+                            {
+                                case 1:
+                                case 2:
+                                    return true;
+                            }
+                        }
+                    }
+                    else
+                    {
+                        // high byte not 40-7f
+                        if (lowBitSet)
+                        {
+                            // high byte not 40-7f
+                            // low bit set
+                            // N:4                            
+                        }
+                        else
+                        {
+                            // high byte not 40-7f
+                            // low bit reset
+                            // N:1, C:3
+                            switch (T)
+                            {
+                                case 2:
+                                    return true;
+                            }
+                        }
+                    }
+                    break;
+
                 case MachineType.ZXSpectrum128:
                 case MachineType.ZXSpectrum128Plus2:
-
                     if ((lastPortAddr & 0xc000) == 0x4000)
                         highByte407f = true;
 
@@ -299,6 +355,8 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
 
                 case MachineType.ZXSpectrum128Plus2a:
                 case MachineType.ZXSpectrum128Plus3:
+                    // No contention occurs as the ULA only applies contention when the Z80 MREQ line is active
+                    // (which is not during an IO operation)
                     break;
             }
 
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ULA.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ULA.cs
index e953e2a9e0..0bec3a194c 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ULA.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ULA.cs
@@ -88,6 +88,16 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
         /// </summary>
         public int InterruptLength;
 
+        /// <summary>
+        /// Arbitrary offset into the contention array (for memory ops)
+        /// </summary>
+        public int MemoryContentionOffset;
+
+        /// <summary>
+        /// Arbitrary offset into the contention array (for port ops)
+        /// </summary>
+        public int PortContentionOffset;
+
         /// <summary>
         /// The time in T-States for one scanline to complete
         /// </summary>
@@ -776,8 +786,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
         /// <returns></returns>
         public int GetContentionValue(int tstate)
         {
-            int off = 5;
-            tstate += off;
+            tstate += MemoryContentionOffset;
             if (tstate >= FrameCycleLength)
                 tstate -= FrameCycleLength;
 
@@ -793,8 +802,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
         /// <returns></returns>
         public int GetPortContentionValue(int tstate)
         {
-            int off = 5;
-            tstate += off;
+            tstate +=  PortContentionOffset;
             if (tstate >= FrameCycleLength)
                 tstate -= FrameCycleLength;
 
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.Screen.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.Screen.cs
index cbd98f1128..547e0fab72 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.Screen.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.Screen.cs
@@ -20,6 +20,9 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             InterruptLength = 36;
             ScanlineTime = 228;
 
+            MemoryContentionOffset = 5;
+            PortContentionOffset = 5;
+
             BorderLeftTime = 24;
             BorderRightTime = 24;
 
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus2a/ZX128Plus2a.Screen.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus2a/ZX128Plus2a.Screen.cs
index e9026ce5d7..949ca558ce 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus2a/ZX128Plus2a.Screen.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus2a/ZX128Plus2a.Screen.cs
@@ -20,6 +20,9 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             InterruptLength = 32;
             ScanlineTime = 228;
 
+            MemoryContentionOffset = 7;
+            PortContentionOffset = 7;
+
             BorderLeftTime = 24;
             BorderRightTime = 24;
 
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.Screen.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.Screen.cs
index 96219e04db..5af98f944a 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.Screen.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.Screen.cs
@@ -20,6 +20,9 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             InterruptLength = 32;
             ScanlineTime = 224;
 
+            MemoryContentionOffset = 5;
+            PortContentionOffset = 5;
+
             BorderLeftTime = 24;
             BorderRightTime = 24;