diff --git a/src/main/java/de/neemann/digital/core/switching/Switch.java b/src/main/java/de/neemann/digital/core/switching/Switch.java
index 89a1a27a4..8fe81a74d 100644
--- a/src/main/java/de/neemann/digital/core/switching/Switch.java
+++ b/src/main/java/de/neemann/digital/core/switching/Switch.java
@@ -76,8 +76,19 @@ public class Switch implements Element, NodeInterface {
                     constant = in2.searchConstant();
                     if (constant != null)
                         switchModel = new SimpleSwitch(constant, output1);
-                    else
-                        switchModel = new RealSwitch(in1, in2);
+                    else {
+                        // not a constant
+                        boolean def1 = in1.isAlwaysDefined();
+                        boolean def2 = in2.isAlwaysDefined();
+                        if (def1 == def2)
+                            switchModel = new RealSwitch(in1, in2);
+                        else {
+                            if (def1)
+                                switchModel = new SimpleSwitch(input1, output2);
+                            else
+                                switchModel = new SimpleSwitch(input2, output1);
+                        }
+                    }
                 }
             } else {
                 switchModel = new SimpleSwitch(input1, output2);
diff --git a/src/main/java/de/neemann/digital/core/wiring/bus/CommonBusValue.java b/src/main/java/de/neemann/digital/core/wiring/bus/CommonBusValue.java
index f4dea1338..470700d9e 100644
--- a/src/main/java/de/neemann/digital/core/wiring/bus/CommonBusValue.java
+++ b/src/main/java/de/neemann/digital/core/wiring/bus/CommonBusValue.java
@@ -63,7 +63,7 @@ public final class CommonBusValue extends ObservableValue implements NodeInterfa
     /**
      * Returns true if this net is a constant
      *
-     * @return true if this is a constant
+     * @return the constant if this is a constant, null otherwise
      */
     public ObservableValue searchConstant() {
         for (ObservableValue i : inputs)
@@ -72,6 +72,19 @@ public final class CommonBusValue extends ObservableValue implements NodeInterfa
         return null;
     }
 
+    /**
+     * Checks if this net is always defined.
+     * This means it can never be in a high z state.
+     *
+     * @return true if this net is always defined
+     */
+    public boolean isAlwaysDefined() {
+        for (ObservableValue i : inputs)
+            if (!i.supportsHighZ())
+                return true;
+        return false;
+    }
+
     @Override
     public String toString() {
         return "CommonBusValue{"
diff --git a/src/test/resources/dig/test/fet/cmos10TFullAdder.dig b/src/test/resources/dig/test/fet/cmos10TFullAdder.dig
index 226b60599..0c63bfaef 100644
--- a/src/test/resources/dig/test/fet/cmos10TFullAdder.dig
+++ b/src/test/resources/dig/test/fet/cmos10TFullAdder.dig
@@ -4,7 +4,10 @@
   <attributes>
     <entry>
       <string>Description</string>
-      <string>10 Transistor Full Adder</string>
+      <string>10 Transistor Full Adder
+The drivers represent p-channel fets. Needed to 
+avoid the resistance problem 
+(see https://github.com/hneemann/Digital/issues/30).</string>
     </entry>
   </attributes>
   <visualElements>
@@ -51,10 +54,6 @@
     <visualElement>
       <elementName>Testcase</elementName>
       <elementAttributes>
-        <entry>
-          <string>Label</string>
-          <string>Failing</string>
-        </entry>
         <entry>
           <string>Testdata</string>
           <testData>
@@ -140,16 +139,6 @@
       </elementAttributes>
       <pos x="780" y="560"/>
     </visualElement>
-    <visualElement>
-      <elementName>PFET</elementName>
-      <elementAttributes/>
-      <pos x="840" y="320"/>
-    </visualElement>
-    <visualElement>
-      <elementName>PFET</elementName>
-      <elementAttributes/>
-      <pos x="900" y="320"/>
-    </visualElement>
     <visualElement>
       <elementName>NFET</elementName>
       <elementAttributes/>
@@ -215,6 +204,34 @@
       <elementAttributes/>
       <pos x="1020" y="360"/>
     </visualElement>
+    <visualElement>
+      <elementName>DriverInvSel</elementName>
+      <elementAttributes>
+        <entry>
+          <string>rotation</string>
+          <rotation rotation="3"/>
+        </entry>
+        <entry>
+          <string>flipSelPos</string>
+          <boolean>true</boolean>
+        </entry>
+      </elementAttributes>
+      <pos x="860" y="340"/>
+    </visualElement>
+    <visualElement>
+      <elementName>DriverInvSel</elementName>
+      <elementAttributes>
+        <entry>
+          <string>rotation</string>
+          <rotation rotation="3"/>
+        </entry>
+        <entry>
+          <string>flipSelPos</string>
+          <boolean>true</boolean>
+        </entry>
+      </elementAttributes>
+      <pos x="920" y="340"/>
+    </visualElement>
   </visualElements>
   <wires>
     <wire>
@@ -229,22 +246,10 @@
       <p1 x="1080" y="320"/>
       <p2 x="1100" y="320"/>
     </wire>
-    <wire>
-      <p1 x="880" y="320"/>
-      <p2 x="900" y="320"/>
-    </wire>
-    <wire>
-      <p1 x="800" y="320"/>
-      <p2 x="840" y="320"/>
-    </wire>
     <wire>
       <p1 x="1100" y="320"/>
       <p2 x="1120" y="320"/>
     </wire>
-    <wire>
-      <p1 x="960" y="240"/>
-      <p2 x="1160" y="240"/>
-    </wire>
     <wire>
       <p1 x="1120" y="480"/>
       <p2 x="1140" y="480"/>
@@ -257,6 +262,26 @@
       <p1 x="1120" y="260"/>
       <p2 x="1140" y="260"/>
     </wire>
+    <wire>
+      <p1 x="1100" y="460"/>
+      <p2 x="1160" y="460"/>
+    </wire>
+    <wire>
+      <p1 x="780" y="620"/>
+      <p2 x="1160" y="620"/>
+    </wire>
+    <wire>
+      <p1 x="960" y="240"/>
+      <p2 x="1160" y="240"/>
+    </wire>
+    <wire>
+      <p1 x="800" y="340"/>
+      <p2 x="840" y="340"/>
+    </wire>
+    <wire>
+      <p1 x="880" y="340"/>
+      <p2 x="900" y="340"/>
+    </wire>
     <wire>
       <p1 x="780" y="500"/>
       <p2 x="860" y="500"/>
@@ -271,12 +296,16 @@
     </wire>
     <wire>
       <p1 x="780" y="280"/>
-      <p2 x="860" y="280"/>
+      <p2 x="800" y="280"/>
     </wire>
     <wire>
       <p1 x="860" y="280"/>
       <p2 x="880" y="280"/>
     </wire>
+    <wire>
+      <p1 x="800" y="280"/>
+      <p2 x="860" y="280"/>
+    </wire>
     <wire>
       <p1 x="800" y="440"/>
       <p2 x="860" y="440"/>
@@ -325,29 +354,21 @@
       <p1 x="1060" y="540"/>
       <p2 x="1120" y="540"/>
     </wire>
-    <wire>
-      <p1 x="1100" y="460"/>
-      <p2 x="1160" y="460"/>
-    </wire>
-    <wire>
-      <p1 x="780" y="620"/>
-      <p2 x="1160" y="620"/>
-    </wire>
     <wire>
       <p1 x="800" y="260"/>
-      <p2 x="800" y="280"/>
-    </wire>
-    <wire>
-      <p1 x="800" y="320"/>
-      <p2 x="800" y="440"/>
-    </wire>
-    <wire>
-      <p1 x="800" y="280"/>
       <p2 x="800" y="320"/>
     </wire>
+    <wire>
+      <p1 x="800" y="320"/>
+      <p2 x="800" y="340"/>
+    </wire>
+    <wire>
+      <p1 x="800" y="340"/>
+      <p2 x="800" y="440"/>
+    </wire>
     <wire>
       <p1 x="880" y="280"/>
-      <p2 x="880" y="320"/>
+      <p2 x="880" y="340"/>
     </wire>
     <wire>
       <p1 x="880" y="380"/>
@@ -474,4 +495,5 @@
       <p2 x="1100" y="460"/>
     </wire>
   </wires>
+  <measurementOrdering/>
 </circuit>
\ No newline at end of file
diff --git a/src/test/resources/dig/test/fet/cmosSERFFullAdder.dig b/src/test/resources/dig/test/fet/cmosSERFFullAdder.dig
index 31f1e88e3..84e6d5485 100644
--- a/src/test/resources/dig/test/fet/cmosSERFFullAdder.dig
+++ b/src/test/resources/dig/test/fet/cmosSERFFullAdder.dig
@@ -4,7 +4,10 @@
   <attributes>
     <entry>
       <string>Description</string>
-      <string>Static Energy Recovery Full Adder (SERF)</string>
+      <string>Static Energy Recovery Full Adder (SERF)
+The drivers represent n-channel fets. Needed to 
+avoid the resistance problem 
+(see https://github.com/hneemann/Digital/issues/30).</string>
     </entry>
   </attributes>
   <visualElements>
@@ -33,21 +36,11 @@
       <elementAttributes/>
       <pos x="840" y="320"/>
     </visualElement>
-    <visualElement>
-      <elementName>NFET</elementName>
-      <elementAttributes/>
-      <pos x="800" y="400"/>
-    </visualElement>
     <visualElement>
       <elementName>PFET</elementName>
       <elementAttributes/>
       <pos x="840" y="260"/>
     </visualElement>
-    <visualElement>
-      <elementName>NFET</elementName>
-      <elementAttributes/>
-      <pos x="880" y="400"/>
-    </visualElement>
     <visualElement>
       <elementName>PFET</elementName>
       <elementAttributes/>
@@ -63,11 +56,6 @@
       <elementAttributes/>
       <pos x="1080" y="380"/>
     </visualElement>
-    <visualElement>
-      <elementName>NFET</elementName>
-      <elementAttributes/>
-      <pos x="1120" y="460"/>
-    </visualElement>
     <visualElement>
       <elementName>In</elementName>
       <elementAttributes>
@@ -83,11 +71,6 @@
       <elementAttributes/>
       <pos x="1120" y="580"/>
     </visualElement>
-    <visualElement>
-      <elementName>NFET</elementName>
-      <elementAttributes/>
-      <pos x="1040" y="460"/>
-    </visualElement>
     <visualElement>
       <elementName>Out</elementName>
       <elementAttributes>
@@ -111,10 +94,6 @@
     <visualElement>
       <elementName>Testcase</elementName>
       <elementAttributes>
-        <entry>
-          <string>Label</string>
-          <string>Failing</string>
-        </entry>
         <entry>
           <string>Testdata</string>
           <testData>
@@ -210,8 +189,60 @@
       <elementAttributes/>
       <pos x="1100" y="300"/>
     </visualElement>
+    <visualElement>
+      <elementName>Driver</elementName>
+      <elementAttributes>
+        <entry>
+          <string>rotation</string>
+          <rotation rotation="1"/>
+        </entry>
+      </elementAttributes>
+      <pos x="900" y="420"/>
+    </visualElement>
+    <visualElement>
+      <elementName>Driver</elementName>
+      <elementAttributes>
+        <entry>
+          <string>rotation</string>
+          <rotation rotation="1"/>
+        </entry>
+      </elementAttributes>
+      <pos x="1140" y="480"/>
+    </visualElement>
+    <visualElement>
+      <elementName>Driver</elementName>
+      <elementAttributes>
+        <entry>
+          <string>rotation</string>
+          <rotation rotation="1"/>
+        </entry>
+      </elementAttributes>
+      <pos x="1060" y="480"/>
+    </visualElement>
+    <visualElement>
+      <elementName>Driver</elementName>
+      <elementAttributes>
+        <entry>
+          <string>rotation</string>
+          <rotation rotation="1"/>
+        </entry>
+      </elementAttributes>
+      <pos x="820" y="420"/>
+    </visualElement>
   </visualElements>
   <wires>
+    <wire>
+      <p1 x="1100" y="480"/>
+      <p2 x="1120" y="480"/>
+    </wire>
+    <wire>
+      <p1 x="1020" y="480"/>
+      <p2 x="1040" y="480"/>
+    </wire>
+    <wire>
+      <p1 x="780" y="480"/>
+      <p2 x="900" y="480"/>
+    </wire>
     <wire>
       <p1 x="740" y="320"/>
       <p2 x="780" y="320"/>
@@ -224,10 +255,6 @@
       <p1 x="780" y="320"/>
       <p2 x="840" y="320"/>
     </wire>
-    <wire>
-      <p1 x="780" y="480"/>
-      <p2 x="900" y="480"/>
-    </wire>
     <wire>
       <p1 x="1140" y="640"/>
       <p2 x="1180" y="640"/>
@@ -236,6 +263,14 @@
       <p1 x="1020" y="640"/>
       <p2 x="1100" y="640"/>
     </wire>
+    <wire>
+      <p1 x="860" y="420"/>
+      <p2 x="880" y="420"/>
+    </wire>
+    <wire>
+      <p1 x="780" y="420"/>
+      <p2 x="800" y="420"/>
+    </wire>
     <wire>
       <p1 x="740" y="260"/>
       <p2 x="760" y="260"/>
@@ -276,22 +311,6 @@
       <p1 x="760" y="720"/>
       <p2 x="1140" y="720"/>
     </wire>
-    <wire>
-      <p1 x="1100" y="500"/>
-      <p2 x="1120" y="500"/>
-    </wire>
-    <wire>
-      <p1 x="1020" y="500"/>
-      <p2 x="1040" y="500"/>
-    </wire>
-    <wire>
-      <p1 x="860" y="440"/>
-      <p2 x="880" y="440"/>
-    </wire>
-    <wire>
-      <p1 x="780" y="440"/>
-      <p2 x="800" y="440"/>
-    </wire>
     <wire>
       <p1 x="1060" y="440"/>
       <p2 x="1100" y="440"/>
@@ -409,7 +428,7 @@
       <p2 x="1100" y="440"/>
     </wire>
     <wire>
-      <p1 x="1100" y="500"/>
+      <p1 x="1100" y="480"/>
       <p2 x="1100" y="520"/>
     </wire>
     <wire>
@@ -425,7 +444,7 @@
       <p2 x="860" y="260"/>
     </wire>
     <wire>
-      <p1 x="860" y="440"/>
+      <p1 x="860" y="420"/>
       <p2 x="860" y="460"/>
     </wire>
     <wire>
@@ -434,22 +453,22 @@
     </wire>
     <wire>
       <p1 x="780" y="320"/>
-      <p2 x="780" y="440"/>
+      <p2 x="780" y="420"/>
     </wire>
     <wire>
-      <p1 x="780" y="440"/>
+      <p1 x="780" y="420"/>
       <p2 x="780" y="480"/>
     </wire>
     <wire>
       <p1 x="1020" y="380"/>
-      <p2 x="1020" y="500"/>
+      <p2 x="1020" y="480"/>
     </wire>
     <wire>
       <p1 x="1020" y="540"/>
       <p2 x="1020" y="640"/>
     </wire>
     <wire>
-      <p1 x="1020" y="500"/>
+      <p1 x="1020" y="480"/>
       <p2 x="1020" y="540"/>
     </wire>
   </wires>
diff --git a/src/test/resources/dig/test/fet/nchan.dig b/src/test/resources/dig/test/fet/nchan.dig
index 30a02d52f..d0370b3f1 100644
--- a/src/test/resources/dig/test/fet/nchan.dig
+++ b/src/test/resources/dig/test/fet/nchan.dig
@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <circuit>
   <version>1</version>
+  <attributes/>
   <visualElements>
     <visualElement>
       <elementName>NFET</elementName>
@@ -50,6 +51,10 @@
           <string>Label</string>
           <string>S</string>
         </entry>
+        <entry>
+          <string>isHighZ</string>
+          <boolean>true</boolean>
+        </entry>
       </elementAttributes>
       <pos x="500" y="280"/>
     </visualElement>
diff --git a/src/test/resources/dig/test/fet/pchan.dig b/src/test/resources/dig/test/fet/pchan.dig
index cdcca4c48..d329c99fa 100644
--- a/src/test/resources/dig/test/fet/pchan.dig
+++ b/src/test/resources/dig/test/fet/pchan.dig
@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <circuit>
   <version>1</version>
+  <attributes/>
   <visualElements>
     <visualElement>
       <elementName>In</elementName>
@@ -45,6 +46,10 @@
           <string>Label</string>
           <string>D</string>
         </entry>
+        <entry>
+          <string>isHighZ</string>
+          <boolean>true</boolean>
+        </entry>
       </elementAttributes>
       <pos x="500" y="280"/>
     </visualElement>