diff --git a/src/main/java/de/neemann/digital/core/io/PowerSupply.java b/src/main/java/de/neemann/digital/core/io/PowerSupply.java
index f9e41cf27..f12a17f64 100644
--- a/src/main/java/de/neemann/digital/core/io/PowerSupply.java
+++ b/src/main/java/de/neemann/digital/core/io/PowerSupply.java
@@ -14,7 +14,7 @@ import static de.neemann.digital.core.element.PinInfo.input;
  * Enforces a power supply
  * Created by hneemann on 15.07.17.
  */
-public class PowerSupply implements Element {
+public class PowerSupply extends Node implements Element {
 
     /**
      * Enforces a power supply
@@ -24,6 +24,9 @@ public class PowerSupply implements Element {
             .addAttribute(Keys.ROTATE)
             .addAttribute(Keys.LABEL);
 
+    private ObservableValue vcc;
+    private ObservableValue gnd;
+
     /**
      * Creates a new instance
      *
@@ -34,13 +37,8 @@ public class PowerSupply implements Element {
 
     @Override
     public void setInputs(ObservableValues inputs) throws NodeException {
-        check(inputs.get(0).checkBits(1, null, 0), 1);
-        check(inputs.get(1).checkBits(1, null, 1), 0);
-    }
-
-    private void check(ObservableValue val, int expected) throws NodeException {
-        if (!val.isConstant() || val.getValue() != expected)
-            throw new NodeException(Lang.get("err_errorInPowerSupply"), val);
+        vcc = inputs.get(0).checkBits(1, null, 0).addObserverToValue(this);
+        gnd = inputs.get(1).checkBits(1, null, 1).addObserverToValue(this);
     }
 
     @Override
@@ -49,6 +47,15 @@ public class PowerSupply implements Element {
     }
 
     @Override
-    public void registerNodes(Model model) {
+    public void readInputs() throws NodeException {
+        if (vcc.getValue() != 1 || vcc.isHighZ())
+            throw new NodeException(Lang.get("err_errorInPowerSupply", "VCC"), vcc);
+        if (gnd.getValue() != 0 || gnd.isHighZ())
+            throw new NodeException(Lang.get("err_errorInPowerSupply", "GND"), gnd);
     }
+
+    @Override
+    public void writeOutputs() throws NodeException {
+    }
+
 }
diff --git a/src/main/java/de/neemann/digital/gui/Main.java b/src/main/java/de/neemann/digital/gui/Main.java
index db3ad858d..f0b6d667c 100644
--- a/src/main/java/de/neemann/digital/gui/Main.java
+++ b/src/main/java/de/neemann/digital/gui/Main.java
@@ -9,6 +9,7 @@ import de.neemann.digital.core.element.ElementAttributes;
 import de.neemann.digital.core.element.Key;
 import de.neemann.digital.core.element.Keys;
 import de.neemann.digital.core.io.In;
+import de.neemann.digital.core.io.InValue;
 import de.neemann.digital.core.io.Out;
 import de.neemann.digital.core.io.PowerSupply;
 import de.neemann.digital.core.memory.ROM;
@@ -732,6 +733,7 @@ public final class Main extends JFrame implements ClosingWindowListener.ConfirmS
                             .setShapeFactory(shapeFactory)
                             .setAttribute(Keys.LABEL, "VCC")
                             .setAttribute(Keys.PINNUMBER, maxNum)
+                            .setAttribute(Keys.INPUT_DEFAULT, new InValue(1))
                             .setPos(new Vector(0, 0)));
                     list.add(new VisualElement(In.DESCRIPTION.getName())
                             .setShapeFactory(shapeFactory)
diff --git a/src/main/resources/lang/lang_de.xml b/src/main/resources/lang/lang_de.xml
index e9db528c8..06eeb6280 100644
--- a/src/main/resources/lang/lang_de.xml
+++ b/src/main/resources/lang/lang_de.xml
@@ -555,7 +555,7 @@ Sind evtl. die Namen der Variablen nicht eindeutig?</string>
     <string name="err_nameUsedTwice_N">Signal {0} wurde mehrfach verwendet!</string>
     <string name="err_errorParsingTestdata">Fehler beim Einlesen der Testdaten.</string>
     <string name="err_backtrackOf_N_isImpossible">Die Modelkomponente {0} kann nicht analysiert werden.</string>
-    <string name="err_errorInPowerSupply">Fehler in der Stromversorgung.</string>
+    <string name="err_errorInPowerSupply">Fehler in der Stromversorgung an {0}.</string>
 
     <string name="key_AddrBits">Adress-Bits</string>
     <string name="key_AddrBits_tt">Anzahl der Adress-Bits die verwendet werden.</string>
diff --git a/src/main/resources/lang/lang_en.xml b/src/main/resources/lang/lang_en.xml
index f86c7b0d7..4b57aa35a 100644
--- a/src/main/resources/lang/lang_en.xml
+++ b/src/main/resources/lang/lang_en.xml
@@ -543,7 +543,7 @@ The names of the variables may not be unique.</string>
     <string name="err_nameUsedTwice_N">Signal {0} is used twice!</string>
     <string name="err_errorParsingTestdata">Error parsing the test data.</string>
     <string name="err_backtrackOf_N_isImpossible">The model component {0} can not be analysed.</string>
-    <string name="err_errorInPowerSupply">Error in wiring of power supply.</string>
+    <string name="err_errorInPowerSupply">Error in wiring of power supply at {0}.</string>
 
     <string name="key_AddrBits">Address Bits</string>
     <string name="key_AddrBits_tt">Number of address bits used.</string>