diff --git a/src/main/java/de/neemann/digital/core/Model.java b/src/main/java/de/neemann/digital/core/Model.java
index b93573a06..83f79dd25 100644
--- a/src/main/java/de/neemann/digital/core/Model.java
+++ b/src/main/java/de/neemann/digital/core/Model.java
@@ -66,6 +66,7 @@ public class Model implements Iterable<Node>, SyncAccess {
     private final ArrayList<Signal> signals;
     private final ArrayList<Signal> inputs;
     private final ArrayList<Signal> outputs;
+    private final ArrayList<Signal> testOutputs;
 
     private final ArrayList<Node> nodes;
     private ArrayList<Node> nodesToUpdateAct;
@@ -92,6 +93,7 @@ public class Model implements Iterable<Node>, SyncAccess {
         this.buttonsToMap = new HashMap<>();
         this.signals = new ArrayList<>();
         this.outputs = new ArrayList<>();
+        this.testOutputs = new ArrayList<>();
         this.inputs = new ArrayList<>();
         this.nodes = new ArrayList<>();
         this.nodesToUpdateAct = new ArrayList<>();
@@ -586,8 +588,13 @@ public class Model implements Iterable<Node>, SyncAccess {
      * @param signal the signal
      */
     public void addSignal(Signal signal) {
-        if (signal.isValid())
+        if (signal.isValid()) {
+            if (signals.contains(signal))
+                invalidSignal = signal;
             signals.add(signal);
+            if (signal.isTestOutput())
+                testOutputs.add(signal);
+        }
     }
 
     /**
@@ -623,6 +630,7 @@ public class Model implements Iterable<Node>, SyncAccess {
                 invalidSignal = signal;
             signals.add(signal);
             outputs.add(signal);
+            testOutputs.add(signal);
         } else
             invalidSignal = signal;
     }
@@ -649,6 +657,13 @@ public class Model implements Iterable<Node>, SyncAccess {
         return outputs;
     }
 
+    /**
+     * @return the models outputs
+     */
+    public ArrayList<Signal> getTestOutputs() {
+        return testOutputs;
+    }
+
     /**
      * @return all registered signals
      */
diff --git a/src/main/java/de/neemann/digital/core/Signal.java b/src/main/java/de/neemann/digital/core/Signal.java
index 456e43f7d..b56dc7656 100644
--- a/src/main/java/de/neemann/digital/core/Signal.java
+++ b/src/main/java/de/neemann/digital/core/Signal.java
@@ -16,6 +16,7 @@ public final class Signal implements Comparable<Signal> {
     private String pinNumber;
     private ObservableValue bidirectionalReader;
     private boolean showInGraph;
+    private boolean testOutput;
 
     /**
      * Creates a new Instance
@@ -60,6 +61,23 @@ public final class Signal implements Comparable<Signal> {
         return showInGraph;
     }
 
+    /**
+     * Makes this signal to a test output signal
+     *
+     * @return this for chained calls
+     */
+    public Signal setTestOutput() {
+        testOutput = true;
+        return this;
+    }
+
+    /**
+     * @return true if this signal is a test output
+     */
+    public boolean isTestOutput() {
+        return testOutput;
+    }
+
     /**
      * @return the name
      */
diff --git a/src/main/java/de/neemann/digital/core/flipflops/FlipflopBit.java b/src/main/java/de/neemann/digital/core/flipflops/FlipflopBit.java
index 43fe5bbcf..b2b81d797 100644
--- a/src/main/java/de/neemann/digital/core/flipflops/FlipflopBit.java
+++ b/src/main/java/de/neemann/digital/core/flipflops/FlipflopBit.java
@@ -86,7 +86,7 @@ abstract class FlipflopBit extends Node implements Element {
                 out = v != 0;
                 q.setBool(out);
                 qn.setBool(!out);
-            }));
+            }).setTestOutput());
     }
 
     void setOut(boolean out) {
diff --git a/src/main/java/de/neemann/digital/core/flipflops/FlipflopD.java b/src/main/java/de/neemann/digital/core/flipflops/FlipflopD.java
index 5dfc900ee..da4340bd5 100644
--- a/src/main/java/de/neemann/digital/core/flipflops/FlipflopD.java
+++ b/src/main/java/de/neemann/digital/core/flipflops/FlipflopD.java
@@ -120,7 +120,7 @@ public class FlipflopD extends Node implements Element, Countable {
                 value = v;
                 q.setValue(value);
                 qn.setValue(~value);
-            }));
+            }).setTestOutput());
     }
 
     /**
diff --git a/src/main/java/de/neemann/digital/core/io/Probe.java b/src/main/java/de/neemann/digital/core/io/Probe.java
index 9465f9682..fbcd0da9d 100644
--- a/src/main/java/de/neemann/digital/core/io/Probe.java
+++ b/src/main/java/de/neemann/digital/core/io/Probe.java
@@ -56,7 +56,7 @@ public class Probe implements Element {
 
     @Override
     public void registerNodes(Model model) {
-        model.addOutput(new Signal(label, value).setShowInGraph(showInGraph).setFormat(format));
+        model.addSignal(new Signal(label, value).setShowInGraph(showInGraph).setFormat(format).setTestOutput());
         model.registerGlobalValue(label, value);
     }
 
diff --git a/src/main/java/de/neemann/digital/core/memory/Counter.java b/src/main/java/de/neemann/digital/core/memory/Counter.java
index c3ef06ae4..af11f6df6 100644
--- a/src/main/java/de/neemann/digital/core/memory/Counter.java
+++ b/src/main/java/de/neemann/digital/core/memory/Counter.java
@@ -105,7 +105,7 @@ public class Counter extends Node implements Element, ProgramCounter {
                 boolean o = (counter == maxValue) && enable.getBool();
                 out.setValue(counter);
                 ovf.setBool(o);
-            }));
+            }).setTestOutput());
     }
 
     @Override
diff --git a/src/main/java/de/neemann/digital/core/memory/CounterPreset.java b/src/main/java/de/neemann/digital/core/memory/CounterPreset.java
index fc307359f..8132d67f7 100644
--- a/src/main/java/de/neemann/digital/core/memory/CounterPreset.java
+++ b/src/main/java/de/neemann/digital/core/memory/CounterPreset.java
@@ -146,7 +146,7 @@ public class CounterPreset extends Node implements Element, ProgramCounter {
                 boolean o = getOvfValue(counter, dir.getBool(), enable.getBool());
                 out.setValue(counter);
                 ovf.setBool(o);
-            }));
+            }).setTestOutput());
     }
 
     @Override
diff --git a/src/main/java/de/neemann/digital/core/memory/Register.java b/src/main/java/de/neemann/digital/core/memory/Register.java
index 7443031ac..4c74be678 100644
--- a/src/main/java/de/neemann/digital/core/memory/Register.java
+++ b/src/main/java/de/neemann/digital/core/memory/Register.java
@@ -90,7 +90,7 @@ public class Register extends Node implements Element, Countable, ProgramCounter
             model.addSignal(new Signal(label, q, (v, z) -> {
                 value = v;
                 q.setValue(value);
-            }));
+            }).setTestOutput());
     }
 
     @Override
diff --git a/src/main/java/de/neemann/digital/testing/TestExecutor.java b/src/main/java/de/neemann/digital/testing/TestExecutor.java
index 1fa721212..3504dc810 100644
--- a/src/main/java/de/neemann/digital/testing/TestExecutor.java
+++ b/src/main/java/de/neemann/digital/testing/TestExecutor.java
@@ -119,7 +119,7 @@ public class TestExecutor {
             }
         }
 
-        for (Signal s : model.getOutputs()) {
+        for (Signal s : model.getTestOutputs()) {
             final int index = getIndexOf(s.getName());
             if (index >= 0) {
                 outputs.add(new TestSignal(index, s.getValue()));
diff --git a/src/main/java/de/neemann/digital/testing/parser/Context.java b/src/main/java/de/neemann/digital/testing/parser/Context.java
index 25e976161..dff59bad4 100644
--- a/src/main/java/de/neemann/digital/testing/parser/Context.java
+++ b/src/main/java/de/neemann/digital/testing/parser/Context.java
@@ -49,7 +49,7 @@ public class Context {
                 if (model != null) {
                     // inputs are not supported because there are cases where values
                     // are evaluated and model inputs are not set!
-                    for (Signal s : model.getOutputs())
+                    for (Signal s : model.getTestOutputs())
                         if (s.getName().equals(name))
                             return s.getValue().getValue();
                 }
diff --git a/src/main/java/de/neemann/digital/testing/parser/Parser.java b/src/main/java/de/neemann/digital/testing/parser/Parser.java
index 5515fa60c..847c4f95c 100644
--- a/src/main/java/de/neemann/digital/testing/parser/Parser.java
+++ b/src/main/java/de/neemann/digital/testing/parser/Parser.java
@@ -115,10 +115,15 @@ public class Parser {
                     expect(Tokenizer.Token.IDENT);
                     final String sName = tok.getIdent();
                     expect(Tokenizer.Token.EQUAL);
+                    int sign = 1;
+                    if (tok.peek() == Tokenizer.Token.SUB) {
+                        tok.consume();
+                        sign = -1;
+                    }
                     expect(Tokenizer.Token.NUMBER);
                     long n = convToLong(tok.getIdent());
                     expect(Tokenizer.Token.SEMICOLON);
-                    signalInitMap.put(sName, n);
+                    signalInitMap.put(sName, sign * n);
                     break;
                 case PROGRAM:
                     tok.consume();