diff --git a/misc/pseudo_device.py b/misc/pseudo_device.py
--- a/misc/pseudo_device.py
+++ b/misc/pseudo_device.py
@@ -6,7 +6,7 @@
 #
 # Currently this script only outputs ASCII(comma separated) data.
 #
-# Copyright © 2015 Hasan Yavuz Özderya
+# Copyright © 2018 Hasan Yavuz Özderya
 #
 # This file is part of serialplot.
 #
@@ -26,6 +26,22 @@
 
 import os, pty, time, struct, math
 
+def ascii_test_str(port):
+    text = """
+1,2,3
+2,4,6
+3,8,11
+1,2,3
+-1,-1,-1
+nana
+0,0,0
+1,na,na
+    """
+    while True:
+        for line in text.splitlines():
+            os.write(port, bytes(line+"\r\n", 'utf8'))
+            time.sleep(1)
+
 def ascii_test(port):
     """Put ASCII test data through pseudo terminal."""
     print("\n")
@@ -109,8 +125,9 @@ def run():
 
     try:
         # float_sine(master)
-        frame_test(master)
+        # frame_test(master)
         # ascii_test(master)
+        ascii_test_str(master)
     finally:
         # close the pseudo terminal files
         os.close(master)
diff --git a/src/asciireader.cpp b/src/asciireader.cpp
--- a/src/asciireader.cpp
+++ b/src/asciireader.cpp
@@ -110,57 +110,59 @@ void AsciiReader::onDataReady()
             continue;
         }
 
-        auto separatedValues = line.split(delimiter, QString::SkipEmptyParts);
-
-        unsigned numReadChannels; // effective number of channels to read
-        unsigned numComingChannels = separatedValues.length();
-
-        if (autoNumOfChannels)
-        {
-            // did number of channels changed?
-            if (numComingChannels != _numChannels)
-            {
-                _numChannels = numComingChannels;
-                updateNumChannels();
-                emit numOfChannelsChanged(numComingChannels);
+        const SamplePack* samples = parseLine(line);
+        if (samples != nullptr) {
+            // update number of channels if in auto mode
+            if (autoNumOfChannels ) {
+                unsigned nc = samples->numChannels();
+                if (nc != _numChannels) {
+                    _numChannels = nc;
+                    updateNumChannels();
+                    // TODO: is `numOfChannelsChanged` signal still used?
+                    emit numOfChannelsChanged(nc);
+                }
             }
-            numReadChannels = numComingChannels;
-        }
-        else if (numComingChannels >= _numChannels)
-        {
-            numReadChannels = _numChannels;
-        }
-        else // there is missing channel data
-        {
-            numReadChannels = separatedValues.length();
-            qWarning() << "Incoming data is missing data for some channels!";
-            qWarning() << "Read line: " << line;
-        }
+
+            Q_ASSERT(samples->numChannels() == _numChannels);
 
-        // parse read line
-        unsigned numDataBroken = 0;
-        SamplePack samples(1, _numChannels);
-        for (unsigned ci = 0; ci < numReadChannels; ci++)
-        {
-            bool ok;
-            samples.data(ci)[0] = separatedValues[ci].toDouble(&ok);
-            if (!ok)
-            {
-                qWarning() << "Data parsing error for channel: " << ci;
-                qWarning() << "Read line: " << line;
-                samples.data(ci)[0] = 0;
-                numDataBroken++;
-            }
-        }
-
-        if (numReadChannels > numDataBroken)
-        {
             // commit data
-            feedOut(samples);
+            feedOut(*samples);
         }
     }
 }
 
+SamplePack* AsciiReader::parseLine(const QString& line) const
+{
+    auto separatedValues = line.split(delimiter, QString::SkipEmptyParts);
+    unsigned numComingChannels = separatedValues.length();
+
+    // check number of channels (skipped if auto num channels is enabled)
+    if ((!numComingChannels) || (!autoNumOfChannels && numComingChannels != _numChannels))
+    {
+        qWarning() << "Line parsing error: invalid number of channels!";
+        qWarning() << "Read line: " << line;
+        return nullptr;
+    }
+
+    // parse data per channel
+    auto samples = new SamplePack(1, numComingChannels);
+    for (unsigned ci = 0; ci < numComingChannels; ci++)
+    {
+        bool ok;
+        samples->data(ci)[0] = separatedValues[ci].toDouble(&ok);
+        if (!ok)
+        {
+            qWarning() << "Data parsing error for channel: " << ci;
+            qWarning() << "Read line: " << line;
+
+            delete samples;
+            return nullptr;
+        }
+    }
+
+    return samples;
+}
+
 void AsciiReader::saveSettings(QSettings* settings)
 {
     _settingsWidget.saveSettings(settings);
diff --git a/src/asciireader.h b/src/asciireader.h
--- a/src/asciireader.h
+++ b/src/asciireader.h
@@ -21,7 +21,9 @@
 #define ASCIIREADER_H
 
 #include <QSettings>
+#include <QString>
 
+#include "samplepack.h"
 #include "abstractreader.h"
 #include "asciireadersettings.h"
 
@@ -50,6 +52,12 @@ private:
 
 private slots:
     void onDataReady() override;
+    /**
+     * Parses given line and returns sample pack.
+     *
+     * Returns `nullptr` in case of error.
+     */
+    SamplePack* parseLine(const QString& line) const;
 };
 
 #endif // ASCIIREADER_H