diff --git a/commandwidget.cpp b/commandwidget.cpp
--- a/commandwidget.cpp
+++ b/commandwidget.cpp
@@ -48,10 +48,38 @@ void CommandWidget::onDeleteClicked()
 
 void CommandWidget::onSendClicked()
 {
-    emit sendCommand(ui->leCommand->text(), ui->pbASCII->isChecked());
+    auto command = ui->leCommand->text();
+
+    if (command.isEmpty())
+    {
+        qWarning() << "Enter a command to send!";
+        ui->leCommand->setFocus(Qt::OtherFocusReason);
+        return;
+    }
+
+    if (!isASCIIMode()) // hex mode
+    {
+        command = command.remove(' ');
+        // check if nibbles are missing
+        if (command.size() % 2 == 1)
+        {
+            qWarning() << "HEX command is missing a nibble at the end!";
+            ui->leCommand->setFocus(Qt::OtherFocusReason);
+            int textSize = ui->leCommand->text().size();
+            ui->leCommand->setSelection(textSize-1, textSize);
+            return;
+        }
+    }
+
+    emit sendCommand(command, ui->pbASCII->isChecked());
 }
 
 void CommandWidget::onASCIIToggled(bool checked)
 {
     ui->leCommand->setMode(checked);
 }
+
+bool CommandWidget::isASCIIMode()
+{
+    return ui->pbASCII->isChecked();
+}
diff --git a/commandwidget.h b/commandwidget.h
--- a/commandwidget.h
+++ b/commandwidget.h
@@ -37,10 +37,16 @@ public:
 
 signals:
     void deleteRequested(CommandWidget* thisWidget); // emitted when delete button is clicked
-    void sendCommand(QString command, bool ascii);   // emitted when send button clicked
+
+    // emitted when send button is clicked
+    //
+    // in case of hex mode, command text should be a hexadecimal
+    // string containing hexadecimal characters only (not even spaces)
+    void sendCommand(QString command, bool ascii);
 
 private:
     Ui::CommandWidget *ui;
+    bool isASCIIMode(); // true: ascii mode, false hex mode
 
 private slots:
     void onDeleteClicked();