diff --git a/commandwidget.cpp b/commandwidget.cpp
--- a/commandwidget.cpp
+++ b/commandwidget.cpp
@@ -20,6 +20,44 @@
 #include "commandwidget.h"
 #include "ui_commandwidget.h"
 
+#include <QRegExp>
+#include <QRegExpValidator>
+
+class HexCommandValidator : public QRegExpValidator
+{
+public:
+    explicit HexCommandValidator(QObject* parent = 0);
+    QValidator::State validate(QString & input, int & pos) const;
+};
+
+HexCommandValidator::HexCommandValidator(QObject* parent) :
+    QRegExpValidator(parent)
+{
+    QRegExp regExp("([0-9A-F]{2}[ ])*");
+    setRegExp(regExp);
+}
+
+QValidator::State HexCommandValidator::validate(QString & input, int & pos) const
+{
+    input = input.toUpper();
+
+    // don't let pos to be altered at this stage
+    int orgPos = pos;
+    auto r = QRegExpValidator::validate(input, pos);
+    pos = orgPos;
+
+    // try fixing up spaces
+    if (r == QValidator::Invalid)
+    {
+        input = input.replace(" ", "");
+        input.replace(QRegExp("([0-9A-F]{2})"), "\\1 ");
+        if (pos == input.size()-1) pos = input.size();
+        r = QRegExpValidator::validate(input, pos);
+    }
+
+    return r;
+}
+
 CommandWidget::CommandWidget(QWidget *parent) :
     QWidget(parent),
     ui(new Ui::CommandWidget)
@@ -28,11 +66,15 @@ CommandWidget::CommandWidget(QWidget *pa
 
     connect(ui->pbDelete, &QPushButton::clicked, this, &CommandWidget::onDeleteClicked);
     connect(ui->pbSend, &QPushButton::clicked, this, &CommandWidget::onSendClicked);
+    connect(ui->pbASCII, &QPushButton::toggled, this, &CommandWidget::onASCIIToggled);
+
+    hexValidator = new HexCommandValidator(this);
 }
 
 CommandWidget::~CommandWidget()
 {
     delete ui;
+    delete hexValidator;
 }
 
 void CommandWidget::onDeleteClicked()
@@ -44,3 +86,15 @@ void CommandWidget::onSendClicked()
 {
     emit sendCommand(ui->leCommand->text(), ui->pbASCII->isChecked());
 }
+
+void CommandWidget::onASCIIToggled(bool checked)
+{
+    if (checked)
+    {
+        ui->leCommand->setValidator(0);
+    }
+    else
+    {
+        ui->leCommand->setValidator(hexValidator);
+    }
+}