diff --git a/x26model.cpp b/x26model.cpp
index 7bed1eb..82b516c 100644
--- a/x26model.cpp
+++ b/x26model.cpp
@@ -64,6 +64,16 @@ QVariant X26Model::data(const QModelIndex &index, int role) const
 				return QVariant();
 		}
 
+	// Error colours from KDE Plasma Breeze (light) theme
+	if (role == Qt::ForegroundRole && triplet.error() != X26Triplet::NoError && index.column() == m_tripletErrors[triplet.error()].columnHighlight)
+		return QColor(252, 252, 252);
+
+	if (role == Qt::BackgroundRole && triplet.error() != X26Triplet::NoError && index.column() == m_tripletErrors[triplet.error()].columnHighlight)
+		return QColor(218, 68, 63);
+
+	if (role == Qt::ToolTipRole && triplet.error() != X26Triplet::NoError)
+		return m_tripletErrors[triplet.error()].message;
+
 	if (role == Qt::DisplayRole || role == Qt::EditRole)
 		switch (index.column()) {
 			case 0:
@@ -94,7 +104,7 @@ QVariant X26Model::data(const QModelIndex &index, int role) const
 
 	if (role == Qt::DisplayRole) {
 		if (index.column() == 2)
-			return (modeTripletName[triplet.modeExt()]);
+			return (m_modeTripletName[triplet.modeExt()]);
 		// Column 3 - describe effects of data/address triplet parameters in plain English
 		switch (triplet.modeExt()) {
 			case 0x01: // Full row colour
diff --git a/x26model.h b/x26model.h
index bf4eee8..99e511c 100644
--- a/x26model.h
+++ b/x26model.h
@@ -51,87 +51,98 @@ private:
 	TeletextWidget *m_parentMainWidget;
 	bool m_listLoaded;
 	TeletextFontBitmap m_fontBitmap;
-};
 
-static const QString modeTripletName[64] {
-	// Row triplet modes
-	"Full screen colour",
-	"Full row colour",
-	"Reserved 0x02",
-	"Reserved 0x03",
+	const QString m_modeTripletName[64] {
+		// Row triplet modes
+		"Full screen colour",
+		"Full row colour",
+		"Reserved 0x02",
+		"Reserved 0x03",
 
-	"Set Active Position",
-	"Reserved 0x05",
-	"Reserved 0x06",
-	"Address row 0",
+		"Set Active Position",
+		"Reserved 0x05",
+		"Reserved 0x06",
+		"Address row 0",
 
-	"PDC origin, source",
-	"PDC month and day",
-	"PDC cursor and start hour",
-	"PDC cursor and end hour",
+		"PDC origin, source",
+		"PDC month and day",
+		"PDC cursor and start hour",
+		"PDC cursor and end hour",
 
-	"PDC cursor local offset",
-	"PDC series ID and code",
-	"Reserved 0x0e",
-	"Reserved 0x0f",
+		"PDC cursor local offset",
+		"PDC series ID and code",
+		"Reserved 0x0e",
+		"Reserved 0x0f",
 
-	"Origin modifier",
-	"Invoke active object",
-	"Invoke adaptive object",
-	"Invoke passive object",
+		"Origin modifier",
+		"Invoke active object",
+		"Invoke adaptive object",
+		"Invoke passive object",
 
-	"Reserved 0x14",
-	"Define active object",
-	"Define adaptive object",
-	"Define passive object",
+		"Reserved 0x14",
+		"Define active object",
+		"Define adaptive object",
+		"Define passive object",
 
-	"DRCS mode",
-	"Reserved 0x19",
-	"Reserved 0x1a",
-	"Reserved 0x1b",
+		"DRCS mode",
+		"Reserved 0x19",
+		"Reserved 0x1a",
+		"Reserved 0x1b",
 
-	"Reserved 0x1c",
-	"Reserved 0x1d",
-	"Reserved 0x1e",
-	"Termination marker",
+		"Reserved 0x1c",
+		"Reserved 0x1d",
+		"Reserved 0x1e",
+		"Termination marker",
 
-	// Column triplet modes
-	"Foreground colour",
-	"G1 character",
-	"G3 character, level 1.5",
-	"Background colour",
+		// Column triplet modes
+		"Foreground colour",
+		"G1 character",
+		"G3 character, level 1.5",
+		"Background colour",
 
-	"Reserved 0x04",
-	"Reserved 0x05",
-	"PDC cursor, start end min",
-	"Additional flash functions",
+		"Reserved 0x04",
+		"Reserved 0x05",
+		"PDC cursor, start end min",
+		"Additional flash functions",
 
-	"Modified G0/G2 character set",
-	"G0 character",
-	"Reserved 0x0a",
-	"G3 character, level 2.5",
+		"Modified G0/G2 character set",
+		"G0 character",
+		"Reserved 0x0a",
+		"G3 character, level 2.5",
 
-	"Display attributes",
-	"DRCS character",
-	"Font style",
-	"G2 character",
+		"Display attributes",
+		"DRCS character",
+		"Font style",
+		"G2 character",
 
-	"G0 character no diacritical",
-	"G0 character diacritical 1",
-	"G0 character diacritical 2",
-	"G0 character diacritical 3",
-	"G0 character diacritical 4",
-	"G0 character diacritical 5",
-	"G0 character diacritical 6",
-	"G0 character diacritical 7",
-	"G0 character diacritical 8",
-	"G0 character diacritical 9",
-	"G0 character diacritical A",
-	"G0 character diacritical B",
-	"G0 character diacritical C",
-	"G0 character diacritical D",
-	"G0 character diacritical E",
-	"G0 character diacritical F"
+		"G0 character no diacritical",
+		"G0 character diacritical 1",
+		"G0 character diacritical 2",
+		"G0 character diacritical 3",
+		"G0 character diacritical 4",
+		"G0 character diacritical 5",
+		"G0 character diacritical 6",
+		"G0 character diacritical 7",
+		"G0 character diacritical 8",
+		"G0 character diacritical 9",
+		"G0 character diacritical A",
+		"G0 character diacritical B",
+		"G0 character diacritical C",
+		"G0 character diacritical D",
+		"G0 character diacritical E",
+		"G0 character diacritical F"
+	};
+
+	struct tripletErrorShow {
+		QString message;
+		int columnHighlight;
+	};
+
+	const tripletErrorShow m_tripletErrors[3] {
+		{ "", 0 }, // No error
+		{ "Active Position can't move up", 0 },
+		{ "Active Position can't move left within row", 1 }
+	};
 };
 
 #endif
diff --git a/x26triplets.cpp b/x26triplets.cpp
index fcad995..286277a 100644
--- a/x26triplets.cpp
+++ b/x26triplets.cpp
@@ -52,22 +52,117 @@ void X26Triplet::setAddressColumn(int addressColumn)
 }
 
 
+void X26TripletList::updateInternalData(int r)
+{
+	ActivePosition activePosition;
+	X26Triplet *triplet;
+
+	if (r != 0) {
+		activePosition.setRow(m_list[r-1].m_activePositionRow);
+		activePosition.setColumn(m_list[r-1].m_activePositionColumn);
+	}
+
+	for (int i = r; i < m_list.size(); i++) {
+		triplet = &m_list[i];
+		triplet->m_error = X26Triplet::NoError;
+		if (triplet->isRowTriplet()) {
+			switch (m_list.at(i).modeExt()) {
+				case 0x00: // Full screen colour
+					if (activePosition.isDeployed())
+						// TODO more specific error needed
+						triplet->m_error = X26Triplet::ActivePositionMovedUp;
+					break;
+				case 0x01: // Full row colour
+					if (!activePosition.setRow(triplet->addressRow()))
+						triplet->m_error = X26Triplet::ActivePositionMovedUp;
+					break;
+				case 0x04: // Set Active Position;
+					if (!activePosition.setRow(triplet->addressRow()))
+						triplet->m_error = X26Triplet::ActivePositionMovedUp;
+					else if (triplet->data() >= 40 || !activePosition.setColumn(triplet->data()))
+						triplet->m_error = X26Triplet::ActivePositionMovedLeft;
+					break;
+				case 0x07: // Address row 0
+					if (activePosition.isDeployed())
+						triplet->m_error = X26Triplet::ActivePositionMovedUp;
+					else {
+						activePosition.setRow(0);
+						activePosition.setColumn(8);
+					}
+					break;
+				case 0x15 ... 0x17: // Define Object
+					activePosition.reset();
+			};
+		// Column triplet: make sure that PDC and reserved triplets don't affect the Active Position
+		} else if (triplet->modeExt() != 0x24 && triplet->modeExt() != 0x25 && triplet->modeExt() != 0x26 && triplet->modeExt() != 0x2a)
+			if (!activePosition.setColumn(triplet->addressColumn()))
+				triplet->m_error = X26Triplet::ActivePositionMovedLeft;
+		triplet->m_activePositionRow = activePosition.row();
+		triplet->m_activePositionColumn = activePosition.column();
+	}
+}
+
 void X26TripletList::append(const X26Triplet &value)
 {
 	m_list.append(value);
+	updateInternalData(m_list.size()-1);
 }
 
 void X26TripletList::insert(int i, const X26Triplet &value)
 {
 	m_list.insert(i, value);
+	updateInternalData(i);
 }
 
 void X26TripletList::removeAt(int i)
 {
 	m_list.removeAt(i);
+	if (m_list.size() != 0 && i < m_list.size())
+		updateInternalData(i);
 }
 
 void X26TripletList::replace(int i, const X26Triplet &value)
 {
 	m_list.replace(i, value);
+	updateInternalData(i);
 }
+
+
+X26TripletList::ActivePosition::ActivePosition()
+{
+	m_row = m_column = -1;
+}
+
+void X26TripletList::ActivePosition::reset()
+{
+	m_row = m_column = -1;
+}
+
+bool X26TripletList::ActivePosition::setRow(int row)
+{
+	if (row < m_row)
+		return false;
+	if (row > m_row) {
+		m_row = row;
+		m_column = -1;
+	}
+	return true;
+}
+
+bool X26TripletList::ActivePosition::setColumn(int column)
+{
+	if (column < m_column)
+		return false;
+	if (m_row == -1 and column >= 0)
+		m_row = 0;
+	m_column = column;
+	return true;
+}
+/*
+bool X26TripletList::ActivePosition::setRowAndColumn(int newRow, int newColumn)
+{
+	if (!setRow(newRow))
+		return false;
+	return setColumn(newColumn);
+}
+*/
diff --git a/x26triplets.h b/x26triplets.h
index 422ca0e..fd1a625 100644
--- a/x26triplets.h
+++ b/x26triplets.h
@@ -25,6 +25,8 @@
 class X26Triplet
 {
 public:
+	enum X26TripletError { NoError, ActivePositionMovedUp, ActivePositionMovedLeft };
+
 	X26Triplet() {}
 //	X26Triplet(const X26Triplet &other);
 
@@ -46,8 +48,17 @@ public:
 	void setAddressRow(int);
 	void setAddressColumn(int);
 
+	int activePositionRow() const { return m_activePositionRow; }
+	int activePositionColumn() const { return m_activePositionColumn; }
+	X26TripletError error() const { return m_error; }
+
+	friend class X26TripletList;
+
 private:
 	int m_address, m_mode, m_data;
+	int m_activePositionRow = -1;
+	int m_activePositionColumn = -1;
+	X26TripletError m_error = NoError;
 };
 
 class X26TripletList
@@ -66,7 +77,27 @@ public:
 	int size() const { return m_list.size(); }
 
 private:
+	void updateInternalData(int);
+
 	QList<X26Triplet> m_list;
+
+	class ActivePosition
+	{
+	public:
+		ActivePosition();
+		void reset();
+//		int row() const { return (m_row == -1) ? 0 : m_row; }
+//		int column() const { return (m_column == -1) ? 0 : m_column; }
+		int row() const { return m_row; }
+		int column() const { return m_column; }
+		bool isDeployed() const { return m_row != -1; }
+		bool setRow(int);
+		bool setColumn(int);
+//		bool setRowAndColumn(int, int);
+
+	private:
+		int m_row, m_column;
+	};
 };
 
 #endif