diff --git a/mux_signal.go b/mux_signal.go
new file mode 100644
index 0000000..ca35871
--- /dev/null
+++ b/mux_signal.go
@@ -0,0 +1,129 @@
+package acmelib
+
+import (
+	"errors"
+	"slices"
+)
+
+type MultiplexerSignal0 struct {
+	*signal
+
+	muxSignals     *set[EntityID, Signal]
+	muxSigNames    *set[string, EntityID]
+	muxSigGroupIDs *set[EntityID, []int]
+
+	fixedSignals *set[EntityID, bool]
+
+	groupCount int
+	groupSize  int
+	groups     []*signalPayload
+}
+
+func NewMultiplexerSignal0(name string, groupCount, groupSize int) *MultiplexerSignal0 {
+	groups := make([]*signalPayload, groupCount)
+	for i := 0; i < groupCount; i++ {
+		groups[i] = newSignalPayload(groupSize)
+	}
+
+	return &MultiplexerSignal0{
+		signal: newSignal(name, SignalKindMultiplexer),
+
+		muxSignals:     newSet[EntityID, Signal]("multiplexed signal"),
+		muxSigNames:    newSet[string, EntityID]("multiplexed signal name"),
+		muxSigGroupIDs: newSet[EntityID, []int]("multiplexer signal group id"),
+
+		fixedSignals: newSet[EntityID, bool]("fixed signals"),
+
+		groupCount: groupCount,
+		groupSize:  groupSize,
+		groups:     groups,
+	}
+}
+
+func (ms *MultiplexerSignal0) addMuxSignal(sig Signal) {
+	id := sig.EntityID()
+	name := sig.Name()
+
+	ms.muxSignals.add(id, sig)
+	ms.muxSigNames.add(name, id)
+
+	if ms.hasParent() {
+		parent := ms.Parent()
+		for parent != nil {
+			switch parent.GetSignalParentKind() {
+			case SignalParentKindMultiplexerSignal:
+				muxParent, err := parent.ToParentMultiplexerSignal()
+				if err != nil {
+					panic(err)
+				}
+				parent = muxParent.Parent()
+
+			case SignalParentKindMessage:
+				msgParent, err := parent.ToParentMessage()
+				if err != nil {
+					panic(err)
+				}
+
+				msgParent.signals.add(id, sig)
+				msgParent.signalNames.add(name, id)
+				return
+			}
+		}
+	}
+}
+
+func (ms *MultiplexerSignal0) GetSize() int {
+	return ms.groupSize + calcSizeFromValue(ms.groupCount)
+}
+
+func (ms *MultiplexerSignal0) InsertMuxSignal(signal Signal, startBit int, groupIDs ...int) error {
+	// TODO! checks on groupIDs and signal name
+
+	groupIDs = slices.Compact(groupIDs)
+
+	prevGroupIDs := []int{}
+	if ms.muxSigGroupIDs.hasKey(signal.EntityID()) {
+		tmpIDs, err := ms.muxSigGroupIDs.getValue(signal.EntityID())
+		if err != nil {
+			panic(err)
+		}
+		prevGroupIDs = tmpIDs
+	}
+
+	for _, groupID := range groupIDs {
+		if slices.Contains(prevGroupIDs, groupID) {
+			return ms.errorf(errors.New("dupl group id"))
+		}
+
+		if err := ms.groups[groupID].insert(signal, startBit); err != nil {
+			return err
+		}
+	}
+
+	ms.addMuxSignal(signal)
+
+	signal.setParent(nil)
+
+	return nil
+}
+
+func (ms *MultiplexerSignal0) InsertFixedSignal(signal Signal, startBit int) error {
+	// TODO! check signal name
+
+	for i := 0; i < ms.groupCount; i++ {
+		if err := ms.groups[i].insert(signal, startBit); err != nil {
+			return err
+		}
+	}
+
+	ms.addMuxSignal(signal)
+	ms.fixedSignals.add(signal.EntityID(), true)
+
+	signal.setParent(nil)
+
+	return nil
+}
+
+func (ms *MultiplexerSignal0) RemoveSignal(muxSignalEntityID EntityID) error {
+	return nil
+}
diff --git a/signal.go b/signal.go
index 7508661..bc2efcd 100644
--- a/signal.go
+++ b/signal.go
@@ -460,6 +460,10 @@ type MultiplexerSignal struct {
 
 	totalSize  int
 	selectSize int
+
+	groupCount int
+	groupSize  int
+	muxGroups  []*signalPayload
 }
 
 // NewMultiplexerSignal creates a new [MultiplexerSignal] with the given name,