diff --git a/sointu.go b/sointu.go
index b185878..942303f 100644
--- a/sointu.go
+++ b/sointu.go
@@ -268,6 +268,20 @@ var UnitTypes = map[string]([]UnitParameter){
 		{Name: "channel", MinValue: 0, MaxValue: 6, CanSet: true, CanModulate: false}},
 }
 
+var Ports = make(map[string]([]string))
+
+func init() {
+	for name, unitType := range UnitTypes {
+		unitPorts := make([]string, 0)
+		for _, param := range unitType {
+			if param.CanModulate {
+				unitPorts = append(unitPorts, param.Name)
+			}
+		}
+		Ports[name] = unitPorts
+	}
+}
+
 type Song struct {
 	BPM            int
 	RowsPerPattern int
@@ -358,6 +372,46 @@ func (s *Song) Validate() error {
 	return nil
 }
 
+func (p *Patch) FindSendTarget(i, u int) (int, int, int, error) {
+	if i < 0 || i >= len(p.Instruments) {
+		return -1, -1, -1, errors.New("instrument index out of range")
+	}
+	instr := p.Instruments[i]
+	if u < 0 || u >= len(instr.Units) {
+		return -1, -1, -1, errors.New("unit index out of range")
+	}
+	unit := instr.Units[u]
+	var targetInstrIndex int
+	if unit.Parameters["voice"] == 0 {
+		targetInstrIndex = i
+	} else {
+		var err error
+		targetInstrIndex, err = p.InstrumentForVoice(unit.Parameters["voice"] - 1)
+		if err != nil {
+			return -1, -1, -1, errors.New("the target voice was out of range")
+		}
+	}
+	targetInstr := p.Instruments[targetInstrIndex]
+	targetUnitIndex := unit.Parameters["unit"]
+	if targetUnitIndex < 0 || targetUnitIndex >= len(targetInstr.Units) {
+		return targetInstrIndex, -1, -1, errors.New("the target unit was out of range")
+	}
+	targetUnit := targetInstr.Units[targetUnitIndex]
+	port := unit.Parameters["port"]
+	if port < 0 {
+		return targetInstrIndex, targetUnitIndex, -1, errors.New("the target port was out of range")
+	}
+	for k, param := range UnitTypes[targetUnit.Type] {
+		if param.CanModulate {
+			port--
+			if port < 0 {
+				return targetInstrIndex, targetUnitIndex, k, nil
+			}
+		}
+	}
+	return targetInstrIndex, targetUnitIndex, -1, errors.New("the target port was out of range")
+}
+
 func (s *Song) ParamHintString(instrIndex, unitIndex int, param string) string {
 	if instrIndex < 0 || instrIndex >= len(s.Patch.Instruments) {
 		return ""
diff --git a/tracker/uniteditor.go b/tracker/uniteditor.go
index 20504d2..af171ed 100644
--- a/tracker/uniteditor.go
+++ b/tracker/uniteditor.go
@@ -67,6 +67,19 @@ func (t *Tracker) layoutUnitSliders(gtx C) D {
 			}
 			value = params[name]
 			min, max = ut[index].MinValue, ut[index].MaxValue
+			if u.Type == "send" && name == "voice" {
+				max = t.song.Patch.TotalVoices()
+			} else if u.Type == "send" && name == "unit" { // set the maximum values depending on the send target
+				instrIndex, _, _, _ := t.song.Patch.FindSendTarget(t.CurrentInstrument, t.CurrentUnit)
+				if instrIndex != -1 {
+					max = len(t.song.Patch.Instruments[instrIndex].Units) - 1
+				}
+			} else if u.Type == "send" && name == "port" { // set the maximum values depending on the send target
+				instrIndex, unitIndex, _, _ := t.song.Patch.FindSendTarget(t.CurrentInstrument, t.CurrentUnit)
+				if instrIndex != -1 && unitIndex != -1 {
+					max = len(sointu.Ports[t.song.Patch.Instruments[instrIndex].Units[unitIndex].Type]) - 1
+				}
+			}
 			hint := t.song.ParamHintString(t.CurrentInstrument, t.CurrentUnit, name)
 			if hint != "" {
 				valueText = fmt.Sprintf("%v / %v", value, hint)
@@ -77,6 +90,9 @@ func (t *Tracker) layoutUnitSliders(gtx C) D {
 		if !t.ParameterSliders[index].Dragging() {
 			t.ParameterSliders[index].Value = float32(value)
 		}
+		if max < min {
+			max = min
+		}
 		sliderStyle := material.Slider(t.Theme, t.ParameterSliders[index], float32(min), float32(max))
 		sliderStyle.Color = t.Theme.Fg
 		return layout.Flex{Axis: layout.Horizontal, Alignment: layout.Middle}.Layout(gtx,