diff --git a/gex/Client.py b/gex/Client.py
index 0e8eae0..366205d 100644
--- a/gex/Client.py
+++ b/gex/Client.py
@@ -79,6 +79,10 @@ class Client:
 
     def get_callsign(self, name:str, type:str = None) -> int:
         """ Find unit by name and type """
+
+        if name not in self.unit_lu:
+            raise Exception("No %s unit called \"%s\"" % (type or '*', name))
+
         u = self.unit_lu[name]
 
         if type is not None:
diff --git a/gex/__init__.py b/gex/__init__.py
index d1b5d1f..24ab0a8 100644
--- a/gex/__init__.py
+++ b/gex/__init__.py
@@ -8,6 +8,7 @@ from gex.Client import Client
 
 # import all the units
 from gex.units.Pin import Pin
+from gex.units.DOut import DOut
 
 
 # General, low level
diff --git a/gex/units/DOut.py b/gex/units/DOut.py
new file mode 100644
index 0000000..fe0115e
--- /dev/null
+++ b/gex/units/DOut.py
@@ -0,0 +1,38 @@
+import gex
+
+class DOut(gex.Unit):
+    """
+    Digital output port.
+    Pins are represented by bits of a control word, right-aligned.
+
+    For example, if pins C6, C5 and C0 are selected for the unit,
+    calling the "set" function with a word 0b111 will set all three to 1,
+    0b100 will set only C6.
+    """
+
+    def _type(self):
+        return 'DO'
+
+    def write(self, pins:int):
+        """ Set pins to a value """
+        pb = gex.PayloadBuilder()
+        pb.u16(pins)
+        self.send(0x00, pb.close())
+
+    def set(self, pins:int):
+        """ Set pins high """
+        pb = gex.PayloadBuilder()
+        pb.u16(pins)
+        self.send(0x01, pb.close())
+
+    def clear(self, pins:int):
+        """ Set pins low """
+        pb = gex.PayloadBuilder()
+        pb.u16(pins)
+        self.send(0x02, pb.close())
+
+    def toggle(self, pins:int):
+        """ Toggle pins """
+        pb = gex.PayloadBuilder()
+        pb.u16(pins)
+        self.send(0x03, pb.close())
diff --git a/main.py b/main.py
index 319e67a..33a649e 100644
--- a/main.py
+++ b/main.py
@@ -14,9 +14,24 @@ if False:
 
     client.bulk_write(gex.MSG_INI_WRITE, buf)
 
-if True:
+if False:
     led = gex.Pin(client, 'LED')
 
     for i in range(0,10):
         led.toggle()
         time.sleep(.1)
+
+if True:
+    leds = gex.DOut(client, 'TST')
+
+    for i in range(0,0x41):
+        leds.write(i&0x3F)
+        time.sleep(.1)
+
+if False:
+    leds = gex.DOut(client, 'TST')
+
+    for i in range(0, 0x41):
+        #leds.write(i & 0x3F)
+        leds.toggle(0xFF)
+        time.sleep(.1)
\ No newline at end of file