--- nand.py-
+++ nand.py
@@ -9,25 +9,57 @@
 		if parent:
 			setattr(parent, name, self)
 
+	def dbgout(self, s, always=False):
+		dbgout( '{} {}'.format( self.get_name(), s ), always )
+
 	def get_name(self):
 		me = self.name if self.name else ''
 		pa = self.parent.get_name() + '.' if self.parent else ''
 		return pa + me
 
+class Pin(Obj):
+	def __init__(self, parent, name, direc='IO', v=None):
+		Obj.__init__(self, parent, name)
+		self.direc = direc
+		self.v = v
+		self.conn = None
+
+	def connect(self, other):
+		self.conn = other
+		if other and other.conn != self:
+			other.conn = self
+
+	def set(self, v):
+		if v == self.v:
+			return
+		self.v = v
+		self.dbgout( str_v(v) )
+		if 'O' in self.direc and self.conn:
+			self.conn.set(v)
+		if 'I' in self.direc and self.parent and hasattr(self.parent, 'update'):
+			sched.enque(None, self.parent.update)
+
 class Foo(Obj):
 	def __init__(self, parent, latency=None):
 		Obj.__init__(self, parent, 'foo')
 		self.latency = latency
 		self.v = 0
+
+		Pin(self, 'inp', 'I')
+		Pin(self, 'out', 'O')
+
 		sched.enque( latency, self.add, (0, ) )
 
 	def add(self, a):
 		self.v += a
-		msg = '{} v={}'.format( self.get_name(), self.v )
-		dbgout(msg)
+		self.dbgout( 'v={}'.format(self.v) )
 		if self.v < 55:
 			sched.enque( self.latency, self.add, (a+1,) )
 
+	def update(self):
+		v = not self.inp.v
+		sched.enque( self.latency, self.out.set, (v,) )
+
 class Sched:
 	def __init__(self):
 		self.que = []
@@ -52,6 +84,9 @@
 			self.now = tm
 			f(*args)
 
+def str_v(v):
+	return { True: 'H', False: 'L', None: 'Hi-Z' }.get(v, '?')
+
 def dbgout(s, always=False):
 	if always or '-v' in sys.argv:
 		print '{} {}'.format( sched.now, s )
@@ -69,6 +104,10 @@
 	slow_12 = Obj(o, 'slow_12')
 	Foo(slow_12, 12)
 
+	o.foo.out.connect( o.slow.foo.inp )
+	o.slow.foo.out.connect( o.slow_12.foo.inp )
+	sched.enque( 200, o.foo.out.set, (True,) )
+
 	sched.main_loop()
 
 # EOF