diff --git a/lib/rcircuit/vcd.rb b/lib/rcircuit/vcd.rb
new file mode 100644
index 0000000..0773815
--- /dev/null
+++ b/lib/rcircuit/vcd.rb
@@ -0,0 +1,106 @@
+# creates VCD file from port states
+
+require "set"
+
+def create_vcd_id(index)
+  #VCD uses short id codes for signals.
+  #Generate a unique id given a numeric index, 'a' to 'z'
+  #then 'aa', ab',...
+  new_id = ""
+  loop do
+    new_id = ('a'.ord + index%26).chr + new_id
+    index = (index - index%26)
+    break if index < 1
+    index = index / 26 - 1
+  end
+  return new_id
+end
+
+def test_vcd_id
+  [0, 1, 25, 26, 30, 26*26-1, 26*26, 27*26, 27*26+1].each do |i|
+    puts "#{i} -> #{create_vcd_id(i)}"
+  end
+end
+
+#test_vcd_id()
+
+class VCD
+  def initialize(filename, timescale="1ps")
+    @fd = File.open(filename, 'w')
+    @time=0
+    @id_index = 0
+    @changeset = Set.new()
+    @timescale = timescale
+    @portmap = {}
+    @idmap = {}
+    write_header
+  end
+  
+  def start
+    @portmap.keys.each do |portname|
+      port = @portmap[portname]
+      id = @idmap[portname]
+      @fd.write("$var wire #{port.width} #{id} #{portname} $end\n")
+    end
+    @fd.write("$dumpvars\n")
+    @portmap.keys.each do |portname|
+      write_port_state(portname)
+    end
+    @fd.write("$end\n")
+    @fd.write("\#0\n")
+    return self
+  end
+  
+  def finish
+    @fd.close
+  end
+  
+  def attach(port, portname=nil)
+    if portname == nil
+      portname = port.to_s
+    end
+    raise RunTimeError.new("Duplicate port name '#{portname}'") if @portmap.has_key?(portname)
+    @idmap[portname] = create_vcd_id(@id_index)
+    @id_index += 1
+    @portmap[portname] = port
+    port.add_callback do |value|
+      @changeset.add(portname)
+    end
+    return self
+  end
+  
+  def advance(timesteps)
+    @time += timesteps
+    @fd.write("\##{@time.to_s}\n")
+    return self
+  end
+  
+  def write
+    #write out any changes and clear the changed set
+    @changeset.each do |portname|
+      write_port_state(portname)
+    end
+    @changeset.clear
+  end
+  
+  def write_header
+    @fd.write("$date\n#{Time.now.asctime}\n$end\n")
+    @fd.write("$version\nRCircuit VCD Generator Version 0.0\n$end\n")
+    @fd.write("$timescale #{@timescale} $end\n")
+  end
+
+  def write_port_state(portname)
+    #writes VCD line for state of port, using id
+    port = @portmap[portname]
+    if port.width == 1
+      state = port.val.to_s
+    else
+      state = "b#{port.val.to_s(2)} "  #include a space
+    end
+    @fd.write(state + @idmap[portname] + "\n")
+  end
+    
+end
+  
+
+  
\ No newline at end of file
diff --git a/vcd_test.rb b/vcd_test.rb
new file mode 100644
index 0000000..8296ea7
--- /dev/null
+++ b/vcd_test.rb
@@ -0,0 +1,27 @@
+#xor_test.rb
+
+require_relative "lib/rcircuit/port.rb"
+require_relative "lib/rcircuit/gate.rb"
+require_relative "lib/rcircuit/xor.rb"
+require_relative "lib/rcircuit/vcd.rb"
+
+in_a = Port.new(1, "A")
+in_b = Port.new(1, "B")
+gate = XorGate.new(in_a, in_b)
+
+vcd = VCD.new("vcd_test.vcd").attach(in_a)
+                             .attach(in_b)
+                             .attach(gate.out, "OUT")
+
+in_a.setval(0)
+in_b.setval(0)
+vcd.start
+in_a.setval(1)
+vcd.advance(1000).write
+in_b.setval(1)
+vcd.advance(1000).write
+in_a.setval(0)
+vcd.advance(1000).write
+vcd.advance(1000)
+vcd.finish
+