diff --git a/.travis.yml b/.travis.yml
index 35fbacdb..914ebe81 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,13 +9,19 @@ python:
   - "3.4"
   - "3.5"
 
+# binary install as per travis instructions
+# used to install latest version of ghdl
+before_script:
+  - ./scripts/install_ghdl.sh
+  - export PATH=$PATH:$PWD/ghdl-0.33/bin/
+
 addons:
   apt:
-    sources:
-      - pgavin-ghdl
+    # sources:
+    #  - pgavin-ghdl
     packages:
       - iverilog
-      - ghdl
+    #  - ghdl
 
 install:
   - pip install .
diff --git a/doc/source/manual/reference.rst b/doc/source/manual/reference.rst
index 38ce3afa..53ef0163 100644
--- a/doc/source/manual/reference.rst
+++ b/doc/source/manual/reference.rst
@@ -41,6 +41,14 @@ A :class:`Simulation` object has the following method:
    Run the simulation forever (by default) or for a specified duration.
 
 
+.. method:: Simulation.quit()
+
+   Quit the simulation after it has run for a specified duration. The method should
+   be called (the simulation instance must be quit) before another simulation
+   instance is created. The method is called by default when the simulation is run
+   forever.
+
+
 .. _ref-simsupport:
 
 Simulation support functions
@@ -90,6 +98,11 @@ Waveform tracing
       This attribute is used to set the directory to which VCD files are written. By
       default, the current working directory is used.
 
+   .. attribute:: filename
+
+      This attribute is used to set the filename to which VCD files are written. By
+      default, the name attribbute is used.
+
    .. attribute:: timescale
 
       This attribute is used to set the timescale corresponding to unit steps,
diff --git a/example/cookbook/dff/dff.py b/example/cookbook/dff/dff.py
index ecd18d71..e69fcd45 100644
--- a/example/cookbook/dff/dff.py
+++ b/example/cookbook/dff/dff.py
@@ -33,6 +33,7 @@ def simulate(timesteps):
     tb = traceSignals(test_dff)
     sim = Simulation(tb)
     sim.run(timesteps)
+    sim.quit()
 
 simulate(2000)
 
diff --git a/example/cookbook/dffa/dffa.py b/example/cookbook/dffa/dffa.py
index ac6e66bb..be0da586 100644
--- a/example/cookbook/dffa/dffa.py
+++ b/example/cookbook/dffa/dffa.py
@@ -44,6 +44,7 @@ def simulate(timesteps):
     tb = traceSignals(test_dffa)
     sim = Simulation(tb)
     sim.run(timesteps)
+    sim.quit()
 
 simulate(20000)
 
diff --git a/example/cookbook/latch/latch.py b/example/cookbook/latch/latch.py
index 9ab1b3b6..7439357d 100644
--- a/example/cookbook/latch/latch.py
+++ b/example/cookbook/latch/latch.py
@@ -33,6 +33,7 @@ def simulate(timesteps):
     tb = traceSignals(test_latch)
     sim = Simulation(tb)
     sim.run(timesteps)
+    sim.quit()
 
 simulate(20000)
 
diff --git a/myhdl/_Simulation.py b/myhdl/_Simulation.py
index 80ec07c1..07014b8d 100644
--- a/myhdl/_Simulation.py
+++ b/myhdl/_Simulation.py
@@ -45,6 +45,7 @@ class _error:
 _error.ArgType = "Inappriopriate argument type"
 _error.MultipleCosim = "Only a single cosimulator argument allowed"
 _error.DuplicatedArg = "Duplicated argument"
+_error.MultipleSim = "Only a single Simulation instance is allowed"
             
 class Simulation(object):
 
@@ -54,6 +55,7 @@ class Simulation(object):
     run -- run a simulation for some duration
 
     """
+    _no_of_instances = 0
 
     def __init__(self, *args):
         """ Construct a simulation object.
@@ -65,13 +67,16 @@ class Simulation(object):
         _simulator._time = 0
         arglist = _flatten(*args)
         self._waiters, self._cosim = _makeWaiters(arglist)
+        if Simulation._no_of_instances > 0:
+            raise SimulationError(_error.MultipleSim)
+        Simulation._no_of_instances += 1
         if not self._cosim and _simulator._cosim:
             warn("Cosimulation not registered as Simulation argument")
         self._finished = False
         del _futureEvents[:]
         del _siglist[:]
-        
-        
+
+
     def _finalize(self):
         cosim = self._cosim
         if cosim:
@@ -85,9 +90,12 @@ class Simulation(object):
         # clean up for potential new run with same signals
         for s in _signals:
             s._clear()
+        Simulation._no_of_instances = 0
         self._finished = True
-            
-        
+
+    def quit(self):
+        self._finalize()
+
     def runc(self, duration=0, quiet=0):
         simrunc.run(sim=self, duration=duration, quiet=quiet)
 
@@ -201,7 +209,7 @@ class Simulation(object):
                     self._finalize()
                 # now reraise the exepction
                 raise
-                
+
 
 def _makeWaiters(arglist):
     waiters = []
@@ -231,4 +239,4 @@ def _makeWaiters(arglist):
         if hasattr(sig, '_waiter'):
             waiters.append(sig._waiter)
     return waiters, cosim
-        
+
diff --git a/myhdl/_traceSignals.py b/myhdl/_traceSignals.py
index abd8dd5a..df4cc4ec 100644
--- a/myhdl/_traceSignals.py
+++ b/myhdl/_traceSignals.py
@@ -50,6 +50,7 @@ class _TraceSignalsClass(object):
 
     __slot__ = ("name",
                 "directory",
+                "filename",
                 "timescale",
                 "tracelists"
                 )
@@ -57,6 +58,7 @@ class _TraceSignalsClass(object):
     def __init__(self):
         self.name = None
         self.directory = None
+        self.filename = None
         self.timescale = "1ns"
         self.tracelists = True
 
@@ -89,8 +91,13 @@ class _TraceSignalsClass(object):
             else:
                 directory = self.directory
 
+            if self.filename is None:
+                filename = name
+            else:
+                filename = str(self.filename)
+
             h = _HierExtr(name, dut, *args, **kwargs)
-            vcdpath = os.path.join(directory, name + ".vcd")
+            vcdpath = os.path.join(directory, filename + ".vcd")
             if path.exists(vcdpath):
                 backup = vcdpath + '.' + str(path.getmtime(vcdpath))
                 shutil.copyfile(vcdpath, backup)
diff --git a/myhdl/conversion/_analyze.py b/myhdl/conversion/_analyze.py
index c8d94957..744db3a0 100644
--- a/myhdl/conversion/_analyze.py
+++ b/myhdl/conversion/_analyze.py
@@ -578,10 +578,14 @@ class _AnalyzeVisitor(ast.NodeVisitor, _ConversionMixin):
             node.obj = int(0) # XXX
         elif f is bool:
             node.obj = bool()
-        elif f in _flatten(integer_types, ord):
+        elif f in _flatten(integer_types):
             node.obj = int(-1)
 ##         elif f in (posedge , negedge):
 ##             node.obj = _EdgeDetector()
+        elif f is ord:
+            node.obj = int(-1)
+            if not (isinstance(node.args[0], ast.Str) and (len(node.args[0].s) == 1)):
+                self.raiseError(node, _error.NotSupported, "ord: expect string argument with length 1")
         elif f is delay:
             node.obj = delay(0)
         ### suprize: identity comparison on unbound methods doesn't work in python 2.5??
@@ -915,6 +919,8 @@ class _AnalyzeVisitor(ast.NodeVisitor, _ConversionMixin):
                         s = s[m.end():]
                         continue
                     self.raiseError(node, _error.UnsupportedFormatString, "%s" % s)
+            elif isinstance(n, ast.Str):
+                f.append(n.s)
             else:
                 f.append(defaultConvSpec)
                 a.append(n)
diff --git a/myhdl/conversion/_toVHDL.py b/myhdl/conversion/_toVHDL.py
index dca792e1..dcb9c4a1 100644
--- a/myhdl/conversion/_toVHDL.py
+++ b/myhdl/conversion/_toVHDL.py
@@ -602,9 +602,9 @@ class _ConvertVisitor(ast.NodeVisitor, _ConversionMixin):
                     pre, suf = "", "(0)"
                 else:
                     pre, suf = "stdl(", ")"
-        elif isinstance(vhd, vhd_string):
-            if isinstance(ori, vhd_enum):
-                pre, suf = "%s'image(" % ori._type._name, ")"
+        # elif isinstance(vhd, vhd_string):
+        #     if isinstance(ori, vhd_enum):
+        #         pre, suf = "%s'image(" % ori._type._name, ")"
 
         return pre, suf
 
@@ -654,10 +654,6 @@ class _ConvertVisitor(ast.NodeVisitor, _ConversionMixin):
             self.shiftOp(node)
         elif isinstance(node.op, (ast.BitAnd, ast.BitOr, ast.BitXor)):
             self.BitOp(node)
-        elif isinstance(node.op, ast.Mod) and (self.context == _context.PRINT):
-            self.visit(node.left)
-            self.write(", ")
-            self.visit(node.right)
         else:
             self.BinOp(node)
 
@@ -953,11 +949,10 @@ class _ConvertVisitor(ast.NodeVisitor, _ConversionMixin):
             return
         elif f is ord:
             opening, closing = '', ''
-            if isinstance(node.args[0], ast.Str):
-                if len(node.args[0].s) > 1:
-                    self.raiseError(node, _error.UnsupportedType, "Strings with length > 1" )
-                else:
-                    node.args[0].s = ord(node.args[0].s)
+            v = ord(node.args[0].s)
+            node.args[0].s = v
+            self.write(v)
+            return
         elif f in integer_types:
             opening, closing = '', ''
             pre, suf = self.inferCast(node.vhd, node.vhdOri)
@@ -1361,15 +1356,9 @@ class _ConvertVisitor(ast.NodeVisitor, _ConversionMixin):
                         a.vhd = vhd_boolean()
                     elif isinstance(a.vhdOri, vhd_enum):
                         a.vhd = vhd_string()
-                self.write("write(L, ")
-                self.context = _context.PRINT
+                self.write("write(L, to_string(")
                 self.visit(a)
-                self.context = None
-                if s.justified == 'LEFT':
-                    self.write(", justified=>LEFT")
-                if s.width:
-                    self.write(", field=>%s" % s.width)
-                self.write(")")
+                self.write("))")
                 self.write(';')
             self.writeline()
         self.write("writeline(output, L);")
diff --git a/myhdl/conversion/_toVerilog.py b/myhdl/conversion/_toVerilog.py
index be213ea7..b533054a 100644
--- a/myhdl/conversion/_toVerilog.py
+++ b/myhdl/conversion/_toVerilog.py
@@ -151,7 +151,7 @@ class _ToVerilogConvertor(object):
         genlist = _analyzeGens(arglist, h.absnames)
         siglist, memlist = _analyzeSigs(h.hierarchy)
         _annotateTypes(genlist)
-        
+
         intf = _analyzeTopFunc(func, *args, **kwargs)
         intf.name = name
         doc = _makeDoc(inspect.getdoc(func))
@@ -750,11 +750,7 @@ class _ConvertVisitor(ast.NodeVisitor, _ConversionMixin):
             return
         elif f is ord:
             opening, closing = '', ''
-            if isinstance(node.args[0], ast.Str):
-                if len(node.args[0].s) > 1:
-                    self.raiseError(node, _error.UnsupportedType, "Strings with length > 1")
-                else:
-                    node.args[0].s = str(ord(node.args[0].s))
+            node.args[0].s = str(ord(node.args[0].s))
         elif f in integer_types:
             opening, closing = '', ''
             # convert number argument to integer
diff --git a/myhdl/conversion/_verify.py b/myhdl/conversion/_verify.py
index 34e4192f..caac3098 100644
--- a/myhdl/conversion/_verify.py
+++ b/myhdl/conversion/_verify.py
@@ -22,7 +22,7 @@ _simulators = {}
 sim = namedtuple('sim', 'name hdl analyze elaborate simulate skiplines skipchars ignore')
 
 
-def registerSimulator(name=None, hdl=None, analyze=None, elaborate=None, simulate=None, 
+def registerSimulator(name=None, hdl=None, analyze=None, elaborate=None, simulate=None,
                       skiplines=None, skipchars=None, ignore=None):
     if not isinstance(name, str) or (name.strip() == ""):
         raise ValueError("Invalid simulator name")
@@ -41,8 +41,8 @@ def registerSimulator(name=None, hdl=None, analyze=None, elaborate=None, simulat
 registerSimulator(
     name="ghdl",
     hdl="VHDL",
-    analyze="ghdl -a --workdir=work pck_myhdl_%(version)s.vhd %(topname)s.vhd",
-    elaborate="ghdl -e --workdir=work -o %(unitname)s %(topname)s",
+    analyze="ghdl -a --std=08 --workdir=work pck_myhdl_%(version)s.vhd %(topname)s.vhd",
+    elaborate="ghdl -e --std=08 --workdir=work %(unitname)s",
     simulate="ghdl -r --workdir=work %(unitname)s"
     )
 
@@ -96,7 +96,7 @@ class  _VerificationClass(object):
     __slots__ = ("simulator", "_analyzeOnly")
 
     def __init__(self, analyzeOnly=False):
-        self.simulator = None 
+        self.simulator = None
         self._analyzeOnly = analyzeOnly
 
 
@@ -178,7 +178,7 @@ class  _VerificationClass(object):
             if ret != 0:
                 print("Elaboration failed", file=sys.stderr)
                 return ret
-            
+
         g = tempfile.TemporaryFile(mode='w+t')
         #print(simulate)
         ret = subprocess.call(simulate, stdout=g, shell=True)
diff --git a/myhdl/test/bugs/test_issue_104.py b/myhdl/test/bugs/test_issue_104.py
new file mode 100644
index 00000000..9734d29b
--- /dev/null
+++ b/myhdl/test/bugs/test_issue_104.py
@@ -0,0 +1,43 @@
+from __future__ import print_function
+import pytest
+from myhdl import Simulation, delay, SimulationError, instance, now
+from myhdl._Simulation import _error
+from helpers import raises_kind
+
+def test():
+  @instance
+  def tbstim():
+    yield delay(10)
+    print("{0:<8d} ".format(now()))
+    yield delay(1000)
+    print("{0:<8d} ".format(now()))
+    for _ in range(10):
+      yield delay(1000)
+
+  return tbstim
+
+
+def issue_104_quit_method():
+    sim = Simulation(test())
+    sim.run(1000)
+    sim.run(500)
+    sim.quit()
+    return sim._finished
+    
+def issue_104_multiple_instance():
+    sim1 = Simulation(test())
+    sim1.run(1000)
+    # sim1 is "puased"
+
+    # try and create a second, third, forth simulation instance
+    for ii in range(4):
+        with raises_kind(SimulationError, _error.MultipleSim):
+              another_sim = Simulation(test())
+    # generating more sims should have failed
+    sim1.run(1000)
+    sim1.quit()
+
+def test_issue_104():
+
+    assert issue_104_quit_method() == True
+    issue_104_multiple_instance()
diff --git a/myhdl/test/core/test_traceSignals.py b/myhdl/test/core/test_traceSignals.py
index 7f79c00a..1fea552a 100644
--- a/myhdl/test/core/test_traceSignals.py
+++ b/myhdl/test/core/test_traceSignals.py
@@ -155,12 +155,16 @@ class TestTraceSigs:
         assert not path.exists(psub)
 
     def testTristateTrace(self, vcd_dir):
-        Simulation(topTristate()).run(100, quiet=QUIET)
+        sim = Simulation(topTristate())
+        sim.run(100, quiet=QUIET)
+        sim.quit()
 
     def testBackupOutputFile(self, vcd_dir):
         p = "%s.vcd" % fun.__name__
         dut = traceSignals(fun)
-        Simulation(dut).run(1000, quiet=QUIET)
+        sim = Simulation(dut)
+        sim.run(1000, quiet=QUIET)
+        sim.quit()
         _simulator._tf.close()
         _simulator._tracing = 0
         size = path.getsize(p)
diff --git a/scripts/install_ghdl.sh b/scripts/install_ghdl.sh
new file mode 100755
index 00000000..ef655d42
--- /dev/null
+++ b/scripts/install_ghdl.sh
@@ -0,0 +1,5 @@
+#!/bin/bash
+set -evx
+wget https://sourceforge.net/projects/ghdl-updates/files/Builds/ghdl-0.33/ghdl-0.33-x86_64-linux.tgz -O /tmp/ghdl.tar.gz
+mkdir ghdl-0.33
+tar -C ghdl-0.33 -xvf /tmp/ghdl.tar.gz